METHOD FOR DEVELOPING AN OIL BEARING FORMATION

Abstract

Rows of horizontal production wells and rows of horizontal injection wells are drilled and alternating in a formation. The horizontal production wellbores and the horizontal injection wellbores are placed in the direction of a minimum horizontal stress in the formation. Within casing strings of the wells, at least two hydraulic frac ports are installed and multi-stage fracturing is performed through them in the production wells and in the injection wells in such way that fractures are formed along each production well and along each injection well in a direction perpendicular to the horizontal wellbore, the hydraulic fractures in the injection wells are offset from the hydraulic fractures in the production wells. The production and the injection wells are put into operation by injecting a fluid into the injection wells, a flow rate and/or a volume of the injected fluid are controlled in such a way that the injection pressure is below a fracturing pressure.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Russian Application No. 2016112172 filed Mar. 31, 2016, which is incorporated herein by reference in its entirety.

BACKGROUND

This invention is related to oilfield industry and can find use in secondary recovery methods such as water flooding (or other methods of reservoir pressure support), where the field development pattern design uses horizontal wells with multi stage fractures (HWMSF) that are being drilled and completed in low to mid permeability oil bearing reservoirs (k<100 md).

The completion design, including hydraulic fractures and the wellbore hardware, the position, azimuthal orientation and spacing of injector and producer wells are critical to the optimization of hydrocarbon production and the recovery of reserves.

There are various current production-injection vertical well pattern versions (5, 7, 9-spot) and modifications including vertical wells, vertical fracture wells and horizontal production wells with multi stage fractures (HWMSF).

The industry further evaluated a series of potential field development patterns for fields under water flooding in relation to the fracture propagation azimuth in order to achieve better hydrocarbon recovery.

Thus, in SPE 162031 (I. S. Afanasiev et al., “Analysis of multiple fracture horizontal well application of Priobskoe field” ROGEPT Conference and Exhibition, Oct. 16-18, 2012) a direct line drive pattern is described that has been designed with HWMSF placed along the preferred fracture plane (wells and hydraulic fractures are aligned to the maximum horizontal stress, σ_max). The HWMSF contains multiple longitudinal placed hydraulic fractures spaced at a distance along the length of the horizontal well section. Vertical water injection wells are drilled and hydraulically fractured, in a row located at a distance from the row of the production wells. The injection wells can be intentionally hydraulically fractured or are fractured unintentionally during water injection, when the water injection occurs at formation face pressures above the fracture gradient.

Additional vertical production wells are located within the injection row and hydraulically fractured, but later in the production life of the reservoir will be converted to injection wells.

The position of the fractures in the HWMSF and the injection wells are not positively controlled and there are no specific requirements on the exact spacing of the fractures. This is particularly valid for open hole horizontal well completion systems where the horizontal well section is not cemented. There are no specific requirements for the open hole packers spaced between the frac ports.

The lower the permeability of a formation under secondary recovery mechanism (such as for example—water flooding) the well of above described pattern type becomes less effective and initial production rates are lower compared to HWMSF with perpendicularly oriented fractures.

Also a pattern is known (N. A. Veremko “Optimization formation production in Western Siberia using HW MSF” Lukoil SPE Moscow Section Presentation, 7th February, 2012) consisting of HWMSF that contains a number of perpendicularly placed multiple hydraulic fractures spaced at a distance along the horizontal wellbore length. The pattern contains further vertical injection wells and any additional vertical production wells and vertical hydraulically fractured production wells in the injection row at a distance from the production well row.

This field development pattern is a regular approach of changing the existing field development pattern with vertical hydraulic fractured wells, to HWMSF pattern, where a row of vertical fractured injection wells is spaced in-between HWMSF. Between the injection wells vertical or vertical hydraulic fractured production wells may remain until later stage of field development when some or all of the production may be converted to injection wells.

The position of the fractures in the HWMSF and the injection wells are not positively controlled and there are no specific requirements on the exact spacing of the fractures. This is particularly valid for open hole horizontal well completion systems where the horizontal well section is not cemented. There are no specific requirements for the open hole packers spaced between the frac ports.

As a result of the above development pattern a high initial production rate is expected. The drawback of the pattern is that the reservoir pressure maintenance is poorly supported, and the reservoir pressure is declining rapidly, and such the later production rates. A water breakthrough from the injection wells to production wells is very likely, unless the injection rates are maintained under fracturing pressure.

