SYSTEM AND METHOD FOR PUMPING A PARTICLE-LADEN FLUID, SUCH AS PRESSURIZED FRACKING FLUID

Abstract

A system for delivering pressurized fracking fluid to a well includes a high-pressure pump pumping proppant-free fluid, and a double-piston pressure transfer arrangement for transferring pressure from the proppant-free fluid to a fracking fluid. The pressure transfer arrangement includes first and second cylinder assemblies, each having a piston in sliding engagement within a first hollow cylinder so as to define first and second chambers of the first cylinder and third and fourth chambers of the second cylinder. A piston rod interconnects the pistons. A flow selector alternately directs high-pressure proppant-free fluid to the first chamber and the third chamber so as to act on the pistons, thereby applying pressure to fracking fluid within the second and fourth chambers, respectively, for delivery to the well in alternate power strokes. The faces of the pistons facing the first and third chambers have a smaller effective surface area than the faces towards the second and fourth chambers such that pressure in the fracking fluid remains lower than pressure in the proppant-free fluid.

Description

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to mining and, in particular, it concerns a system and method for pumping a particle-laden fluid, such as pressurized fracking fluid delivered to a well.

Well completion operations in the oil and gas industry often involve hydraulic fracturing (often referred to as fracking or fracing) to increase the release of oil and gas in rock formations. Hydraulic fracturing involves pumping a fluid (a “frac fluid” or “fracking fluid”) containing a combination of water, chemicals, and proppant (e.g., sand, ceramics) into a well at high pressures. The high pressures of the fluid increase crack size and crack propagation through the rock formation, thereby releasing more oil and gas, while the proppant prevents the cracks from closing once the fluid is depressurized. Fracturing operations use high-pressure pumps to increase the pressure of the fracking fluid. Unfortunately, the proppant in the fracking fluid increases wear and maintenance on the high-pressure pumps.

US Patent Application Publication No. 2015/0096739 A1 proposes the use of pressure transfer between two fluids to allow the high pressure pumps to operate on a “clean” fluid without proppant, and then uses the pressure of the clean fluid to drive the proppant-laden fluid. The proposed systems may however suffer from leakage of proppant-laden fluid into the clean fluid, leading to wear on the pressure-transfer system and contamination of the clean fluid.

SUMMARY OF THE INVENTION

The present invention is a system and method for pumping particle-laden fluid.

According to the teachings of the present invention there is provided, a system for delivering pressurized fracking fluid to a well, the system comprising: (a) a source of proppant-laden fracking fluid; (b) a high-pressure pump connected to a source of proppant-free fluid; (c) a double-piston pressure transfer arrangement, the pressure transfer arrangement comprising: (i) a first cylinder assembly comprising a first hollow cylinder, and a first piston in sliding engagement within the first hollow cylinder so as to at least partially define a first chamber and a second chamber; (ii) a second cylinder assembly comprising a second hollow cylinder, and a second piston in sliding engagement within the second hollow cylinder so as to at least partially define a third chamber and a fourth chamber; and (iii) at least one piston rod interconnecting the first piston and the second piston; and (d) a flow selector associated with the high-pressure pump, the first chamber of the first cylinder and the third chamber of the second cylinder, the flow selector assuming alternately: (i) a first selector state in which the proppant-free fluid from the high-pressure pump is directed to the first chamber so as to act on the first piston, thereby applying pressure to a quantity of the fracking fluid within the second chamber for delivery to the well in a first power stroke, the first power stroke also expelling a quantity of the proppant-free fluid from the third chamber and taking in a quantity of the fracking fluid to the fourth chamber; and (ii) a second selector state in which the proppant-free fluid from the high-pressure pump is directed to the third chamber so as to act on the second piston, thereby applying pressure to a quantity of the fracking fluid within the fourth chamber for delivery to the well in a second power stroke, the second power stroke also expelling the quantity of the proppant-free fluid from the first chamber and taking in a quantity of the fracking fluid to the second chamber, wherein the first and second pistons have an inner face facing, respectively, the first and third chambers and an outer face facing, respectively, the second and fourth chamber, the outer face having larger effective surface area than the inner face, such that pressure in the fracking fluid remains lower than pressure in the proppant-free fluid.

According to a further feature of an embodiment of the present invention, the first cylinder arrangement further comprises a pressure vessel enveloping substantially the entirety of the first hollow cylinder, the pressure vessel defining at least one enveloping volume selected from the group consisting of: a first enveloping volume in fluid flow communication so as to form an extension of the first chamber; and a second enveloping volume in fluid flow communication so as to form an extension of the second chamber, and wherein the second cylinder arrangement further comprises a pressure vessel enveloping substantially the entirety of the second hollow cylinder, the pressure vessel defining at least one enveloping volume selected from the group consisting of: a third enveloping volume in fluid flow communication so as to form an extension of the third chamber; and a fourth enveloping volume in fluid flow communication so as to form an extension of the fourth chamber.

According to a further feature of an embodiment of the present invention, there is also provided: (a) a bidirectional hydraulic actuator associated with the flow selector for switching the flow selector between the first selector state and the second selector state; and (b) a hydraulic switch having a switch inlet port for the inflow of a pressurized hydraulic fluid and having two hydraulic connections to the hydraulic actuator for actuating the hydraulic actuator, wherein the hydraulic switch is deployed to be actuated by motion of the first and second pistons to switch the hydraulic connections of the hydraulic switch, thereby subsequently actuating the bidirectional hydraulic actuator to switch the flow selector between the first and second selector states.

According to a further feature of an embodiment of the present invention, the hydraulic actuator includes an actuator piston displaceable within an actuator cylinder.

According to a further feature of an embodiment of the present invention, the switch inlet port is connected to receive proppant-free fluid from the high-pressure pump.

According to a further feature of an embodiment of the present invention, the double-piston pressure transfer arrangement is referred to as a master pressure transfer arrangement, and wherein the flow selector is referred to as a master flow selector, the system further comprising: (a) a slave double-piston pressure transfer arrangement, the pressure transfer arrangement comprising: (i) a first cylinder assembly comprising a first hollow cylinder, and a first piston in sliding engagement within the first hollow cylinder so as to at least partially define a first chamber and a second chamber; (ii) a second cylinder assembly comprising a second hollow cylinder, and a second piston in sliding engagement within the second hollow cylinder so as to at least partially define a third chamber and a fourth chamber; and (iii) at least one piston rod interconnecting the first piston and the second piston; and (b) a slave flow selector associated with the high-pressure pump, the first chamber of the first cylinder and the third chamber of the second cylinder, the flow selector assuming alternately: (i) a first selector state in which the proppant-free fluid from the high-pressure pump is directed to the first chamber so as to act on the first piston, thereby applying pressure to a quantity of the fracking fluid within the second chamber for delivery to the well in a first power stroke, the first power stroke also expelling a quantity of the proppant-free fluid from the third chamber and taking in a quantity of the fracking fluid to the fourth chamber; and (ii) a second selector state in which the proppant-free fluid from the high-pressure pump is directed to the third chamber so as to act on the second piston, thereby applying pressure to a quantity of the fracking fluid within the fourth chamber for delivery to the well in a second power stroke, the second power stroke also expelling the quantity of the proppant-free fluid from the first chamber and taking in a quantity of the fracking fluid to the second chamber; and (c) a bidirectional slave hydraulic actuator associated with the slave flow selector for switching the slave flow selector between the first selector state and the second selector state, wherein the slave hydraulic actuator is associated with the master flow selector or the master pressure transfer arrangement such that, during the first power stroke of the master pressure transfer arrangement, proppant-free fluid from the high pressure pump is delivered to the slave hydraulic actuator so as to switch the slave flow selector from the second selector state to the first selector state and, during the second power stroke of the master pressure transfer arrangement, proppant-free fluid from the high pressure pump is delivered to the slave hydraulic actuator so as to switch the slave flow selector from the first selector state to the second selector state.

