Method for exploiting coal bed gas by using under-pressure coal reservoir horizontal well group

Abstract

The invention discloses a method for exploiting coal bed gas by using an underpressure coal reservoir horizontal well group, which comprises the following steps that the horizontal well group is arranged, the horizontal well group comprises a preset number of horizontal wells, each horizontal well comprises a horizontal well section, and each horizontal well section is positioned in a coal reservoir of the same series of strata; each horizontal well section is divided into a plurality of fracturing sections with preset lengths, and perforation positions are selected in each fracturing section for perforation to form fracturing points; fracturing construction is carried out on each fracturing section, the fracturing construction comprises a liquid injection and energy charging fracturing stage and a main body fracturing stage, in the liquid injection and energy charging fracturing stage, a first part of liquid is injected into the coal reservoir through fracturing points at preset displacement and preset construction pressure so as to fill natural fractures and cleats of the coal reservoir, and in the main body fracturing stage, a second part of liquid is injected into the coal reservoir through the fracturing points at preset displacement and preset construction pressure so as to fill the natural fractures and cleats of the coal reservoir. Injecting a second part of liquid into the coal reservoir through the fracturing point to form an artificial fracture in the coal reservoir, wherein the preset construction pressure does not exceed the formation fracturing pressure of the area where the horizontal well group is located; and after fracturing of all the fracturing sections is completed, coal bed gas exploitation is conducted.

Description

Technical Field

[0001] The invention relates to the technical field of coalbed methane exploitation, and in particular to a method for exploiting coalbed methane using a horizontal well group in an underpressured coal reservoir. Background Art

[0002] The proportion of under-pressure coal reservoirs currently used to exploit coalbed methane is increasing. Under-pressure coal reservoirs refer to: for example, a coalbed methane block in a certain place is buried at a depth of 1000-1200m, and the overall coal reservoir pressure range of the gas block is 0.7~0.9MPa / 100m (coal reservoir pressure every 100 meters from the ground). The overall coal reservoir pressure range of the gas block is lower than the normal reservoir pressure value (1MPa / 100m).

[0003] At present, the main way to develop coal reservoirs is to use conventional oil and gas fracturing to increase production. However, this method is not targeted enough for coal reservoirs with characteristics such as underpressure, undersaturation, multiple fracture systems and easy compression. That is, this method does not fully consider the impact or damage to coal reservoirs with characteristics such as underpressure, undersaturation, multiple fracture systems and easy compression. For example, it is very easy to cause damage to the natural fractures and cleats of the coal reservoir while improving the permeability. That is, the current method fails to fully protect and utilize the natural fractures and cleats of the coal reservoir. In addition, the current development of coal reservoirs is still mainly based on single-well fracturing design and construction, lacking the design of the overall transformation of the well group, and the overall utilization rate is low, which cannot fully meet the overall fracturing construction of the horizontal well group when currently exploiting coalbed methane. Summary of the invention

[0004] To solve the above technical problems, the present invention proposes a method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir, which can solve the technical problem that existing conventional fracturing methods fail to fully protect and utilize the natural fractures and cleat systems of the coal reservoir.

[0005] The embodiment of the present invention discloses a method for exploiting coalbed methane from a horizontal well group in an underpressure coal reservoir, comprising the following steps:

[0006] Arranging a horizontal well group, wherein the horizontal well group includes a predetermined number of horizontal wells, each of the horizontal wells includes a horizontal well section, and each of the horizontal well sections is located in a coal reservoir of the same formation;

[0007] Dividing each of the horizontal well sections into a plurality of fracturing sections, selecting a perforation position in each of the fracturing sections for perforation to form a fracturing point;

[0008] Performing fracturing construction on each of the fracturing sections, the fracturing construction includes a liquid injection and energy fracturing stage and a main fracturing stage, in which a first portion of liquid is injected into the coal reservoir through the fracturing point at a predetermined displacement and a predetermined construction pressure to fill natural fractures and cleats in the coal reservoir, and in the main fracturing stage, a second portion of liquid is injected into the coal reservoir through the fracturing point to form artificial fractures in the coal reservoir;

[0009] After all the fracturing stages are completed, coalbed methane extraction is carried out.

[0010] According to one embodiment of the present invention, each of the horizontal well sections is arranged in parallel, and the direction of the horizontal well section is perpendicular to the direction of the maximum principal stress in the horizontal plane of the coal reservoir or is oblique to the direction of the maximum principal stress at an angle of 60° to 90°.

[0011] According to one embodiment of the present invention, the order of fracturing construction for each of the horizontal well sections is: first, simultaneously fracturing each of the fracturing sections in the two horizontal well sections located on both sides of the horizontal well group, and then sequentially fracturing each of the fracturing sections in the horizontal well section located in the middle of the horizontal well group, until the fracturing construction of all the horizontal well sections is completed.

[0012] According to one embodiment of the present invention, when performing fracturing construction on each of the fracturing stages, the pump is stopped to expand the pressure after the fluid injection and energy charging fracturing stage, and then the main fracturing stage is continued.

[0013] According to one embodiment of the present invention, the step of selecting perforation positions for perforation to form fracturing points includes: selecting native structural coal seams in each of the fracturing sections for perforation to form fracturing points, and the fracturing points in two adjacent horizontal well sections are arranged in an alternating manner.

[0014] According to one embodiment of the present invention, the gas measurement value of the native structural coal seam is greater than 50% and the gamma value is less than 35 API.