Another known development pattern perusing the knowledge of mechanical rock properties and geomechanics is described in the RF Patent No 22515628 C1. The method is based on the knowledge of the state of stress in Western Siberia, where the horizontal in-situ stresses have very low difference in magnitude (σ_Hmax-σ_Hmin<3%), creating a low anisotropy environment under initial reservoir conditions. The method is using the condition of changing reservoir pressure under production and injection to target the timing of placing wells under injection to maximize the effect of hydrocarbon sweep with the water from water injection.

The drawback of the method is the subsurface complexity and the modeling of the same for realistic conditions, and the repeated conditions injection wells injecting water above the fracturing pressure of the formation.

For the few above and other analyzed patterns where HWMSF completed wells are used in the field/sector development, the difficulty is to optimize a total oil recovery when adjusting initial and late production rate.

It is an object of the present disclosure to provide a method and a pattern for developing oil bearing formations with the use of HWMSF completions in which the aforementioned disadvantages of existing patterns are avoided.

SUMMARY

The disclosure provides for maximum hydrocarbon recovery for a field or a field sector at initial and subsequent stages by providing best reservoir contact through vertical and lateral coverage of low permeability hydrocarbon bearing zones, it also provides for highest production/injection rates, in other words the highest productivity index (PI), by providing the lowest draw-down requirements (in production wells) and the lowest injection pressure (in injection wells). The method also provides the lowest risk of premature water breakthrough, an option to control a water injection rate and an option for potential re-fracturing of the initial completion in case such requirement arise in the process of the well exploitation.

According to the disclosed method rows of horizontal production wells and rows of horizontal injection wells are drilled in an oil-bearing formation, both horizontal production wellbores and horizontal injection wellbores are arranged in a direction of a minimum horizontal stress in the formation. The rows of horizontal production wells and the rows of horizontal injection wells are alternating and are placed at a first distance from each other.

At casing strings in the injection wells and in the production wells at least two frac ports are disposed, the frac ports provide fluid communication between the wells and the formation and are spaced at a second distance from each other.

Multi-stage hydraulic fracturing of the production wells and the injection wells is performed through the frac ports so that multiple fractures are placed along each horizontal production well and each injection well perpendicularly to the direction of the horizontal wellbores, the fractures are spaced at the second distance from each other and the hydraulic fractures of the injection wells are offset from the hydraulic fractures of the production wells by a third distance.

Then the production and the injection wells are put into operation by injecting a fluid into the injection wells and controlling a fluid injection rate and/or a fluid injection volume so that an injection pressure is maintained below a fracturing pressure.

According one embodiment the frac ports are capable of repeated opening and closing, and the fluid injection rate and/or the fluid injection volume are controlled by opening and closing the frac ports.

According to another embodiment of the disclosure the injection fractures are offset from the production fractures by a half of the distance between the fractures.

According to another embodiment of the invention in case of an open hole completion each frac port is placed between two frac packers so that a distance between the two frac packers is at least twenty times smaller than the distance between the frac ports.

The frac ports can be opened and closed with a coiled tubing, rigid wire or/and a wireline tractor.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure is explained by the figures where FIG. 1 shows a field development pattern in accordance with one embodiment of the disclosure; FIG. 2 shows an example of disposing frac ports in an open hole horizontal wellbore; FIG. 3 shows an example of disposing frac ports in a cemented cased hole horizontal wellbore; and FIG. 4 shows an example of production profiles and water cut for the HWMSF according to the disclosure and to the prior art methods.

DETAILED DESCRIPTION

Hydraulic fracturing is a primary tool for enhancing well productivity by creating highly permeable artificial fractures between a wellbore and a reservoir. Conventional methods of hydraulic fracturing in general case are divided into acid, in which a permeable fracture is created by chemical etching, or propped, in which permeability is maintained using proppants which can be artificial (ceramic, bauxite, plastics or other materials) or natural (quartz sand). The conductivity is achieved mainly by selecting proppants having desired concentration, size and qualitative characteristics or by creating almost infinite number of channels between propped fracture pillars. The disclosed method is applicable for all types of hydraulic fracturing.

The disclosure suggests a field development pattern that embodies HWMSF completions. HWMSF are horizontal wellbores containing multiple hydraulic fractures along a horizontal section of the wells. The arrangement of the fractures depend on the azimuth of a minimum horizontal stress and position of the horizontal wellbore in relation to the minimum horizontal stress. There is a multitude of the completion options ensuring creation of hydraulic fractures in the formation. A distinction is generally made between wells with an open hole wellbore and wells with a cemented horizontal wellbore.

According to the disclosed method, rows of horizontal production wells and rows of horizontal injection wells are drilled in an oil bearing formation, horizontal wellbores of the production wells and of the injection wells are arranged in a direction of a minimum horizontal stress in the formation so as to provide propagation of hydraulic fractures perpendicular to the direction of horizontal wellbores.

