ELECTRICAL CONTROLLER FOR ANTI-STALL TOOLS FOR DOWNHOLE DRILLING ASSEMBLIES

Abstract

An anti-stall tool in an oil well drilling assembly that controls reciprocation of the drill bit by an electrical controller that alters weight-on-bit (WOB) depending upon measured downhole pressure or torque at the downhole motor. The electrical controller receives preset high and low working pressure limits for the downhole motor and activates a hydraulic valve system to keep the drill bit rotating by maintaining WOB during normal drilling operations, increasing WOB if sensed working pressure indicates that drill bit loading or torque is undesirably low, and reversing WOB by retracting the drill bit if excessive working pressure or torque is sensed.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to U.S. Provisional Application No. 61/405,066, filed Oct. 20, 2010, which is incorporated herein by reference.

BACKGROUND

The present invention provides an improvement in anti-stall tool controllers, and more specifically is an electrical controller to operate an anti-stall tool (AST) for controlling weight-on-bit during drilling operations.

Coiled tubing drilling requires the use of a downhole positive displacement motor (PDM) to rotate the drill bit. During drilling operations, the unloaded PDM rotates at a constant RPM and achieves a “freespin” motor pressure, with respect to the fluid flow rate. As the drill bit encounters the bottom of the hole and force is transferred to the bit, referred to as weight-on-bit (WOB), the motor will sense an increase in torque. This increase in torque is a result of increased resistance to rotating at the constant RPM (assuming a constant flow rate). In turn, the PDM requires additional pressure to turn the motor at the constant RPM while under increased resistance. If the resistance increases to a condition which prohibits the PDM from rotating (i.e. excessive WOB), a motor stall is encountered. During a motor stall, the motor stops turning, the downhole fluid path is severely restricted, and the surface pump pressure dramatically increases. This event can eventually cause a motor failure, which requires the drilling process to be stopped, and the coiled tubing to be fatigue-cycled as the bit is pulled off bottom and run back into the hole to start drilling again. An anti-stall tool (AST) is described in U.S. Patent Publication No. 2009/0173540 to Mock, et al.

A downhole tool that monitors motor pressure and sharply reduces the occurrence of motor stalls will increase overall drilling efficiency by:

(1) Increasing the average rate of penetration. This is achieved by reducing the occurrences of pulling off-bottom every time the motor stalls.

(2) Decreasing the damage to PDMs through repeated motor stalls, thereby decreasing occurrence of downhole failure.

(3) Decreasing the fatigue cycles on the coiled tubing. This increases the number of wells a coiled tubing string can service.

By achieving a more efficient drilling operation, the operators can substantially increase the cost savings of drilling a well.

The present invention provides an electrical controller for an anti-stall tool that controls WOB during drilling operations, resulting in improved overall drilling efficiency.

SUMMARY OF THE INVENTION

Briefly, the invention comprises an electrical controller for an anti-stall tool for use in a downhole assembly near the bottom of the tubing adjacent a positive displacement motor (PDM) and the drill bit. In one embodiment, the tubing comprises a coiled tubing, although the invention also can be used in rotary drilling applications. The electrical controller controls the force applied to the drill bit during drilling to prevent the drill bit from stalling under load. A working pressure range of the PDM is sensed during use by a hydraulic valve control system and is used as an input to the controller. The controller alters weight-on-bit (WOB) if the downhole pressure goes beyond either end of a preset working pressure range of the system. The controller keeps the drill bit rotating by (1) maintaining WOB during normal drilling operations, (2) increasing WOB if sensed PDM working pressure indicates that drill bit loading is low, and (3) reducing WOB which reduces PDM back-pressure to retract the drill bit from the bottom if excessive working pressure is sensed due to increased torque at the PDM.

The anti-stall tool generally comprises one or more hydraulic cylinders for applying an axial force either in a forward direction or a reverse direction. The electrical controller comprises electronics and a system of hydraulic valves adapted to control piston force in either the forward or reverse directions. An active stage of the anti-stall tool reacts to the PDM producing low downhole pressures (e.g. below a preset low pressure) by actuating one or more of the pistons in the downhole direction to increase WOB which increases PDM back-pressure. When the PDM is operating within its normal operating pressure range, the controller locks the pistons in a passive mode, in which the pistons are sealed and the anti-stall tool transfers force from the tubing to the drill bit. If the controller senses a preset high pressure or greater due to high torque at the PDM, the valve system reverses hydraulic flow to the pistons, which reduces WOB to force the drill bit away from the bottom to reduce PDM back-pressure.

One embodiment of the invention comprises an anti-stall method for controlling drilling operations in a downhole assembly which includes a tubing that extends downhole, a drill bit carried on the tubing, a positive displacement motor (PDM) for rotating the drill bit, and an anti-stall tool adjacent the PDM. The method comprises sensing pressure in the PDM, providing a range of operating pressures for the PDM defined by high and low limits of operating pressures, and operating the anti-stall tool in: (1) an active stage for increasing WOB forces in the downhole direction when the low limit of operating pressure is sensed, (2) a reverse stage for providing a WOB force in the reverse direction when the high limit of operating pressure is sensed, and (3) an optional passive stage in which the anti-stall tool is locked to transfer WOB directly from the tubing to the drill bit when the PDM is operating within the limits of its normal operating pressure range.