There is also provided according to the teachings of an embodiment of the present invention, a method for delivering pressurized fracking fluid to a well, the method comprising the steps of: (a) providing a double-piston pressure transfer arrangement, the pressure transfer arrangement comprising: (i) a first cylinder assembly comprising a first hollow cylinder, and a first piston in sliding engagement within the first hollow cylinder so as to at least partially define a first chamber and a second chamber; (ii) a second cylinder assembly comprising a second hollow cylinder, and a second piston in sliding engagement within the second hollow cylinder so as to at least partially define a third chamber and a fourth chamber; and (iii) at least one piston rod interconnecting the first piston and the second piston; and (b) directing a proppant-free fluid from a high-pressure pump to the first chamber so as to act on the first piston, thereby applying pressure to a quantity of the fracking fluid within the second chamber for delivery to the well in a first power stroke, the first power stroke also expelling a quantity of the proppant-free fluid from the third chamber and taking in a quantity of the fracking fluid to the fourth chamber; and (c) directing the proppant-free fluid from the high-pressure pump to the third chamber so as to act on the second piston, thereby applying pressure to a quantity of the fracking fluid within the fourth chamber for delivery to the well in a second power stroke, the second power stroke also expelling the quantity of the proppant-free fluid from the first chamber and taking in a quantity of the fracking fluid to the second chamber, wherein the first and second pistons have an inner face facing, respectively, the first and third chambers and an outer face facing, respectively, the second and fourth chamber, the outer face having larger effective surface area than the inner face, such that pressure in the fracking fluid remains lower than pressure in the proppant-free fluid.

According to a further feature of an embodiment of the present invention, the first cylinder arrangement further comprises a pressure vessel enveloping substantially the entirety of the first hollow cylinder, the pressure vessel defining at least one enveloping volume selected from the group consisting of: a first enveloping volume in fluid flow communication so as to form an extension of the first chamber; and a second enveloping volume in fluid flow communication so as to form an extension of the second chamber, and wherein the second cylinder arrangement further comprises a pressure vessel enveloping substantially the entirety of the second hollow cylinder, the pressure vessel defining at least one enveloping volume selected from the group consisting of: a third enveloping volume in fluid flow communication so as to form an extension of the third chamber; and a fourth enveloping volume in fluid flow communication so as to form an extension of the fourth chamber.

There is also provided according to the teachings of an embodiment of the present invention, a system for pumping a particle-laden fluid from a fluid source, the system comprising: (a) a pump connected to a source of clean fluid; (b) a double-piston pressure transfer arrangement, the pressure transfer arrangement comprising: (i) a first cylinder assembly comprising a first hollow cylinder, and a first piston in sliding engagement within the first hollow cylinder so as to at least partially define a first chamber and a second chamber; (ii) a second cylinder assembly comprising a second hollow cylinder, and a second piston in sliding engagement within the second hollow cylinder so as to at least partially define a third chamber and a fourth chamber; and (iii) at least one piston rod interconnecting the first piston and the second piston; and (c) a flow selector associated with the pump, the first chamber of the first cylinder and the third chamber of the second cylinder, the flow selector assuming alternately: (i) a first selector state in which the clean fluid from the pump is directed to the first chamber so as to act on the first piston, thereby applying pressure to a quantity of the particle-laden fluid within the second chamber for delivery through an outlet in a first power stroke, the first power stroke also expelling a quantity of the clean fluid from the third chamber and taking in a quantity of the particle-laden fluid to the fourth chamber; and (ii) a second selector state in which the clean fluid from the pump is directed to the third chamber so as to act on the second piston, thereby applying pressure to a quantity of the particle-laden fluid within the fourth chamber for delivery through an outlet in a second power stroke, the second power stroke also expelling the quantity of the clean fluid from the first chamber and taking in a quantity of the particle-laden fluid to the second chamber, wherein the first and second pistons have an inner face facing, respectively, the first and third chambers and an outer face facing, respectively, the second and fourth chamber, the outer face having larger effective surface area than the inner face, such that pressure in the particle-laden fluid remains lower than pressure in the clean fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1A is a schematic diagram showing a high-pressure fluid injection system employing a paired cylinder arrangement according to an embodiment of the invention;

FIG. 1B is an enlarged view of a paired cylinder arrangement according to an embodiment of the invention;

FIG. 2A is a schematic diagram showing a paired cylinder arrangement including a hydraulic switch and flow selector with hydraulic actuator according to an embodiment of the invention;

FIG. 2B is an enlarged view of a paired cylinder arrangement including a hydraulic switch and flow selector with hydraulic actuator according to an embodiment of the invention;

FIG. 3A is a cross-sectional view of a flow selector according to an embodiment of the invention;

FIG. 3B is an enlarged view of a flow selector according to an embodiment of the invention;

FIG. 4 is a cross-sectional view of a hydraulic switch according to an embodiment of the invention;

FIG. 5A is a view of some of the elements of a hydraulic switch according to an embodiment of the invention;

FIG. 5B is a cross-sectional view of elements of a hydraulic switch according to an embodiment of the invention;

FIGS. 6A-6C show a schematic diagram of the state transitions of a flow selector and a hydraulic switch according to an embodiment of the invention;

FIG. 7 is a schematic diagram showing a controlled out of phase supply of pressurized fluid by a plurality of paired cylinder arrangements according to an embodiment of the invention;

FIG. 8 is a schematic diagram showing a master paired cylinder arrangement in connection with a number of successive slave paired cylinder arrangements for controlling a supply of pressurized fluid according to an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a system and method for pumping a particle-laden fluid, such as fracking fluid delivered at high pressure to a well.

The principles and operation of systems and methods according to the present invention may be better understood with reference to the drawings and the accompanying description.

Referring now to the drawings, FIG. 1A shows a schematic overview of a system, generally designated 100, constructed and operative according to an embodiment of the present invention applied to the field of fracking, for delivering pressurized fracking fluid to a well so as to fracture and prop-open cracks in rock 102.

Generally speaking, in the exemplary case of fracking, system 100 includes a source of proppant-laden fracking fluid 104, a high-pressure pump 106 connected to a source 108 of proppant-free fluid, and a double-piston pressure transfer arrangement 110. Pressure transfer arrangement 110 includes a first cylinder assembly 200-1 having a first hollow cylinder 204-1, and a first piston 208-1 in sliding engagement within first hollow cylinder 204-1 so as to at least partially define a first chamber 214-1 and a second chamber 216-1. Pressure transfer arrangement 110 further includes a second cylinder assembly 200-2 having a second hollow cylinder 204-2, and a second piston 208-2 in sliding engagement within second hollow cylinder 204-2 so as to at least partially define a third chamber 214-2 and a fourth chamber 216-2. At least one piston rod 218 interconnects first piston 208-1 and second piston 208-2.

A flow selector 300, associated with high-pressure pump 106, first chamber 214-1 and third chamber 214-2, assumes alternately:

• • A first selector state in which the proppant-free fluid from high-pressure pump 106 is directed to first chamber 214-1 so as to act on first piston 208-1, thereby applying pressure to a quantity of the fracking fluid within second chamber 216-1 for delivery to the well in a first power stroke. This first power stroke also expels a quantity of the proppant-free fluid from third chamber 214-1 and takes in a quantity of the fracking fluid to fourth chamber 216-2.
  • A second selector state in which the proppant-free fluid from the high-pressure pump 106 is directed to third chamber 214-2 so as to act on second piston 208-2, thereby applying pressure to a quantity of the fracking fluid within fourth chamber 216-2 for delivery to the well in a second power stroke. This second power stroke also expels the quantity of proppant-free fluid from first chamber 214-1 and takes in a quantity of the fracking fluid to second chamber 216-1, ready for a next cycle.