The rows of the injection wells and the rows of the production wells are alternating and are placed at a certain distance (hereinafter referred to as the first) from each other.

The horizontal wellbores of the injection and the production wells are placed in the direction of the minimal horizontal stress in the formation, i.e parallel to the minimum horizontal stress or at angle close to it, so as to provide propagation of hydraulic fractures perpendicular to the direction of the horizontal wellbores. This placement ensures hydraulic fractures in the production wells and the injection wells at the initial wellbore completion with HSMSF in a perpendicular orientation to the orientation of the horizontal wellbore, thus ensuring high initial production rates and a high hydrocarbon recovery factor. The angle between the direction of the minimum horizontal stress and the direction of the rows of horizontal wellbores depends on properties of rocks of the formation, formation pressure and thickness and is an acute angle not more than 20° to the direction of the minimal horizontal stress.

Then, at least two frac ports spaced at a distance (hereinafter referred to as a second distance) from each other are created in casing strings disposed in the injection wells and in the production wells. The frac ports provide fluid communication between the wells and the formation and are capable of repeated opening and closing. Multi-stage hydraulic fracturing of the production wells and the injection wells is made through the frac ports so that multiple fractures are created along each horizontal production well and each injection well perpendicularly to the direction of the horizontal wellbore.

A field development plan usually involves drilling wells in a special arrangement, so-called pattern, for which a location and a number of production and injection wells is selected in accordance with an enhanced oil recovery project. The pattern is generated based on the location of existing wells, reservoir size and shape, rock properties, reservoir fluids, cost of new wells and the recovery increase associated with the various possible injection and production wells in a pattern.

A distance between the hydraulic fractures and accordingly between the frac ports, and a number of fractures are selected based on the formation properties (permeability and porosity), the completion type (cemented or open hole) and a length of a well. Usually seven to eight hydraulic fractures are created, for which an appropriate number of frac ports are installed, but the number of fractures may reach 15-20 and even more.

The distance between the rows of the injector and the production wells is selected based on certain formation properties (permeability and porosity), and characteristics of the hydraulic fractures (fracture length and conductivity).

Common injection patterns are direct line drive, staggered line drive, two-spot, three-spot, four-spot, five-spot, seven-spot and nine-spot. The patterns are called normal or regular when they include only one production well per pattern. Patterns are described as inverted when they include only one injection well per pattern.

FIG. 1 represents an embodiment of the disclosure with a direct line drive pattern having two rows of production wells and one row of injection wells disposed between the two rows of the production wells.

Each production well 1 in the two rows of the production wells has several perpendicular fractures of multiple hydraulic fracturing (f₁, f₂. . . f_n), each fracture has a length 2. The fractures are located at a certain (second) distance 3 from each other along a length 4 of the horizontal wellbore.

Each injection well 5 in the row of the injection wells has several perpendicular fractures of multiple hydraulic fracturing (i₁, i₂. . . i_n), each fracture has a length 6. The length 6 of each fracture in the injection well is equal to the length 2 of each fracture in the production well. The fractures are spaced at a distance 7 along a length 8 of the horizontal wellbore, the distance 7 is equal to the distance 3 between the fractures of the production wells. The row of the injection wells 5 is located at a distance 9 with respect to each row of the production wells 1.

The fractures in the injection wells 5 are offset from the fractures in the production wells 1 by a distance 10 ensuring that the injection and production fractures do not overlap.

This is achieved by:

cementing the wells and perforating/jet blasting openings through the casing and cement into the formation at the exact predetermined spot in the cemented horizontal wells or,

placing frac ports in an open hole completion system at the casing string within the wellbore at the exact predetermined spot. The frac ports are isolated from the remaining horizontal wellbore by open hole packers spaced closely to the frac port.

Thus accurate placement of initiated and created fractures in the horizontal wellbore is provided, and therefore a direct connection of the fractures of the injection and the production wells is excluded, which minimizes the risk of early water breakthrough into the production well.

The length 2 of the hydraulic fractures, the distance 7 between the fractures and the distance 9 between the rows of the wells are typically optimized based on a horizontal permeability. The hydraulic fractures in the horizontal section of the injection wells are offset from the hydraulic fractures in the horizontal section of the production wells by approximately half of the distance between the fractures in the wellbore. In the currently used development patterns this is not controlled and therefore the fractures from the injection wells may be at a small distance from the fractures from the production wells and hence may connect with them, resulting in early water breakthrough into the production well.