Another embodiment of the invention comprises a spring-operated anti-stall tool adapted for use in a downhole assembly which comprises a tubing for extending downhole, a drill bit carried on the tubing, and a positive displacement motor (PDM) adjacent the drill bit for rotating the drill bit during drilling operations. A spring-operated anti-stall tool is carried on the tubing and positioned adjacent the PDM for preventing stalling of the PDM due to excessive loads on the drill bit. The spring-operated anti-stall tool comprises at least one piston in a cylinder having a forward piston area and a reverse piston area, and an electrical controller comprising electronics and a hydraulic valve system for controlling operation of the piston. The forward piston area receives hydraulic fluid to produce a force in the downhole direction. The reverse piston area contains a load spring adapted to apply an upward spring force on the piston. The electrical controller adjusts WOB in response to sensed PDM sets operating pressure. The controller inputs a desired range of operating pressures for the PDM, including an upper limit and a lower limit. The controller operates a valve system to: (1) supply hydraulic fluid to the forward piston area to increase WOB force in the downhole direction when operating pressure in the PDM surpasses the lower limit; this compresses the load spring as the cylinder moves in the downhole direction; (2) vent the piston volume in the forward piston area so the compressed spring will push the tool uphole, to reduce WOB when operating pressure in the PDM exceeds the upper limit; and (3) optionally lock the piston in a passive state when the PDM is operating within its normal operating pressure range.

Another embodiment comprises an improved anti-stall tool which produces a controlled translational motion of the drill bit that increases drilling efficiency. The anti-stall tool controls the force applied to the drill bit during drilling to prevent the drill bit from stalling under load. The anti-stall tool comprises one or more hydraulic cylinders for applying an axial force in either a forward or reverse direction, and an electrical controller adapted to control the force applied by the one or more hydraulic cylinders to the drill bit in response to sensed working pressure of the drive motor during drilling operations. The controller comprises a system for adjusting WOB when working pressure exceeds either end of a working pressure range of the drive motor. The system includes (1) a passive stage for maintaining WOB when working pressure is within a preset normal operating range, (2) an active stage for applying pressure to the one or more cylinders to increase WOB when sensed working pressure is below a preset limit, and (3) a reverse stage for reversing pressure to the one or more cylinders to reduce WOB and thereby retract the drill bit from the bottom when sensed working pressure is above a preset limit. The tool is normally controlled to apply WOB at pressures within a desired wide range of pressures. When reaching a preset anti-stall pressure, the tool is reversed to reduce WOB and does not resume applying WOB over a preset wide range of PDM back-pressure drop.

In another embodiment, the tool can apply WOB during the wide range of operating pressures via at least two stages, one where pressure is increasing up to a set desired operating pressure, and then switches the tool to a locked position at that pressure and higher up to a preset anti-stall limit at which flow to the pistons is reversed to lift the drill bit. The two stages can be operated as active/reverse stages as well.

These and other aspects of the invention, including additional embodiments, will be more fully understood by referring to the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a downhole assembly containing an anti-stall tool according to principles of this invention;

FIG. 2 shows a cross-sectional view of one embodiment of a hydraulic-operated anti-stall tool;

FIG. 3 is an elevational view showing a further embodiment of an anti-stall tool;

FIG. 4 is a cross-sectional view showing the anti-stall tool of FIG. 3 along with a schematic view of an improved controller;

FIG. 5 shows a cross-sectional view of another alternative embodiment spring-operated anti-stall tool;

FIG. 6 is a cross-sectional view of an electrically controlled anti-stall tool which includes a 2-position, 4-way pilot valve with a pressure control valve according to principles of this invention;

FIG. 7 shows an alternative embodiment electrically controlled anti-stall tool which includes a 3-position, 4-way valve; and

FIG. 8 shows a cross-sectional view of the electrically controlled AST, which includes an electronics package and electrical downhole motor contained in the AST.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram illustrating a coiled tubing drilling system 10 for drilling a well bore in an underground formation 12. The coiled tubing drilling system can include a coiled tubing reel 14, a gooseneck tubing guide 16, a tubing injector 18, a coiled tubing 20, a coiled tubing connector 21, and a drill bit 22 at the bottom of the well bore. FIG. 1 also shows a control cab 24, a power pack 26, and an alignment of other BHA tools at 27. A tractor (not shown), such as that described in U.S. Pat. No. 7,343,982, may be used to move downhole equipment within the bore. The '982 patent is incorporated herein in its entirety by this reference. During drilling, the downhole equipment includes a downhole motor 28, such as a positive displacement motor (PDM), for rotating the drill bit. An anti-stall tool (AST) 30, according to principles of this invention, is positioned near the bottom of the coiled tubing, upstream from the downhole motor and the drill bit. In one embodiment, hydraulic back pressure produced within the coiled tubing is measured at the surface. Torque produced at the drill bit during drilling operations is directly related to back-pressure. As a result, hydraulic back-pressure measurements can be sensed and used as inputs to a hydraulic control valve system contained in the anti-stall tool.