First and second pistons 208-1 and 208-2 have inner faces 210-1 and 210-2, respectively, facing first and third chambers 214-1 and 214-2, and outer faces 212-1 and 212-2, respectively, facing second and fourth chambers 216-1 and 216-2. Outer faces 212-1 and 212-2 have larger effective surface areas than inner faces 210-1 and 210-2, such that pressure in the fracking fluid remains lower than pressure in the proppant-free fluid.

At this stage, it will already be appreciated that, according to certain preferred embodiments of the present invention, by maintaining the pressure of the proppant-free fluid higher than that of the fracking fluid, any leakage around pistons 208-1 and 208-2 occurs only in the forward direction, from the clean fluid into the fracking fluid, thereby reducing proppant-induce wear or mechanical disruption of the pistons, and avoiding contamination of the proppant-free fluid which is recycled through the high pressure pump. This and other advantages of certain embodiments of the present invention will be better understood by reference to the detailed description herein.

The present invention is applicable to any situation in which a fluid containing solid particles as a suspension, slurry or admixture of any sort, or with particularly abrasive or corrosive properties, is to be pumped from one location to another, or to be delivered at high pressure to a target location or volume. Examples of relevant fields to which the present invention may be applied include, but are not limited to: hydraulic fracturing (“fracking”), pumped hydroelectric energy storage, and geothermal energy recovery. In each case, pressure applied to a “clean fluid” (i.e., a fluid without a mechanically-significant content of particles) is transferred via the pressure-transfer arrangement of the present invention to the particle-laden fluid. The detailed description herein is given in the non-limiting exemplary context of fracking, where particularly high pressures are required. The application of the invention to other scenarios in which solid-particle-laden water or other liquids are to be pumped will be self-explanatory by analogy to the relevant features described herein.

In the example of fracking, the invention provides a solution for high pressure applications. “High pressure” for this purpose is defined as any pressure above about 20 bar, although fracking applications typically reach pressures in excess of 100 bar, and often several hundred bar, up to 1000 bar or more. In fracking processes, high pressure fluid with addition of a proppant is delivered into rock in order to open up fissures and prop them open. Fracking has applications in the field of mining for enhancing the rate at which petroleum (oil) and natural gas can be recovered from subterranean natural reservoirs, and may also be used to improve accessibility to groundwater wells or to provide access routes for various other industrial or experimental processes. The word “well” is used herein in the description and claims to refer to any and all scenarios in which a subterranean (or under-sea) channel is used to provide access for extraction of, or access to, a subterranean natural resource. The fracking fluid is typically water with an admixture of proppant particles and chemical additives to provide various desired physical and chemical properties, providing properties such as increased viscosity, corrosion resistance, resistance to freezing etc., all as is known in the art. Fluids other than water may also be used as the basis for the fracking fluid. The proppant particles may be any suitable granular material, most commonly silica sand or ceramic particles.

Thus, in a typical fracking implementation as illustrated in FIG. 1A, source 104 of fracking fluid includes a low-pressure pump 113 drawing water from a tank 114 or other water source. A mixer device 116 delivers the proppant into the fluid flow, as is known in the art, before it is delivered to the double-piston pressure transfer arrangement 110. Source 104 typically provides the low-pressure fracking fluid supply to second and fourth 216-1 and 216-2 via two respective check valves 118-1 and 118-2 that are typically part of a valve block 120 which also includes check valves 122-1 and 122-2 through which high pressure fracking fluid is delivered during first and second power strokes, respectively, from chambers 216-1 and 216-2 to the well.

The proppant-free fluid is typically water, but other fluids may also be used. As mentioned above, various preferred implementations of the present invention maintain a pressure differential from the proppant-free fluid to the fracking fluid, such that any leakage around the pistons occurs from the clean fluid to the fracking fluid. The choice of proppant-free fluid is therefore preferably compatible with the primary fluid component of the fracking fluid so that any leakage of the proppant-free fluid into the fracking fluid does not adversely affect the fracking fluid properties.

In order to ensure smooth operation of high pressure pump 106 without cavitation at its inlet, a low pressure pump 124 preferably feeds the proppant-free fluid from tank 108 to high pressure pump 106. Fluid expelled from the first and third chambers 214-1 and 214-2 during their respective return strokes is preferably returned via selector 300 to tank 108 for recycling as part of a closed-loop system, which may be periodically or continuously topped up if necessary, or may start with sufficient volume of liquid to accommodate any likely leakage losses. The structure and operation of selector 300 will be detailed further below.

An implementation of double-piston pressure transfer arrangement 110 is shown in more detail in FIG. 1B. Cylinders 204-1 and 204-2 have an interior surface defining an interior volume and are open at a single end 206-1 and 206-2. Hollow cylinders 204-1 and 204-2 are preferably positioned inside respective pressure vessels 202-1 and 202-2, thereby lowering the pressure differential across the walls of cylinders 204-1 and 204-2. The envelopment of cylinders 204-1 and 204-2 by respective pressure vessels 202-1 and 202-2 defines enveloping volumes in the respective spaces between cylinders 204-1 and 204-2 and pressure vessels 202-1 and 202-2. It is preferable that substantially the entirety of the surface area of cylinders 204-1 and 204-2 is enveloped by respective pressure vessels 202-1 and 202-2. In this configuration, the entire surface areas of cylinders 204-1 and 204-2 are exposed to high pressure from within respective pressure vessels 202-1 and 202-2. This allows for cylinders 204-1 and 204-2 to be made from a lower cost and/or thinner material chosen for its dimensional stability and wear resistance. Particularly preferred materials include stainless steel, composite materials and various polymer materials. Although the depiction of pressure vessels 202-1 and 202-2 in FIGS. 1A-1B shows a cylindrical shape with flat ends, other suitable shapes can be used, such as, for example, a tubular shape with hemispherical ends. Pressure vessels 202-1 and 202-2 are made of a material or combination of materials suitable for withstanding high pressure differentials including, but not limited to, steel and composite materials.

As mentioned above, each cylinder 204-1 and 204-2 contains a reciprocally movable piston 208-1 and 208-2 for moving within the respective internal surfaces of cylinders 204-1 and 204-2. Pistons 208-1 and 208-2 have inner faces 210-1 and 210-2 and outer faces 212-1 and 212-2. Pistons 208-1 and 208-2 effectively subdivide the internal volumes of cylinders 204-1 and 204-2 into volumes forming part of chambers 216-1 and 216-2 for the fracking fluid and volumes forming a part of chambers 214-1 and 214-2 for the proppant-free fluid. Preferably, outer faces 212-1 and 212-2 are of larger effective surface area than inner faces 210-1 and 210-2. This difference in the effective surface area provides a step-down pressure difference so that a given pressure of clean fluid introduced into chamber 214-1 or 214-2 results in a somewhat lower output pressure of fracking fluid from the corresponding chamber 216-1 or 216-2. The cylinder arrangement of hollow cylinders 204-1 and 204-2 inside of respective pressure vessels 202-1 and 202-2 are hereinafter referred to as cylinder arrangements 200-1 and 200-2. It is preferable that cylinder arrangements 200-1 and 200-2 are connected by a mechanical arrangement, thereby creating a paired cylinder arrangement, as detailed further below.

Fracking fluid inlet-outlet ports 220-1 and 220-2 are here implemented as connections to pressure vessels 202-1 and 202-2. An additional advantage of the double cylinder arrangements 200-1 and 200-2 is that inlet-outlet ports 220-1 and 220-2 may be placed far from respective open ends 206-1 and 206-2. This allows for the grouping of fracking fluid inlet-outlet ports near the center of arrangement 110 as shown in FIGS. 1A-1B, thereby minimizing the required length of high pressure pipes and avoiding stress caused by the cyclic longitudinal expansion and contraction along the length of pressure vessels 202-1 and 202-2 which could otherwise lead to connection reliability issues. Furthermore, the use of double cylinder arrangements ensures that the pressure vessels themselves are not subject to mechanical wear from the moving pistons.