Frac ports disposed in a wellbore are being used to create hydraulic fractures. The frac ports used in accordance with one embodiment of the disclosure allow for multiple closing and opening as needed. Opening and closing of the frac ports are performed by special tools delivered by a flexible pipe of a coiled tubing or by a wireline tractor. Frac ports from various manufacturers are described in literature (patent application USA No 20140332228; patent application USA No 20110204273; IPTC-18104 <<Case Study: A challenging Large-scale fracturing in Sichuan basin>>, Yuan F. et al., December 2014 r.; SPE163935 <<Reducing Water Volume in Multistage Fracturing Using sliding Sleeves and CT deployed resettable frac Isolation>>, Schlosser D. et al., March 2013 r.; <<Hydraulic Fracturing innovations target strategic fracture placement, re-fracturing of existing wells for next bump in recovery>>, Katie Mazerov, Drilling Contractor, Jan. 27, 2015, prospect Schlumberger <<Reclosable frac Sleeve>> http://www.slb.com/˜/media/Files/stimulation/product_sheets/broadband/broadband_reclosable_fracturing_sleeve_ps.pdf, 2014).

In other cases the frac ports can be closed by cementing, installation of the plates on the casings or other known methods.

In the case of open hole completions (FIG. 2) a frac port 11 is placed on a casing 13 between two external packers 12 to ensure initiation of a hydraulic fracture at a certain point of the horizontal wellbore and to control position of the fracture. The ports in the injection and in the production wells are disposed in such way that the fractures created in the injection wells are offset from the fractures of the production wells by a third distance.

A distance 14 between the packers located near the frac port 11, is relatively small (approximately 2-5 m) compared to a distance 16 between the frac ports, and accordingly to a distance 15 to the next open hole packer at the next frac port (approximately more then 100 m). The recommended distance between the two packers is approximately twenty times less than the distance between the neighboring frac ports. The distance 16 between the frac ports 11 defines the distance between the hydraulic fractures 17 (f1 . . . fn) in the horizontal wellbore. This also allows to select re-fracture position during the well operation. The same is true for the injection wells and their frac ports.

The frac ports are of the type that can be opened and closed multiple times.

The closing and opening operations of the frac ports 11 can be fulfilled with the use of a flexible tube of a coiled tubing and (or) a wireline and (or) a rigid wire or a wireline tractor. On FIG. 2 it is shown that the frac port 11 is open, through this port a hydraulic fracture in the formation was created and water injection 18 takes place. The other frac port 19 is closed after the fracture was created, and water injection is not taking place. In case of extreme water influx the frac port 19 can be closed not only on the injection well, but also the similar opposite frac port in the production well can be closed. In the event of increased water cut in the production well, the water flow can be reduced by closing the responsible frac port on the injector wellbore or the watered out frac port in the production well. This ensures controlled water injection 18 and active formation pressure control, which reduces the risk of increased water cut in the production of the well.

The wellhead injection pressure is determined by calculating the hydrostatic pressure of the injection water column, corrected for the pressure loss from the hydraulic friction pressure caused by the flow from pumping water through the wellbore and through the perforations/ports; the resulting wellhead pressure should be below the breakdown pressure at the bottom of the well. (Rose, S. C., Buckwalter, J. F., and Woodhall, R. J. 1989. The Design Engineering Aspects of Waterflooding, Vol. 11. Richardson, Tex.: monograph series, SPE; Perkins, T. K. and Gonzalez, J. A. 1985. The Effect of Thermoelastic Stresses on Injection Well Fracturing. SPE J. 25 (1): 78-88. SPE-11332-PA). The injection rate at each port or perforation hole can be controlled by logging tools, for example using spinners and other flowmeters, run on a coiled tubing or wireline tractor, or using distributed thermometry obtained from a fiber optic cable. Based on the information obtained, using coiled tubing or wireline tractor, one can selectively close one or several ports through which excessive water was pumped during the injection period, while at the same time redistributing the injection through other ports where the flow rate was lower. As a result, the water front distribution between the injection well and the production well is more uniform.

The pattern shown on FIG. 2 relates to the completion in an open hole HWMSF wells, but is equally valid for cemented horizontal wellbores 21 in HWMSF completed wells (FIG. 3) where position of the frac ports 11 connecting the wellbore with the formation, and the distance 16 between the ports and respectively between the hydraulic fractures 17 should be selected taking into account not only their position within the horizontal wellbore itself, but also the position relative to the next row of the HWMSF.

The pattern created according to the disclosed method allows to optimize the production profile from HWMSF completed wells. On FIG. 4 it is represented with the curve A. The described pattern, where the HWMSF are used as injection and production wells ensures high initial production rates and high rates at later stages. Current patterns can provide either high initial production rates or high later rates, but do not ensure both high initial and later production rates.