The anti-stall tool 30 incorporates use of a series of hydraulic cylinders and as few as three pressure-actuated valves to control the applied weight-on-bit (WOB) while drilling. This tool will virtually create a real time, downhole motor pressure sensor that will alter the WOB to maintain a relatively constant drilling rate of penetration and provide feedback to the coiled tubing operator to adjust coiled tubing injector rates to match the PDM pressure.

The invention uses the working pressure range of the downhole positive displacement motor 28 to alter the WOB if the downhole pressure surpasses either end of the working range. During drilling operations, the AST controls WOB through the use of three distinct operations: active WOB, passive WOB and reverse.

FIG. 2 illustrates one embodiment of the anti-stall tool 30 which includes a series of axially aligned hydraulic cylinders with separate pistons that define piston areas A1 and A2, A3A and A3B, and A3C and A3D. The torque section of the tool is shown at 35. FIG. 2 also schematically shows a hydraulic controller 34 contained in the anti-stall tool. The controller includes a pressure reducing valve 36, a reverser valve 38, and a vent valve 40. Hydraulic control fluid passes through a filter 42.

In the description to follow, specific operating pressure set points or values are related to operative ranges for coiled tubing equipment. Use of the anti-stall tool in rotary drilling operations, for example, would involve use of different operating pressure ranges or control valve set points.

The first stage of the hydraulic anti-stall tool is activated when the unloaded PDM produces low downhole pressures. For example, if the PDM creates a back pressure of 200 psi (adjustable to specific motor requirements), the anti-stall tool will be in the active WOB stage. This causes pressure to be supplied to all pistons that will produce a force in the downhole direction (A1, A3A and possibly A3C). As the WOB is applied, the normal reaction is for the PDM to generate more pressure. As the anti-stall tool senses the increase in pressure to 250 psi (adjustable to specific motor requirements), the pressure reducing valve 36 will shut off additional flow to the pistons and hydraulically lock the pistons in the passive WOB stage.

In the passive WOB stage, the anti-stall tool transfers the force from the tubing to the bit. The tool is acting as a rigid member and is monitoring the PDM back-pressure. The pressure reducing valve 36 is closed and is sealing the fluid in the pistons (A3A and possibly A3C) that produce a force in the downhole direction. All of the resultant pressure from the WOB will be contained in the sealed piston volumes.

During the final stage of the anti-stall tool, the back pressure due to high torque in the PDM triggers the reverser valve 38 and vent valve 40 to reduce WOB. Once the back pressure reaches 1,000 psi (adjustable to specific motor requirements), the reverser valve 38 switches the flow of fluid to the pistons that produce force in the uphole direction (A2, A3B, A3D). At the same time, the vent valve 40 vents the opposite side of those pistons. This allows the tool to travel uphole, reducing WOB and thereby reducing the PDM back pressure. As the PDM back pressure falls below the reverser valve setting (including hysteresis) the reverser valve 38 will switch back to its original position.

The anti-stall tool is designed to be in the fully expanded position at low pressures. This bias allows the tool to have the full length of stroke available to retract as much as needed until the PDM back-pressure reduces below the lower limit of the vent valve. The anti-stall tool will then try to fully expand, but the pressure may rise to the pressure control valve setting or higher and limit the expansion. Therefore, the long stroke length will allow several retraction steps before the stroke length is used up. The coiled tubing operator can adjust the input speed of the coiled tubing into the hole to prevent the anti-stall tool from fully retracting. The operator will see a change in pump pressure with each retraction to signal the need to reduce the coiled tubing input speed.

The anti-stall tool operates as an open loop system. Drilling fluid from the surface is pumped down the bore in the tubing through the tool, to the motor for rotating the drill bit. Most of the fluid flow in the system is used for driving the drill bit. A small amount of the fluid is used for the controller and is jetted out to the sides and into the annulus during use.

The anti-stall tool includes splines in a torque section 44 which contains an outer spline housing and splines contained internally on the piston housing. The splines allow the BHA to maintain its orientation relative to the motor and drill bit, without undesired twisting. The splines allow the tool to be used with a steerable BHA. Steerable BHAs can be controlled to drill the hole to a desired location, while changing the direction of the hole while drilling to achieve this goal. The splines allow the PDM and bit to maintain alignment with the orienting tools that would be uphole of the anti-stall tool. The torque load is transferred from the PDM across the outermost housings and across the spline of the anti-stall tool to the tools uphole of the anti-stall tool. The inner shafts do not see direct loading due to torque. The spline section functions in both the expansion and retraction of the anti-stall tool.

FIGS. 3 and 4 show an improved anti-stall tool 30′ which produces a three-stage controlled translational motion to the drill bit that increases drilling efficiency.