Although the configuration shown here has the fracking fluid chamber extending around the inner cylinder, a similar reduction of the pressure differential across the walls of cylinders 204-1 and 204-2 may be achieved by extending the clean fluid chambers 214-1 and 214-2 to envelope some or all of the exterior of respective cylinders 204-1 and 204-2. Where the enveloping volume is provided in part by the fracking fluid chamber and in part by the clean fluid chamber, a relatively narrow dividing wall (not shown) is provided to seal around the inner cylinder and subdivide the enveloping volume.

It is most preferable that the aforementioned mechanical arrangement is implemented using at least one connecting rod 218 connecting pistons 208-1 and 208-2 for simultaneous movement. Rod 218 preferably interconnects pistons 208-1 and 208-2 at inner faces 210-1 and 210-2. Rod 218 extends through an aperture or apertures in a central body separating cylinder arrangements 200-1 and 200-2. Due to the pressure difference between the clean fluid and the fracking fluid, leakage of fracking fluid from chambers 216-1 and 216-2 into the clean fluid in chambers 2144 and 214-2 is typically avoided. Nevertheless, pistons 208-1 and 208-2 may advantageously include a structure, such as a seal ring or the like, which prevents mixing in either direction between the clean fluid in chambers 214-1 and 214-2 and the fracking fluid in chambers 216-1 and 216-2. Pistons 208-1 and 208-2 may be described as moving between first and second extreme positions. The first position of piston 208-1 is preferably adjacent inlet-outlet port 222-1, while the first position of piston 208-2 is preferably adjacent inlet-outlet port 222-2, i.e., the positions of the pistons nearest the middle of assembly 110. The second position of piston 208-1 is preferably adjacent open end 206-1 of cylinder 204-1, while the second position of piston 208-2 is preferably adjacent open end 206-2 of cylinder 204-2. The movement of pistons 208-1 and 208-2 from their respective first positions to second positions is referred to as the pressure stroke of pistons 208-1 and 208-2 during which fracking fluid is expelled under pressure for delivery to the well. The movement of pistons 208-1 and 208-2 from their respective second positions to first positions is referred to as the reverse stroke of pistons 208-1 and 208-2. It is apparent that, due to the mechanical linkage via rod(s) 218, the pressure stroke of piston 208-1 coincides with the reverse stroke of piston 208-2, and that the pressure stroke of piston 208-2 coincides with the reverse stroke of piston 208-1.

Although the use of one or more common piston rod rigidly linking pistons 208-1 and 208-2 is a particularly preferred implementation used throughout the description, it should be noted that other forms of mechanical linkage between the pistons to ensure simultaneous and opposite motion may also be used. Such options may be of particular value where a compact side-by-side deployment of the pair of cylinders is preferred to the coaxial arrangement illustrated herein. Such mechanical linkages can readily be implemented using a lever arms configuration with pivotally mounted drive rods linked to the pistons.

As further seen in FIG. 1A, proppant-free fluid inlet-outlet ports 222-1 and 222-2 are provided in chambers 214-1 and 214-2. Proppant-free fluid inlet-outlet ports 222-1 and 222-2 are in fluid flow connection with high-pressure pump 106 via a flow selector 300. Flow selector 300 may be a valve arrangement connected to an actuator 350. Actuator 350 may be connected to a controller 112 which is associated with sensors (illustrated schematically as a sensor 116) for sensing the positions of piston 208-1 and/or piston 208-2. The sensors may include transducers or the like located adjacent to the first and second positions of piston 208-1 and/or piston 208-2, a laser range finder for sensing proximity of one or both pistons to the end of the cylinders, or various other linear encoder or position sensor arrangements for sensing the position of the pistons or piston rods. Sensing the positions of pistons 208-1 and 208-2 is preferably used for timing the stroke reversal of cylinder arrangements 200-1 and 200-2, i.e. the change from pressure stroke to reverse stroke of cylinder arrangement 200-1 and the simultaneous change from reverse stroke to pressure stroke of cylinder arrangement 200-2, and vice versa. Flow selector 300 controls the inflow and outflow of proppant-free fluid to and from inlet-outlet ports 222-1 and 222-2 as actuated by actuator 350. Fracking fluid is correspondingly taken up and expelled under pressure from chambers 216-1 and 216-2 through the check valves of valve block 120. Although described thus far in an implementation using an arrangement of check valves for controlling the inflow and outflow of fracking fluid to and from chambers 216-1 and 216-2, other embodiments are possible in which a switchable valve arrangement (not shown) controls the inflow and outflow of the fracking fluid. In such embodiments, the valve may be actuated by actuator 350 to ensure synchronization between the inflow/outflow of fracking fluid with the inflow/outflow of the proppant-free fluid.

The operation of the system as shown in FIGS. 1A-1B will now be described. Proppant-free fluid from tank 108, e.g., clean water, is supplied via feed pump 124 to high pressure pump 106. Flow selector 300 directs this flow alternately to one of chambers 214-1 and 214-2 while low pressure clean water is drained from the other of these chambers via flow selector 300 back to tank 108. According to the flow paths illustrated by solid arrows in FIG. 1A, high pressure clean water is currently being delivered to chamber 214-1, thereby driving piston 208-1 to the left as shown and delivering fracking fluid from chamber 216-1 at high pressure via outlet 220-1 and check valve 122-1 to be delivered to the target volume (rocks 102 or the well). The power-stroke motion of piston 208-1 is also conveyed via piston rod(s) 218 to displace piston 208-2 to the left in its return stroke, thereby simultaneously expelling from chamber 214-2 the clean water introduced during the previous power stroke via selector 300 and drawing in fracking fluid via check valve 118-2, to fill chamber 216-2.

When piston 208-1 nears its second position, for example as identified by position sensors detecting the position of piston 208-1 and/or piston 208-2, controller 112 preferably sends a control signal to actuator 350 to actuate flow selector 300 to change the inflow and outflow of clean water to and from inlet-outlet ports 222-1 and 222-2. In such an arrangement, flow selector 300 is preferably actuated prior to piston 208-1 reaching the limit of its range of motion, thereby minimizing any momentary dead time which may occur during the direction reversal, and any consequent fluctuation in the output flow rate from the cylinder arrangement. After actuation of flow selector 300, fluid connections to and from cylinder arrangements 200-1 and 200-2 are reversed whereby high-pressure fresh water from pump 106 is supplied to chamber 214-2 to drive piston 208-2 from its first position towards its second position. Fracking fluid from chamber 216-2 is pumped through inlet-outlet port 220-2 and check valve 122-2 for delivery to the target location in the well. New fracking fluid is simultaneously drawn in via check valve 118-1 and inlet-outlet port 220-1 from fracking fluid source 104 to chamber 216-1, and clean water is discharged from chamber 214-1 through inlet-outlet port 222-1. When piston 208-2 nears its second position and piston 208-1 nears its first position, the position of piston 208-1 and/or piston 208-2 are sensed via the position sensors or the like and controller 112 sends a control signal to actuator 350 to actuate flow selector 300 to change the inflow and outflow of clean water to and from inlet-outlet ports 222-1 and 222-2 as shown in the illustrated first position and the pressure transfer cycle repeats.

Parenthetically, although illustrated herein in one particularly preferred implementation (described further below) as a tube-like selector, flow selector 300 may be implemented in any of a number of configurations. By way of one additional non-limiting example, a rotary selector may be used in which arcuate channels alternately connect the high pressure clean water connection and the clean water return connection with each of the pressurized drive-stroke chambers of the cylinder arrangements.