The curve B on FIG. 4 characterizes a production profile of a well with a direct line drive pattern as described in SPE 162031 where drilling of horizontal wells is produced on the basis of the longitudinal orientation of fractures along the horizontal wellbore. This pattern has a lower production rates at the initial stage because of the reduced drainage area, but provides a sufficiently high and stable subsequent debit. Due to more uniform formation pressure maintenance, the pattern also provides a smaller content of the produced water in subsequent stages of operation of the well, as shown by curve D.

The curve C describes the production profile of a well from a pattern as described by G. A. Veremko where horizontal wellbores of the production wells are drilled along the maximum stress and the fractures are perpendicular to the horizontal wellbore. The injection wells are vertical wells placed between the horizontal wells. This pattern will provide higher production rates at the initial because of a larger drainage area. The deficiency of the pattern is an insufficient pressure support from the injector wells which leads to a fast production decline. In addition, the injection leads to an uncontrolled fracture extension into the drainage area of the production wells and to the fast growth of the content of formation water in the production from a certain moment of operation of the wells, as demonstrated by the curve E.

Both patterns do not provide for the use of horizontal wells with multi stage fracturing as injector wells, and the injector wells are typically vertical, slanted or S-shaped wells with or without hydraulic fractures completions. The injection is typically done at a pressure above the fracturing pressure of the formation rock using water/fluids which temperature may be lower than the static formation temperature, which causes a hydraulic and/or temperature fracturing. These fracturing is uncontrolled and not always intentional. It is called an automatic frac (in English literature—“auto-frac”).

The curve A is characterized by the highest initial production rates (compared to curve B and C) which results from the large drainage area and best contact with the formation through vertical and lateral coverage of the low permeability hydrocarbon bearing zones by using HWMSF. The disclosed pattern using HWMSF both for production and injection wells provides the highest productivity along with an optimal pressure maintenance at the maximum replacement ratio of the void volume. As a result, the initial production rate is higher compared to curve C, and the reduction in the production rate is slower compared to the curve B.

Further, the pattern consists of HWMSF production and injection wells. The injection wells are put into operation by injecting water (or brine, or other liquid for pressure support) with control of a fluid injection rate and/or a fluid injection volume to maintain an injection pressure below a fracturing pressure of the formation, the volumes injected are controlled by opening and closing the frac ports.

Within the secondary recovery method, the HWMSF injection wells are drilled and completed at the same time as the production HWMSF, in order to maintain the reservoir pressure after the start of production. For adequate volume replacement of produced oil under matrix flow conditions a suitable secondary recovery method of development is required. Regulation of the injection rate allows to avoid exceeding the fracture gradient and consequently prevents non-controlled fracture extension from the injection wells towards the production wells. This prevents rapid water breakthrough from the injection well into the production well, which is characterized by the curve F.

Claims

1. A method for developing an oil bearing formation, the method comprising:

drilling within the formation alternating rows of horizontal production wells and rows of horizontal injection wells, the rows being placed at a first distance from each other, both horizontal production wellbores and horizontal injection wellbores are arranged in a direction of a minimum horizontal stress in the formation so as to provide hydraulic fractures perpendicular to the direction of the horizontal wellbores,

providing at least two frac ports at casing strings in the injection wells and in the production wells, the frac ports being spaced at a second distance from each other and providing fluid communication between the wells and the formation,

performing multi-stage hydraulic fracturing through the frac ports in the production and the injection wells so that multiple fractures are placed along each horizontal production well and each injection well perpendicularly to the direction of the horizontal wellbore, so that the hydraulic fractures of the injection wells are offset from the hydraulic fractures of the production wells by a third distance,

putting the production and the injection wells into operation by injecting a fluid into the injector wells and controlling a fluid injection rate and/or a fluid injection volume so that an injection pressure is maintained below a fracturing pressure.

2. The method of claim 1, wherein the frac ports are capable of repeated opening and closing, and the fluid injection rate and/or the fluid injection volume are controlled by opening and closing the frac ports.

3. The method of claim 1, wherein the hydraulic fractures of the injection wells are offset from the hydraulic fractures of the production wells by a half of the distance between the hydraulic fractures.

4. The method of claim 1, wherein in case of an open hole completed well each frac port is located between two frac packers is such way that a distance between the frac packers is at least twenty times less that the distance between the two neighboring frac ports.

5. The method of claim 2, wherein the frac ports are opened and closed with a coiled tubing, rigid wire or/and a wireline tractor.