This illustrated embodiment includes a series of axially aligned hydraulic cylinders with pistons that cooperate to form piston areas S1, A1 and A2, and A3A and A3B. The torque section of the tool is shown at 44 along with a hydraulic controller contained in the anti-stall tool and shown schematically at 46. The controller includes a pressure control valve 48, a pilot valve 50, a sequence valve 52, and a vent valve 54. A filter for the hydraulic controller is shown at 56.

In one embodiment, the controller has the three stages of operation: (1) active, (2) passive, and (3) retraction. The control valves contained in the controller area of the tool are shown schematically in FIG. 4: pressure lines are shown as solid lines, pilot lines are shown as dashed lines, and exhaust lines are shown in dotted lines. In the following description, the pressure ranges are used as examples only; they are adjustable to specific motor requirements.

The active stage applies downward force to the drill bit based on motor back-pressure from the positive displacement motor. If pressure is less than 400 psi, for example, the hydraulic pistons apply a downward force which generates more PDM back-pressure. The vent valve 54 of the controller is open and supplies a pilot signal to the pilot valve 50. If pressure reaches 400 psi, the vent valve 54 closes and vents the pilot line for the pilot valve 50. But the detented pilot valve stays in position, and the PDM back-pressure is sensed by the pressure control valve 48. The pistons apply the downward force until sensed downhole pressure reaches 650 psi, for example, which represents a desired working pressure.

The pressure control valve then switches the anti-stall tool to the passive mode when sensed pressure reaches the desired drilling pressure of 650 psi, for example. Here the pressure control valve 48 shuts off flow to the pistons and hydraulically locks the pistons in the passive WOB mode. The pressure control valve 48 is closed and no pressure is sent to the pistons. The pistons are sealed, and existing force is transferred to the drill bit. Motor pressure is not increased. Downhole pressure continues to be monitored in the passive mode via the vent valve 54 and sequence valve 52, which monitor pressure change in the coiled tubing. The passive state continues until sensed back-pressure reaches 800 psi, for example.

Once downhole pressure reaches the 800 psi level, the anti-stall tool switches to the reverse mode. That is, if torque in the PDM increases, it causes an increase in back-pressure. Motor stall is prevented by sensing and reacting to back pressure at a level below motor stall, e.g., 800 psi, or other pressure below that at which stall can occur.

When sensed pressure reaches 800 psi, the normally-closed sequence valve 52 is opened, sending a pilot signal to the pilot valve 50 which reverses flow of hydraulic fluid to the pistons to produce a force in the uphole direction, to reduce WOB.

As back pressure falls below 800 psi, the pilot signal from the sequence valve 52 to the pilot valve 50 is closed. The sequence valve 52 vents the pilot signal, and this continues until sensed PDM pressure falls to 400 psi, where the vent valve 54 opens and sends a pilot signal to the pilot valve 50 to shift back to the active mode, by supplying fluid pressure to the pistons to expand and to apply downward force to increase WOB.

Thus, in this embodiment, the tool is normally controlled to apply WOB when drilling at pressures within a desired wide range of pressures. These can be from 400 to 800 psi, for example. When reaching a preset anti-stall pressure, such as 800 psi, which would be a safe level below the pressure at which stall actually occurs, the tool is reversed and does not resume applying WOB over a preset wide range of pressure drop, before resuming active WOB operations. This wide range of pressure drop can be from about 200 to about 2,000 psi. In the illustrated embodiment, the range of pressure drop is 400 psi (from 800 to 400 psi), before WOB is resumed.

The tool applies WOB during the desired wide range of operating pressures via two stages, one stage where pressure is increasing up to a set desired operating pressure, for example 650 psi, and then switches to a second-stage locked position at that pressure and higher up until an anti-stall limit, of say 800 psi is reached, before reversing flow to the pistons and lifting the drill bit.

A key feature of the anti-stall tool is the single input necessary for the tool to operate. The tool need only sense and respond to the back-pressure created by the PDM. Stated another way, the anti-stall tool operates on constant (although adjustable) working pressure set points. The fixed set points can be fine-tuned to control the thresholds at which the control valves open and close, and as a result, drill bit penetration rate is more uniform.

An alternate embodiment of the invention comprises a two-phase anti-stall method for controlling drilling operations in a downhole assembly, which includes the tubing that extends downhole, the drill bit carried on the tubing, the positive displacement motor (PDM) for rotating the drill bit, and the anti-stall tool adjacent the PDM. This method comprises sensing pressure in the PDM, providing a range of operating pressures for the PDM defined by high and low limits of operating pressures, and operating the anti-stall tool in: (1) an active stage increasing WOB forces in the downhole direction when the low limit of operating pressure is sensed, and (2) a reverse stage providing a force in the reverse direction, reducing the WOB, when the high limit of operating pressure is sensed.

This two-phase anti-stall method can be accomplished by adjusting the setting of the sequence valve 52 equal to or lower than the pressure control valve 48, but still above the setting of the vent valve 54.

The anti-stall tool also can be operated by the two-phase method, combined with a passive range that operates (as described above) between a small range of pressure settings.