In certain preferred implementations, it may be preferable to implement the sensing and actuation process for reversing the direction of pistons 208-1 and 208-2 using a reduced number of electro-mechanical components. In certain such cases, it is preferred that flow selector 300 and actuator 350 are operated by a hydraulic switching arrangement without requiring additional sensors or electronic actuators.

According to certain preferred embodiments, flow selector 300 is hydraulically actuated and includes a hollow flow selector cylinder assembly 301 which extends between first and second ends 302-1 and 302-2, and which is typically assembled from a number of different cylindrical and branched sections, as discussed below. With reference to FIGS. 2A-3B, selector 300 as illustrated here has a high-pressure clean water inlet port 304 in fluid flow connection with pump 106, and first and second clean water drainage outlet ports 306-1 and 306-2 in fluid flow connection with a discharge conduit returning the water to tank 108 (FIG. 1a). First and second clean water inlet-outlet ports 307-1 and 307-2 are in fluid flow connection with clean water inlet-outlet ports 222-1 and 222-2 respectively.

According to certain particularly preferred implementations, flow selector 300 is integrated with a hydraulic actuator 350. In this case, at one end of the selector, an integrated hydraulic actuator arrangement 350 includes a piston 308 displaceable within an actuator cylinder portion 309 so as to displace a selector rod 316 (which may be assembled from a number of separable sections, as shown) between first and second positions within cylinder assembly 301. Piston 308 has an inner face 310 and an outer face 312. It is preferred that piston 308 includes a structure, such as a seal ring or the like, sealing against the wall of cylinder portion 309. Cylinder portion 309 is delineated by first and second non-moveable seals 305-1 and 305-2. The volume between second non-moveable seal 305-2 and outer face 312 is referred to as first volume 303-1. The volume between inner face 310 and first non-moveable seal 305-1 is referred to as second volume 303-2. Piston 308 has a first position preferably adjacent to second non-moveable seal 305-2. Piston 308 has a second position preferably adjacent to first non-moveable seal 305-1.

Optionally, selector 300 may include an extension portion 318 within which moves an extension rod 320, interconnected so as to move together with selector rod 316. Provision of extension rod 320 equalizes the effective surface area on both sides of the piston 308, and provides a convenient location for a sensor for sensing the current state of selector 300 as an input to the control system. It should be noted, however, the extension rod 320 is not essential, and an asymmetry between the surface areas on the two sides of piston 308 is generally not considered problematic.

In the preferred implementation illustrated here, cylinder assembly 301 contains first and second reciprocally movable seals 314-1 and 314-2 for moving within cylinder assembly 301. Movable seals 314-1 and 314-2 may have a piston-like structure, although axially-closing seals are preferred to radially sliding seals, as detailed below with reference to FIG. 3b. Seal 314-1 has a first position preferably interposed between outlet port 306-1 and inlet-outlet port 307-1. Seal 3144 has a second position preferably interposed between inlet-outlet port 307-1 and inlet port 304. Seal 314-2 has a first position preferably interposed between outlet port 306-2 and inlet-outlet port 307-2. Seal 314-2 has a second position preferably interposed between inlet-outlet port 307-2 and inlet port 304. Piston 308 and seals 314-1 and 314-2 are interconnected for simultaneous movement. Selector rod 316 interconnects piston 308 and seals 314-1 and 314-2. Selector rod 316 extends through apertures in non-moveable seal arrangements 305-1 and 305-2. Preferably, a seal ring or the like at the apertures of non-moveable seals 305-1 and 305-2 provides a sealing engagement between rod 316 and non-moveable seals 305-1 and 305-2 while allowing for axial movement of rod 316. As mentioned above, selector rod 316 may be formed from multiple sections, with each section interconnecting different components of flow selector 300. For example, a first section of rod 316 may be used to connect to outer face 312 of piston 308. A second section of rod 316 may then be used to interconnect inner face 310 of piston 308 and seal 314-1. A third section of rod 316 may then be used to interconnect seals 314-1 and 314-2. The movement of piston 308 displacing rod 316 acts as a hydraulic actuator. The simultaneous movement of pistons 308 and seals 314-1 and 314-2 effects the transitioning of flow selector 300 between first and second states. When seal 314-1 is in its first position and seal 314-2 is in its second position as shown in FIGS. 2A, 2B and 3A, flow selector 300 is in a first state. Likewise, when seal 314-1 is in its second position and seal 314-2 is in its first position, flow selector 300 is in its second state. When flow selector 300 is in the first state, clean water from pump 106 is supplied to chamber 214-1 through inlet port 304, inlet-outlet port 307-1 and inlet-outlet port 222-1. Simultaneously, low pressure clean water is discharged from chamber 214-2 through inlet-outlet port 222-2, inlet-outlet port 307-2, outlet port 306-2, and is discharged to tank 108. When flow selector 300 is in the second state, clean water from pump 106 is supplied to chamber 214-2 through inlet port 304, inlet-outlet port 307-2 and inlet-outlet port 222-2. Simultaneously, low pressure clean water is discharged from chamber 214-1 through inlet-outlet port 222-1, inlet-outlet port 307-1, outlet port 306-1, and is discharged to tank 108.

According to a particularly preferred implementation illustrated here, corresponding to a further aspect of the present invention application to flow selectors, the sealing surfaces of sliding seals 314-1 and 314-2 are provided on axially facing closure surfaces of the sliding seals rather than by using radial seals bearing on the inside of a cylinder. Specifically, hollow cylinder assembly 301 is shown here with sections having two differing internal diameters, namely sections 324 having a first internal diameter and sections 326 having a second internal diameter, where the first internal diameter is larger than the second internal diameter. The locations within hollow cylinder assembly 301 in which changes of internal diameter occur form a plurality of steps 328 defining the first and second positions of seals 314-1 and 314-2. As shown in FIGS. 3A-3B, seals 314-1 and 314-2 are movable within the sections 324 of cylinder assembly 301 having the first diameter and have annular sealing rings 315 on the axially-facing end surfaces of the seals configured for sealing against steps 328, thereby defining the flow paths of the proppant-free fluid through flow selector 300 as previously described. By avoiding use of outwardly-pressing seals against the inner surface of cylinder assembly 301, problems of reliability due to wear of seals 314-1 and 314-2 passing over lateral openings 220-1 and 220-2 are greatly reduced.

According to a variant implementations illustrated with reference to FIGS. 2A-2B, 4, 5A-5B and 6A-6C, as an alternative or supplement to sensor(s) 116 and controller 112, in certain preferred embodiments, the hydraulic actuator 350 of flow selector 300 is actuated by a hydraulic switch 400. According to certain particularly preferred implementations illustrated here, hydraulic switch 400 is mechanically integrated with cylinder arrangement 110 so as to be actuated towards the end of each stroke of piston 208-1 or 208-2. It should be noted that the use of a hydraulic switch 400 to trigger operation of an actuator 350 which causes a change in state of flow selector 300 offers significant advantages over direct actuation of a flow selector by motion of the pistons. Most notably, since no change in state of selector 300 occurs until after the state of hydraulic switch 400 has been completely reversed, potential issues of “stalling” at some intermediate position during the state reversal are avoided.