Different orifice adjustments can be used to control the speed at which the tool responds. In FIG. 2, the orifice is not shown. The orifice can be on the exhaust of the reverser valve 38.

Although the schematic in FIG. 4 depicts a single orifice 55, those skilled in the art would understand that the two-position/four-way valve contains two exhaust ports. Each of the ports vents a different piston area, either the piston area to produce downhole force (expand) or uphole force (retract). Using the high and low limits of the operating pressures, the orifice sizes can be calculated to restrict the volumetric flow rate of fluid exhausted through the valve and thereby control the speed at which the tool expands or retracts. The expansion and retraction of the tool can be controlled individually by different orifice sizes.

As an alternative, WOB can be controlled by a combination of control valve settings and adjustments to orifice sizes.

Example

The following specifications illustrate one embodiment of the anti-stall tool:

	Description	Characteristic
	Tool OD	3.00	in
	Tool ID	.75	in
	Length - Expanded	8.1	ft
	Length - Collapsed	7.4	ft
	Stroke	9	in
	Max Temp	300°	F.
	Tensile Strength	50,000	lbs
	Max Motor Torque	2,000	ft-lbs
	Max Dog Leg	25°/100	ft
	Tool Joint	2⅜	PAC

The design is flexible in that the pressure settings and orifice size may be changed to fine-tune the tool. If a much larger WOB change is needed, then the shaft can be replaced to allow installation of additional pistons.

	Total Downhole	Pressure Control	Max WOB from
# of Pistons	Area (sq. in.)	Valve Setting (psi)	AST (lbs)
1	4.8	650	3,055
2	7.9	650	5,135
3	11.0	650	7,150

The anti-stall tool cylinders and valves may be manufactured from various corrosion-resistant materials including tungsten carbide, Inconel, high strength nickel alloyed steel such as MP35, beryllium-copper, and the like.

Examples of improvements provided by the anti-stall tool are:

• (1) Active WOB: The tool will attempt reset into the fully extended position when the pressure falls below 650 psi. If a motor stall has occurred and the AST has pulled the bit off bottom, the Active WOB stage will produce a minimum WOB and thrust the bit downhole until the PDM pressure exceeds 650 psi.
• (2) Passive WOB: Shuts off the Active WOB stage and allows the coiled tubing to transfer WOB to the bit. Prevents excessive WOB that can be developed as PDM pressure rises and acts on the pistons producing force downhole.
• (3) Reverse: Reduces WOB to prevent motor stalls.
• (4) Torque section will transfer torque through the AST into the coiled tubing.

A downhole tool that monitors motor pressure and sharply reduces the occurrence of motor stalls will increase the overall drilling efficiency by:

• (1) Increasing the average rate of penetration. This is achieved reducing the occurrences of pulling off bottom for motor stalls.
• (2) Decreasing the damage to PDMs through repeated motor stalls, thereby decreasing occurrence of downhole failure.
• (3) Decreasing the fatigue cycles on the coiled tubing. The increases the number of wells a coiled tubing string can service.

FIG. 5 illustrates a spring-operated anti-stall tool 130 according to this invention. In the description to follow, motor pressure values are examples only; they are dependent upon and adjustable to specific motor requirements.

The FIG. 5 embodiment includes a series of axially aligned hydraulic cylinders with separate pistons that define piston areas A1 and A2, A3A and A3B, and A3C and A3D. The torque section of the tool is shown at 35. The piston area A3B contains a compression spring that applies a spring force F1 and a piston area A3D which contains a compression spring that applies a spring force F2. FIG. 2 also schematically shows a controller 34 contained in the anti-stall tool. The controller includes a pressure reducing valve 136 and a vent valve 138. Hydraulic fluid passes through a filter 140.

In the description to follow, specific operating pressure set points or values are related to operative ranges for coiled tubing equipment. Use of the anti-stall tool in rotary drilling operations, for example, would involve use of different operating pressure ranges or control valve set points.

The first stage of the spring operated anti-stall tool 130 is activated when the unloaded PDM produces low downhole pressures. For example, if the PDM 20 creates a back pressure of 200 psi, the spring-operated tool will be in the active WOB stage. This causes pressure to be supplied to all pistons that will produce a force in the downhole direction (A1, A3A and possibly A3C). This will compress and load the springs with a spring force F1 and F2. As the WOB is applied, the normal reaction is for the PDM to generate more pressure. As the tool senses the increase in pressure to 250 psi (adjustable to specific motor requirements), the pressure reducing valve 136 will shut off additional flow to the pistons and hydraulically lock the pistons in the passive WOB stage.

In the passive WOB stage, the spring-operated tool transfers the force from the coil to the bit. The tool is acting as a rigid member and is monitoring the PDM back-pressure. The pressure reducing valve 136 is closed and is sealing the fluid in the pistons (A3A and possibly A3C) that produce a force in the downhole direction. All of the resultant pressure from the WOB is contained in the sealed piston volumes.