Referring now to FIGS. 2A-2B, 4, 5A-5B and 6A-6C, one particularly preferred implementation of hydraulic switch 400 as illustrated here includes a hollow hydraulic switch chamber 402 with a chamber wall 404 having first and second ends. Preferably, chamber wall 404 includes a first at least one aperture 406-1, a second at least one aperture 406-2, a third at least one aperture 406-3, a fourth at least one aperture 406-4, and a fifth at least one aperture 406-5. Apertures 406-1, 406-2, 406-3, 406-4 and 406-5 are preferably arranged annularly around chamber wall 404 as depicted in FIGS. 4, 5A and 5B. Chamber wall 404 preferably includes a first switch non-moveable seal 407-1, a second switch non-moveable seal 407-2, a third switch non-moveable seal 407-3, a fourth switch non-moveable seal 407-4, a fifth switch non-moveable seal 407-5, and sixth switch non-moveable seal 407-6. The preferred interposed arrangement of the apertures and non-movable seals is as shown in FIGS. 5A-5B. The first end of chamber wall 404 is adjacent seal 407-1, and second end of chamber wall 404 is adjacent seal 407-6.

A hydraulic fluid inlet port 408 is provided adjacent to aperture 406-1. Pressurized hydraulic fluid, which may be an oil or may be high or low pressurized clean water taken from some suitable point in the system, is supplied to inlet port 408 from a hydraulic fluid source. The hydraulic fluid is preferably pressurized to at least 5 bar. Where a dedicated hydraulic fluid is used, the fluid is pressurized by a suitable device, such as a pressure supply pump 410 or the like. First and second actuator control inlet-outlet ports 412-1 and 412-2 are provided adjacent to apertures 406-3 and 406-2 respectively. Inlet-outlet port 412-1 is in fluid flow connection with outer face 312 of flow selector piston 308. Inlet-outlet port 412-2 is in fluid flow connection with inner face 310 of flow selector piston 308. First and second hydraulic fluid drain outlet ports 414-1 and 414-2 are provided adjacent apertures 406-4 and 406-5 respectively. Outlet ports 414-1 and 414-2 are preferably in fluid flow connection with a hydraulic fluid discharge conduit 416 connected to a hydraulic fluid dump which, in the case of an oil based hydraulic system, is typically the reservoir supplying pump 410. Chamber 402 contains a reciprocally movable switching seal 418 for moving within chamber wall 404. It is preferred that seal 418 has a piston like structure. Switching seal 418 has a first position preferably adjacent inlet-outlet port 412-1 and a second position preferably adjacent inlet-outlet port 412-2. Seal 418 is shown here integrated with a switching rod 420 for movement within chamber wall 404. As shown in FIG. 5B, rod 420 is also integrated with narrow 426, 428 and wide 422, 424 diameter sections. Wide diameter sections 422 and 424 act as sliding seals within chamber 402. When seal 418 is in its first position, seal 422 prevents the flow of hydraulic fluid from aperture 406-1 to aperture 406-5, thus preventing the flow from inlet port 408 to outlet port 414-2. When seal 418 is in its second position, seal 424 prevents the flow of hydraulic fluid from aperture 406-1 to aperture 406-4, thus preventing the flow from inlet port 408 to outlet port 414-1. Chamber 402 is preferably interposed between chambers 214-1 and 214-2. Seal 422 preferably extends out of the first end of chamber 402 through an aperture when seal 418 is in the second position. Likewise, seal 424 extends through the second end of chamber 402 when seal 418 is in the first position. Thus, rod 420 is moved by contact with pistons 208-1 and 208-2 towards the end of their range of motion. The relative positioning of apertures and switch seals dictates the possible flow paths the hydraulic fluid can traverse through inlet port 408, inlet-outlet ports 412-1 and 412-2, and outlet ports 414-1 and 414-2. The movement of switching seal 418 achieves switching of hydraulic switch 400 between first and second states.

When hydraulic switch 400 is in the first state, pressurized hydraulic fluid from a hydraulic fluid source is supplied to volume 303-2 of cylinder assembly 301 through inlet-outlet port 412-1, aperture 406-3, aperture 406-1, and inlet port 408. Simultaneously, hydraulic fluid is discharged from volume 303-1 through inlet-outlet port 412-2, aperture 406-2, aperture 406-5, outlet port 414-2 and hydraulic fluid discharge conduit 416 connected to a hydraulic fluid dump. When hydraulic switch 400 is in the second state, pressurized hydraulic fluid from a hydraulic fluid source is supplied to volume 303-1 of cylinder assembly 301 through inlet-outlet port 412-2, aperture 406-2, aperture 406-1, and inlet port 408. Simultaneously, hydraulic fluid is discharged from volume 303-2 through inlet-outlet port 412-1, aperture 406-3, aperture 406-4, outlet port 414-1 and hydraulic fluid discharge conduit 416 connected to a hydraulic fluid dump.

As noted, hydraulic switch is interposed between pistons 208-1 and 208-2 such that pistons 208-1 and 208-2 move switching rod 420 back and forth. The arrangement preferably operates as a bistable “flip-flop” where, as seal 418 passes the central position of switch 400 in either direction, pressure from the pressure source carries the seal 418 and its associated switching rod 420 quickly to the end of its motion, thereby triggering the change of state of actuator 350 and hence of selector 300. This arrangement is effective to avoid the risk of stalling of the cylinder arrangement in any “dead” intermediate state during switching from pressure stroke to reverse stroke and vice versa.

Although the switching arrangement described thus far has pertained to a hydraulic switch using a hydraulic fluid such as oil or the like for driving the hydraulic switch, other embodiments are possible in which the hydraulic fluid used to drive hydraulic switch 400 is pressurized water. The pressurized water may be taken from the output of low pressure pump 124, or from high pressure pump 106.

Operation of the cylinder arrangement of FIGS. 2A-2B is essentially similar to that of the system of FIGS. 1A-1B as described above, other than in the autonomous mechano-hydraulic reversal functionality provided by hydraulic switch 400 operating actuator 350 to change the state of selector 300. This functionality is presented schematically with reference to FIGS. 6A-6C. Specifically, as shown in FIG. 6A, as piston 208-1 moves to the left and nears the end of its stroke, piston 208-2 contacts switching rod 420 and begins to displace the switching rod and associated switching seals 418, 422 and 424 to the left. As the central seal passes fluid inlet 408, it is urged by fluid pressure to move to the fully-displaced left hand position as shown in FIG. 6B, thereby connecting pressure inlet 408 to port 412-1, and thereby to volume 303-2. FIG. 6B represents the state immediately after the state of switch 400 has flipped, but before the resulting flow has changed the state of the selector. FIG. 6C shows the state soon thereafter, after the actuator of selector 300 has caused the selector to switch states, thereby beginning the reverse (left-to-right as shown) power stroke.

As noted, outer faces 212-1 and 212-2 are preferably of effective surface area larger than inner faces 210-1 and 210-2, thereby achieving a corresponding slight pressure reduction to prevent leakage of fracking fluid into the proppant-free fluid circuit. Preferably, outer faces 212-1 and 212-2 are 1%-5% larger than inner faces 210-1 and 210-2. The difference in effective surface area typically corresponds to the cross-sectional area of piston rod(s) used to interconnect pistons 208-1 and 208-2, and the number and size of the piston rods is chosen accordingly.

Working pressures for the power strokes of the pressure transfer system in fracking applications are typically in the order of several hundred bar, and may approach or exceed 1000 bar. However, the pressure differential between the clean fluid in chambers 214-1 and 214-2 and the fracking fluid in chambers 216-1 and 216-2 is relatively small, preferably less than 10% of the working output pressure, and typically roughly 5% of the working pressure. The “double-walled” structure of cylinder arrangements 200-1 and 200-2 with substantially the entirety of the cylinders surrounded by pressure vessel chambers allows for cylinders 204-1 and 204-2 to be manufactured from lower strength materials than could be used for cylinders which need to withstand larger pressure differentials. This greatly reduces manufacturing costs by separating the requirements of the precision piston-cylinder geometry from the load-bearing requirements applicable to pressure vessels 202-1 and 202-2. The pressure vessels themselves do not require precision surfaces for sliding contact with a piston, and can therefore be produced by lower precision manufacturing techniques and without high quality surface finishing. As previously noted, the double-walled structure of cylinder arrangements 200-1 and 200-2 also allows for ports 220-1 and 220-2 to be located farther from open ends 206-1 and 206-2, for example, in the middle third of the total length of the combined dual cylinder assembly. This greatly reduces the risks of connection failure due to cyclic expansion and contraction along the length of the cylinder as it experiences cyclic pressure changes.