During the final stage of the spring-operated tool, the back pressure due to high torque in the PDM triggers the vent valve 138 to pull the bit off-bottom. Once the back pressure reaches 1,000 psi (adjustable to specific motor requirements), the vent valve 138 vents piston volumes A3A and A3C. The resultant force F1 and F2 of the compressed springs will push the tool uphole, reducing WOB and thereby reducing the PDM back-pressure. As the PDM back-pressure falls below the vent valve setting (including hysteresis), the tool will switch back to one of its other stages of operation.

FIGS. 1-5 illustrate two AST designs having hydraulic controllers, however FIG. 6 shows an alternative embodiment electrically controlled AST 58 having a 2-position, 4-way pilot valve 60 with pressure control valve 62. An electrical controller could be incorporated into the AST embodiments shown in FIGS. 1-5. The embodiment, as shown in FIG. 6, is based on using the working pressure range of a downhole positive displacement motor (PDM) 28 to alter the weight-on-bit (WOB) if the downhole pressure surpasses either end of the PDM's working range. Alternatively, this embodiment can use the working power range of an electric downhole motor 64 to alter the WOB if the power consumption surpasses either end of the motor's working range. During drilling operations, the anti-stall tool (AST) 58 will control the WOB through the use of three distinct operations; active WOB, passive WOB, and Off Bottom.

The first stage of the AST is activated when the unloaded PDM produces low downhole pressures. For example, if the PDM and drill bit jets create a back pressure of 300 psi, the AST will be in the active WOB stage. This means that pressure will be supplied to all pistons in the AST that will produce a force in the downhole direction (A1, A3A). As the WOB is applied, the normal reaction is for the PDM to generate more pressure. When the pressure reaches 400 psi, for example, the electronics package 66, which receives a pressure-related input from a pressure transducer (P) 68, will signal the motor 64 to shift the pilot valve (PV) 60. The PV is a motor-driven, two position, four way spool valve. The PV controls whether the AST will expand or contract. As the AST senses the increase in pressure to 650 psi (adjustable to specific motor requirements), the pressure control valve (PCV) 62 will shut off additional flow to the pistons and hydraulically lock the pistons in the AST in the passive WOB stage.

In the passive WOB stage, the AST simply transfers the force from the coil tubing to the drill bit. The AST is acting as a rigid member and is simply monitoring the PDM back pressure. The PCV is closed and is sealing the fluid in the pistons (A1, A3A) that produce a force in the downhole direction. All of the resultant pressure from the WOB will be contained in the sealed piston volumes.

During the final stage of the AST, the back pressure due to high torque in the PDM signals the electronics package 66 via the pressure transducer 68 to shift the pilot valve 60 and pull the bit off bottom. Once the back pressure reaches 800 psi (off bottom setting, adjustable to specific motor requirements), the pressure transducer/electronics package will signal the electric downhole motor 64 to shift the PV's position. This switches the flow of fluid to the AST pistons that produce force in the uphole direction (A2, A3B). This allows the tool to travel uphole, reducing WOB and thereby reducing the PDM back pressure. When the pressure falls below the active WOB setting (400 psi), the pressure transducer/electronics package will signal the electric downhole motor to shift the PV's positions. Once the PV switches back to its original position, the AST will return to the active WOB stage. The electronics package 74 receives a pressure input from pressure transducer (P) 76 and signals motor 72 to shift pilot valve 70.

FIG. 7 shows a 3-position, 4-way pilot valve 70. The alternative embodiment, shown in FIG. 7, is based on using a single valve to control the AST. The pilot valve 70 is a motor 72 driven, three-position, four-way spool valve. The center position of the pilot valve generates the hydraulic lock necessary for the passive WOB stage.

FIG. 8 illustrates a cross sectional view of the electrically controlled AST 58 which includes a torque transducer 78 (or alternative sensing method), electronics package 66 and the electric motor 64 driven pilot valve 60 within the housing.

Example

Specifications for One Embodiment AST Illustrated in FIGS. 6-8

	Description	Characteristic
	Tool OD	3.00	in
	Tool ID	.75	in
	Length - Expanded	8.1	ft
	Length - Collapsed	7.4	ft
	Stroke	9	in
	Max Temp	300°	F.
	Tensile Strength	50,000	lbs
	Max Motor Torque	2,000	ft-lbs
	Max Dog Leg	25°/100	ft
	Tool Joint	2⅜	PAC

The design of an electrically controlled AST is flexible in that the pressure settings may be changed to fine tune the AST. Programmable pressure settings may be changed on surface or while in operation. Current available communication techniques include mud pulse telemetry, fiber optic and wireline.

If a large increase in WOB is needed, then the shaft of the AST can be replaced to allow the installation of additional pistons.

	Total Downhole	Pressure Control	Max WOB from
# of Piston	Area (sq. in)	Valve Setting (psi)	AST (lbs)
1	4.8	650	3,055
2	7.9	650	5,135
3	11.0	650	7,150

Features and Benefits:

The following illustrates features of an electrically controlled AST:

Active WOB: The tool will attempt reset into the fully extended position when the pressure falls below 650 psi. If a motor stall has occurred and the AST has pulled the bit off bottom, the Active WOB stage will produce a minimum WOB and thrust the bit downhole until the PDM pressure exceeds 650 psi.

Passive WOB: Shuts off the Active WOB stage and allows the coiled tubing to transfer WOB to the bit. Prevents excessive WOB that can be developed as PDM pressure rises and acts on the pistons producing force downhole.

Off Bottom: Pulls the bit off bottom to prevent motor stalls.

Torque section will transfer torque through the AST into the coiled tubing.

The downhole tool monitors motor pressure and sharply reduces the occurrence of motor stalls to thereby increase the overall drilling efficiency by:

Increasing the average Rate of Penetration (ROP). This is achieved reducing the occurrences of pulling off bottom for motor stalls.

Decreasing the damage to PDMs through repeated motor stalls, thereby decreasing occurrence of downhole failure.

Decreasing the fatigue cycles on the coiled tubing. This increases the number of wells a coiled tubing string can service.

By achieving a more efficient drilling operation, the operators can substantially increase the cost savings of drilling a well.

Alternative Methods of Operations:

Instead of an absolute pressure transducer, the AST may sense:

Differential Pressure between annulus and bore using a Differential Pressure Transducer

Load transferred through the tool or Weight on Bit (WOB) using a Load Cell

Torque transferred through the tool using a Torque Transducer

Rotational deceleration using a Torsional Accelerometer

If the system is connected to an electric motor instead of a hydraulic motor, the AST may sense:

Change in voltage

Change in electric current

The AST system itself may be hydraulically powered, electrically powered, battery powered, mechanically assisted, or any combination.

Although the invention has been described in connection with oil well drilling and use with a coiled tubing, the invention has other applications, including: jointed pipe, or rotary drilling; in operations besides drilling where it is useful to retract a tool at high pressures; or where adjustments to the drill bit are made to keep contact with the formation or to pick up the bit completely off the formation. Although the invention has been described with reference to a drill bit used in drilling oil wells in underground formations, the invention also may be used with other pressure-inducing tools such as high pressure jetting tools.

The anti-stall tool cylinders and valves may be manufactured from various corrosion-resistant materials including tungsten carbide, Inconel, high strength nickel alloyed steel such as MP35, beryllium-copper, and the like.

Claims

1. A spring-operated anti-stall tool adapted for use in a downhole assembly comprising a tubing for extending downhole; a drill bit carried on the tubing; and a drive motor adjacent the drill bit for rotating the drill bit during drilling operations; the spring-operated anti-stall tool carried on the tubing and positioned adjacent the motor for preventing stalling of the motor due to excessive loads on the drill bit, the anti-stall tool including at least one piston in a cylinder having a forward piston area and a reverse piston area, and an electrical controller which signals a hydraulic valve system for controlling operation of the piston, the forward piston area receiving hydraulic fluid to produce a force in the downhole direction, the reverse piston area containing a load spring adapted to apply an upward spring force on the piston, the electrical controller sensing operating pressure of the drive motor and setting a desired range of operating pressures for the motor, including an upper limit and a lower limit, the electrical controller adapted to: (1) supply hydraulic fluid to the forward piston area to increase force in the downhole direction to increase weight-on-bit (WOB) when operating pressure in the motor surpasses the lower limit, thereby compressing the load spring as the cylinder moves in the downhole direction; (2) vent the piston volume in the forward piston area so the compressed spring can expand to push the tool uphole to retract the drill bit, to decrease WOB when operating pressure in the motor exceeds the upper limit; and (3) optionally lock the piston in a passive state when the motor is operating within its normal operating pressure range under the bias of the spring.

2. Apparatus according to claim 1 in which the electrical controller comprises an electronics package which receives a pressure-related input from a transducer to signal a motor to shift the hydraulic valve system.

3. Apparatus according to claim 1 in which the hydraulic valve system includes a pilot valve and a pressure control valve.

4. Apparatus according to claim 3 in which the pilot valve is a motor driven, two position, four way spool valve.

5. Apparatus according to claim 1 wherein the hydraulic valve system is a three position, four way pilot valve.

6. Apparatus according to claim 2 wherein the transducer is a torque transducer.

7. Apparatus according to claim 2 wherein the motor is an electric motor.

8. Apparatus according to claim 1 in which the electrical controller comprises an electronics package which receives a differential pressure input between a drilling annulus and a bore hole from a differential pressure transducer to signal a motor to shift the hydraulic valve system.

9. Apparatus according to claim 1 in which the electrical controller comprises an electronics package which receives a load transferred through the anti-stall tool input or a WOB input from a load cell to signal a motor to shift the hydraulic valve system.

10. Apparatus according to claim 1 in which the electrical controller comprises an electronics package which receives a rotational deceleration input from a torsional accelerometer to signal a motor to shift the hydraulic valve system.

11. Apparatus according to claim 1 in which the electrical controller comprises an electronics package which receives a change in voltage input to signal an electric motor to shift the hydraulic valve system.