In cases where a plurality of paired cylinder arrangements HO are operating in parallel, it may be preferable to ensure that the pistons move out-of-phase with each other, i.e., changing directions at different times, in order to minimize the overall effect of any flow fluctuations which may occur during reversal of the piston direction at the end of each stroke. In the case of a system with electronic control of the flow selectors, out-of-phase operation is achieved simply by staggering the control signals for reversal of the different cylinders. As previously described, it is preferred that the positioning of at least one of the pistons of each paired cylinder arrangement is detected by a sensor arrangement. Referring to FIG. 7, as an example, an arrangement of two cylinder arrangement pairs, each with first and second cylinder arrangements 200-1 and 200-2 is considered. In this example, pistons 208-1A and 208-1B are mounted with linear encoders 116A and 116B, respectively, shown here only schematically. In this example, when in phase motion of pistons 208-1A and 208-1B is detected, controller 112 sends a control signal to one of the actuators, for example, actuator 350A, to actuate the corresponding flow selector 300A slightly earlier than would otherwise be required in order to ensure non-simultaneous switching of direction of the two cylinder assemblies.

When employing autonomous hydraulically-actuated stroke reversal, for example according to the system of FIGS. 2A-6C, if it is considered preferable to take precautions against simultaneous reversal of the piston direction for two cylinder assemblies, an alternative approach for ensuring out-of-phase operation of a plurality of energy recovery subsystems obviates the need for electronic control by employing a cascade approach to the hydraulic control where reversal of the stroke direction in a master cylinder assembly triggers sequential reversal of stroke direction in a chain of slave subsystems. This facilitates an implementation with all-hydraulic control. By way of non-limiting example, a chain configuration with three cylinder arrangements is shown in FIG. 8. In such a configuration, a master cylinder arrangement acts to control the stroke reversal of subsequent paired cylinder arrangements. This control cylinder arrangement is referred to herein as the master cylinder arrangement 200A. Master cylinder arrangement 200A preferably operates according to the description of paired cylinder arrangement 110, with stroke direction reversal either by a combination of a flow selector 300 and a hydraulic switch 400 arrangement as previously described or by any other hydraulic or electronic control arrangement suitable for a single paired cylinder arrangement. Each subsequent cylinder arrangement in the chain configuration is referred to herein as a slave cylinder arrangement. Each slave cylinder arrangement is structurally and operationally similarly to master cylinder arrangement 200A, with the exception that slave cylinder arrangements typically do not include a hydraulic switch 400 for actuating flow selectors 300B and 300C. Instead, the state switching of flow selector 300 for each slave subsystem is performed by pressure derived from the power stroke of the preceding cylinder arrangement in the chain which is delivered to the actuator of the selector for the slave cylinder arrangement. With reference to FIG. 8, the hydraulic actuator chambers of the flow selector 300B of the first slave cylinder arrangement 200B are in fluid flow communication, respectively, with proppant-free fluid chambers 214-1A and 214-2A of master cylinder arrangement 200A. In this configuration, master cylinder arrangement 200A actuates flow selector 300B to change the inflow and outflow of proppant-free fluid.

As previously described, the pressure stroke of piston 208-1A coincides with the simultaneous reverse stroke of piston 208-2A. A fluid connection is here provided from chamber 214-1A to volume 303-2B in order to actuate selector 300B to transition from its second state to its first state. This in turn directs high-pressure clean water from pump 106 to chamber 214-1B, thereby moving piston 208-1B in its power stroke from its first position to its second position. When assembly 200A reverses direction, the pressure stroke of piston 208-2A begins when high-pressure proppant-free fluid is delivered to chamber 214-2A, which is also in fluid connection with volume 303-1B in order to actuate selector 300B to transition from its first state to its second state. This in turn switches the stroke direction of assembly 200B, directing proppant-free fluid from pump 106 to chamber 2142B, and thereby moving piston 208-2B in a power stroke from its first position to its second position. The direction switching of pistons 208-1B and 208-2B thus comes at a delay relative to the direction switching of pistons 208-1A and 208-2A corresponding to the time taken for the pressure of the power stroke of assembly 200A to operate the actuator to change the state of the selector of assembly 200B. A similar interconnection is provided between assembly 200B and 200C, such that direction reversal of assembly 200C is triggered by, and occurs just after, direction reversal of assembly 200B. This cascade control scheme can be extended to a large number of cylinder assemblies, and is preferable deployed using 2-6 cylinder assemblies, and most preferably four cylinder assemblies.

The delays between direction switching of successive cylinder arrangements can be adjusted by modifying the flow impedance of the conduits interconnecting flow selectors, but can reasonably be kept as short as possible. Although not depicted in FIG. 8, it should also be apparent that each proppant-free fluid inlet port 304A, 304B, and 304C is in fluid flow connection with high-pressure pump 106, or a number of such pumps operating in parallel.

As previously mentioned, the chain configuration shown in FIG. 8 is advantageous in that it does not require electronic equipment for maintaining average flow with low fluctuation. Furthermore, the proppant-free fluid such as water is used as the motive fluid for operating all slave cylinder arrangements, thereby reducing the need for pressure pumps for hydraulic fluid. As mentioned above, the number of cylinder arrangements is not limited to the number of arrangement depicted in FIG. 8.

It should be noted that the various aspects of the present invention described herein may each be used to advantage independently of other aspects of the invention as presented herein. For example, the various hydraulic control solutions presented herein, although presented in a particularly preferred context of the double-walled cylinder structures of the present invention, may also be used to advantage with otherwise conventional single-walled cylinder constructions.

It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the scope of the present invention as defined in the appended claims.

Claims

1. A system for delivering pressurized fracking fluid to a well, the system comprising: wherein said first and second pistons have an inner face facing, respectively, said first and third chambers and an outer face facing, respectively, said second and fourth chamber, said outer face having larger effective surface area than said inner face, such that pressure in said fracking fluid remains lower than pressure in said proppant-free fluid.

(a) a source of proppant-laden fracking fluid;

(b) a high-pressure pump connected to a source of proppant-free fluid;

(c) a double-piston pressure transfer arrangement, said pressure transfer arrangement comprising: (i) a first cylinder assembly comprising a first hollow cylinder, and a first piston in sliding engagement within said first hollow cylinder so as to at least partially define a first chamber and a second chamber; (ii) a second cylinder assembly comprising a second hollow cylinder, and a second piston in sliding engagement within said second hollow cylinder so as to at least partially define a third chamber and a fourth chamber; and (iii) at least one piston rod interconnecting said first piston and said second piston; and

(d) a flow selector associated with said high-pressure pump, said first chamber of said first cylinder and said third chamber of said second cylinder, said flow selector assuming alternately: (i) a first selector state in which the proppant-free fluid from said high-pressure pump is directed to said first chamber so as to act on said first piston, thereby applying pressure to a quantity of the fracking fluid within said second chamber for delivery to the well in a first power stroke, said first power stroke also expelling a quantity of the proppant-free fluid from said third chamber and taking in a quantity of the fracking fluid to said fourth chamber; and (ii) a second selector state in which the proppant-free fluid from said high-pressure pump is directed to said third chamber so as to act on said second piston, thereby applying pressure to a quantity of the fracking fluid within said fourth chamber for delivery to the well in a second power stroke, said second power stroke also expelling the quantity of the proppant-free fluid from said first chamber and taking in a quantity of the fracking fluid to said second chamber,

2. The system of claim 1, wherein said first cylinder arrangement further comprises a pressure vessel enveloping substantially the entirety of said first hollow cylinder, said pressure vessel defining at least one enveloping volume selected from the group consisting of: a first enveloping volume in fluid flow communication so as to form an extension of said first chamber; and a second enveloping volume in fluid flow communication so as to form an extension of said second chamber, and wherein said second cylinder arrangement further comprises a pressure vessel enveloping substantially the entirety of said second hollow cylinder, said pressure vessel defining at least one enveloping volume selected from the group consisting of: a third enveloping volume in fluid flow communication so as to form an extension of said third chamber; and a fourth enveloping volume in fluid flow communication so as to form an extension of said fourth chamber.