12. Apparatus according to claim 1 in which the electrical controller comprises an electronics package which receives a change in electric current input to signal an electric motor to shift the hydraulic valve system.

13. An anti-stall method for controlling drilling operations in a downhole assembly which includes a tubing that extends downhole, a drill bit carried on the tubing, a drive motor for rotating the drill bit, and a spring-operated anti-stall tool adjacent the motor, the method comprising electrically sensing pressure in the motor, providing a range of operating pressures for the motor defined by high and low limits of operating pressures, and operating the anti-stall tool in: (1) an active stage increasing WOB forces in the downhole direction by applying pressure to the anti-stall tool against the bias of a compression spring therein, when the low limit of operating pressure is sensed, (2) a reverse stage for providing a WOB force in the reverse direction via the compression spring bias, when the high limit of operating pressure is sensed, and (3) an optional passive stage in which the anti-stall tool is locked to transfer WOB directly from the tubing to the drill bit when the drive motor is operating within the limits of its normal operating pressure range.

14. The method according to claim 13 in which electrically sensing pressure is by an electronics package receiving an input from a transducer.

15. The method according to claim 13 in which the anti-stall tool is operated in the active stage, reverse stage and passive stage by a hydraulic valve system responsive to a signal from an electrical controller.

16. A downhole assembly adapted for anti-stall drilling operations, the downhole assembly including a drill bit, a drive motor for rotating the drill bit, a tubing for supplying drilling fluid to the drive motor, and an anti-stall tool positioned between the tubing and the drive motor for controlling the force applied to the drill bit during drilling operations, to thereby prevent the drill bit from stalling under load, the anti-stall tool comprising:

an outer housing,

an internal passageway extending through the housing for transmitting drilling fluid from the tubing to the drive motor for rotating the drill bit,

an open loop controller contained in the outer housing, the open loop controller comprising:

a piston assembly slidably disposed in the outer housing, said internal passageway extending through the piston assembly,

the piston assembly comprising one or more hydraulic cylinders each having a piston therein for applying axial forces in the either the downhole direction or the reverse direction to adjust weight-on-bit (WOB) while drilling,

a hydraulic control valve system contained in the outer housing with an inlet for receiving a supply of hydraulic control fluid from the drilling fluid in the internal passageway, for supplying the hydraulic control fluid to the piston assembly to control WOB, and

an electronics package contained in the outer housing for controlling the hydraulic control valve system,

the hydraulic control valve system including:

an adjustable first set point for indicating a desired lower limit for hydraulic working pressure of the drilling fluid in the tubing, and an adjustable second set point for indicating a desired upper limit for hydraulic working pressure of the drilling fluid in the tubing, the first and second set points representing a desired working pressure range for the drive motor,

an active stage valve assembly for sensing the working pressure in the tubing and supplying the hydraulic control fluid to one or more of the cylinders contained in the piston assembly to apply an axial force in the downhole direction to increase WOB when the sensed working pressure is below the first set point,

a passive stage valve assembly for sensing the working pressure in the tubing and shutting off the hydraulic control fluid supplied to the piston assembly to hydraulically lock the piston assembly in a passive state for maintaining WOB when the sensed working pressure is within the desired working pressure range of the drive motor, and

a reverse stage valve assembly for sensing the working pressure in the tubing and reversing the flow of the hydraulic control fluid supplied to one or more of the cylinders contained in the piston assembly to apply an axial force in the reverse direction to retract the drill bit to decrease WOB when the sensed working pressure reaches or exceeds the second set point.

17. The assembly according to claim 16 in which the electronics package which receives a pressure-related input from a transducer in the outer housing to signal a valve motor in the outer housing to shift the hydraulic control valve system.

18. The assembly according to claim 16 in which the active stage valve assembly, the passive stage valve assembly and the reverse stage valve assembly comprises a pilot valve and a pressure control valve.

19. The assembly according to claim 18 in which the pilot valve is a two position, four way spool valve.

20. The assembly according to claim 18 in which the hydraulic control valve system is a three position, four way pilot valve.

21. The assembly according to claim 17 wherein the transducer is a torque transducer.

22. The assembly according to claim 17 wherein the valve motor is an electric motor.

23. The assembly according to claim 16 wherein the electronics package receives a differential pressure input between a drilling annulus and a bore hole from a differential pressure transducer to signal a motor to shift the hydraulic valve system.

24. The assembly according to claim 16 wherein the electronics package receives a load transferred through the anti-stall tool input or a WOB input from a load cell to signal a motor to shift the hydraulic valve system.

25. The assembly according to claim 16 wherein the electronics package receives a rotational deceleration input from a torsional accelerometer to signal a motor to shift the hydraulic valve system.

26. The assembly according to claim 16 wherein the electronics package receives a change in voltage input to signal an electric motor to shift the hydraulic valve system.

27. The assembly according to claim 16 wherein the electronics package receives a change in electric current input to signal an electric motor to shift the hydraulic valve system.