3. The system of claim 1, further comprising: wherein said hydraulic switch is deployed to be actuated by motion of said first and second pistons to switch said hydraulic connections of said hydraulic switch, thereby subsequently actuating said bidirectional hydraulic actuator to switch said flow selector between said first and second selector states.

(a) a bidirectional hydraulic actuator associated with said flow selector for switching said flow selector between said first selector state and said second selector state; and

(b) a hydraulic switch having a switch inlet port for the inflow of a pressurized hydraulic fluid and having two hydraulic connections to said hydraulic actuator for actuating said hydraulic actuator,

4. The system of claim 3, wherein said hydraulic actuator includes an actuator piston displaceable within an actuator cylinder.

5. The system of claim 3, wherein said switch inlet port is connected to receive proppant-free fluid from said high-pressure pump.

6. The system of claim 1, wherein said double-piston pressure transfer arrangement is referred to as a master pressure transfer arrangement, and wherein said flow selector is referred to as a master flow selector, the system further comprising: wherein said slave hydraulic actuator is associated with said master flow selector or said master pressure transfer arrangement such that, during said first power stroke of said master pressure transfer arrangement, proppant-free fluid from said high pressure pump is delivered to said slave hydraulic actuator so as to switch said slave flow selector from said second selector state to said first selector state and, during said second power stroke of said master pressure transfer arrangement, proppant-free fluid from said high pressure pump is delivered to said slave hydraulic actuator so as to switch said slave flow selector from said first selector state to said second selector state.

(a) a slave double-piston pressure transfer arrangement, said pressure transfer arrangement comprising: (i) a first cylinder assembly comprising a first hollow cylinder, and a first piston in sliding engagement within said first hollow cylinder so as to at least partially define first chamber and a second chamber; (ii) a second cylinder assembly comprising a second hollow cylinder, and a second piston in sliding engagement within said second hollow cylinder so as to at least partially define a third chamber and a fourth chamber; and (iii) at least one piston rod interconnecting said first piston and said second piston; and

(b) a slave flow selector associated with said high-pressure pump, said first chamber of said first cylinder and said third chamber of said second cylinder, said flow selector assuming alternately: (i) a first selector state in which the proppant-free fluid from said high-pressure pump is directed to said first chamber so as to act on said first piston, thereby applying pressure to a quantity of the fracking fluid within said second chamber for delivery to the well in a first power stroke, said first power stroke also expelling a quantity of the proppant-free fluid from said third chamber and taking in a quantity of the fracking fluid to said fourth chamber; and (ii) a second selector state in which the proppant-free fluid from said high-pressure pump is directed to said third chamber so as to act on said second piston, thereby applying pressure to a quantity of the fracking fluid within said fourth chamber for delivery to the well in a second power stroke, said second power stroke also expelling the quantity of the proppant-free fluid from said first chamber and taking in a quantity of the fracking fluid to said second chamber; and

(c) a bidirectional slave hydraulic actuator associated with said slave flow selector for switching said slave flow selector between said first selector state and said second selector state,

7. A method for delivering pressurized fracking fluid to a well, the method comprising the steps of: wherein said first and second pistons have an inner face facing, respectively, said first and third chambers and an outer face facing, respectively, said second and fourth chamber, said outer face having larger effective surface area than said inner face, such that pressure in said fracking fluid remains lower than pressure in said proppant-free fluid.

(a) providing a double-piston pressure transfer arrangement, said pressure transfer arrangement comprising: (i) a first cylinder assembly comprising a first hollow cylinder, and a first piston in sliding engagement within said first hollow cylinder so as to at least partially define a first chamber and a second chamber; (ii) a second cylinder assembly comprising a second hollow cylinder, and a second piston in sliding engagement within said second hollow cylinder so as to at least partially define a third chamber and a fourth chamber; and (iii) at least one piston rod interconnecting said first piston and said second piston; and

(b) directing a proppant-free fluid from a high-pressure pump to said first chamber so as to act on said first piston, thereby applying pressure to a quantity of the fracking fluid within said second chamber for delivery to the well in a first power stroke, said first power stroke also expelling a quantity of the proppant-free fluid from said third chamber and taking in a quantity of the fracking fluid to said fourth chamber; and

(c) directing the proppant-free fluid from the high-pressure pump to said third chamber so as to act on said second piston, thereby applying pressure to a quantity of the fracking fluid within said fourth chamber for delivery to the well in a second power stroke, said second power stroke also expelling the quantity of the proppant-free fluid from said first chamber and taking in a quantity of the fracking fluid to said second chamber,

8. The method of claim 7, wherein said first cylinder arrangement further comprises a pressure vessel enveloping substantially the entirety of said first hollow cylinder, said pressure vessel defusing at least one enveloping volume selected from the group consisting of: a first enveloping volume in fluid flow communication so as to form an extension of said first chamber; and a second enveloping volume in fluid flow communication so as to form an extension of said second chamber, and wherein said second cylinder arrangement further comprises a pressure vessel enveloping substantially the entirety of said second hollow cylinder, said pressure vessel defining at least one enveloping volume selected from the group consisting of: a third enveloping volume in fluid flow communication so as to form an extension of said third chamber; and a fourth enveloping volume in fluid flow communication so as to form an extension of said fourth chamber.

9. A system for pumping a particle-laden fluid from a fluid source, the system comprising: wherein said first and second pistons have an inner face facing, respectively, said first and third chambers and an outer face facing, respectively, said second and fourth chamber, said outer face having larger effective surface area than said inner face, such that pressure in said particle-laden fluid remains lower than pressure in said clean fluid.

(a) a pump connected to a source of clean fluid;

(b) a double-piston pressure transfer arrangement, said pressure transfer arrangement comprising: (i) a first cylinder assembly comprising a first hollow cylinder, and a first piston in sliding engagement within said first hollow cylinder so as to at least partially define a first chamber and a second chamber; (ii) a second cylinder assembly comprising a second hollow cylinder, and a second piston in sliding engagement within said second hollow cylinder so as to at least partially define a third chamber and a fourth chamber; and (iii) at least one piston rod interconnecting said first piston and said second piston; and

(c) a flow selector associated with said pump, said first chamber of said first cylinder and said third chamber of said second cylinder, said flow selector assuming alternately: (i) a first selector state in which the clean fluid from said pump is directed to said first chamber so as to act on said first piston, thereby applying pressure to a quantity of the particle-laden fluid within said second chamber for delivery through an outlet in a first power stroke, said first power stroke also expelling a quantity of the clean fluid from said third chamber and taking in a quantity of the particle-laden fluid to said fourth chamber; and (ii) a second selector state in which the clean fluid from said pump is directed to said third chamber so as to act on said second piston, thereby applying pressure to a quantity of the particle-laden fluid within said fourth chamber for delivery through an outlet in a second power stroke, said second power stroke also expelling the quantity of the clean fluid from said first chamber and taking in a quantity of the particle-laden fluid to said second chamber,