[0015] According to one embodiment of the present invention, the perforating method is to perform directional perforating obliquely upward and / or obliquely downward at a predetermined offset angle relative to the horizontal plane of the coal reservoir.

[0016] According to an embodiment of the present invention, the predetermined misalignment angle is 0-15°.

[0017] According to one embodiment of the present invention, the predetermined construction pressure does not exceed the formation fracture pressure of the area where the horizontal well group is located, and the formation fracture pressure of the area where the horizontal well group is located is obtained by injection and / or pressure drop test combined with adjacent well fracturing curves.

[0018] According to one embodiment of the present invention, the control range of the predetermined displacement is 0.5 to 4 m 3 / min.

[0019] According to one embodiment of the present invention, both the first portion of liquid and the second portion of liquid are coal-phobic liquids.

[0020] According to an embodiment of the present invention, the first portion of liquid and the second portion of liquid are a mixture of KCl and water, wherein the mass fraction of KCl in the mixture is 0.3% to 1%.

[0021] According to one embodiment of the present invention, during the fluid injection and energizing fracturing stage, a quartz sand proppant of 70-100 mesh is added to the first portion of liquid during the injection of the first portion of liquid.

[0022] According to one embodiment of the present invention, in the main fracturing stage, during the injection of the second portion of liquid, quartz sand proppant of 40-70 mesh and quartz sand proppant of 20-40 mesh are sequentially added to the second portion of liquid.

[0023] According to an embodiment of the present invention, after all the fracturing stages have completed fracturing and before coalbed methane extraction, the process further includes: detecting the wellhead pressure of the horizontal well group, and performing pressure-controlled blowout if the wellhead pressure is lower than the fracture closure pressure.

[0024] According to one embodiment of the present invention, if the wellhead pressure is 20-10MPa, a 6mm oil nozzle is used for blowout; if the wellhead pressure is 10MPa-5MPa, a 10mm oil nozzle is used for blowout; if the wellhead pressure is less than 5MPa, the wellhead is opened for blowout.

[0025] According to one embodiment of the present invention, it also includes: when arranging a horizontal well group, obtaining regional geological data to establish a geological model of the horizontal well group, wherein the geological model includes data of the regional geological data; setting the well spacing between two adjacent horizontal well sections, setting the length of the horizontal well section, and selecting the perforation position of the fracturing section according to the data in the geological model.

[0026] According to one embodiment of the present invention, the regional geological data include coal reservoir pressure, average porosity of coal rock, maximum principal stress and direction in the horizontal plane of the coal reservoir, fault and fracture development characteristics, coal body permeability, roof and floor lithology, regional coal body structure distribution, and average comprehensive compression coefficient of coal rock.

[0027] According to one embodiment of the present invention, it also includes: calculating the geological reserve volume used within the horizontal well group according to the data in the geological model, and optimizing the segment length of the fracturing section and the fracturing construction parameters of each fracturing section according to the geological reserve volume and the well spacing by using the regional fracture monitoring data and FracproPT or Meyer software simulation method, wherein the fracturing construction parameters include the maximum injection displacement P of the main fracturing, and the total liquid volume Q injected for the fracturing construction of each fracturing section and the total amount N of proppant added; wherein the total liquid volume Q includes the liquid volume Q injected in the injection and charging fracturing stage 1 and the amount of liquid injected in the main fracturing stage Q 2 The total amount N of the proppant includes the amount N of proppant added in the fluid injection and energy fracturing stage. 1 and the amount of proppant added in the main fracturing stage N 2 .

[0028] According to one embodiment of the present invention, the calculation formula of the geological reserve volume is V 总 =LWh, where L is the length of the horizontal well section, m; W is the overall controlled length of the artificial fractures formed by the horizontal well group, m, and the overall controlled length of the artificial fractures formed by the horizontal well group is the total length of the area occupied by the artificial fractures formed after fracturing in all the fracturing sections; h is the average effective thickness of the coal reservoir, m.

[0029] According to one embodiment of the present invention, the amount of liquid Q in the liquid injection and energy fracturing stage is 1 The calculation formula is Q 1 =V 1 ×φ×C f ×ΔP, where V 1 is the controlled reservoir volume of the natural fractures on both sides of each fracturing stage, m 3 , the controlled reservoir volume is the product of the length of the area expected to be occupied by the natural fractures on both sides of each fracturing stage, the width of the area expected to be occupied by the natural fractures on both sides of each fracturing stage and the average effective thickness of the coal reservoir; φ is the average porosity of the coal rock, %; C f is the average comprehensive compression coefficient of coal rock, MPa -1 ; ΔP is the increase in coal reservoir pressure after the fluid injection and energy fracturing, MPa.

[0030] According to one embodiment of the present invention, the control range of the injection displacement of the main body fracturing is 4 to 20 m 3 / min.

[0031] According to one embodiment of the present invention, the main body fracturing injection rate is set to be 4m 3 / min and then gradually increase the displacement to the maximum injection displacement.

[0032] By adopting the above technical solution, the present invention has at least the following beneficial effects:

[0033] The method for exploiting coalbed methane in a horizontal well group of an underpressured coal reservoir provided by the present invention effectively changes the initial underpressure state of the coal reservoir of the coalbed methane well group by first injecting a first portion of liquid into the coal reservoir to fill the natural fractures and cleats of the coal reservoir, which can replenish the formation energy within the control range of the horizontal well group, greatly improve the overall compressive strength of the coal reservoir in the horizontal well group, dredge the natural fractures and cleats of the coal reservoir and keep the natural fractures and cleats of the coal reservoir open, fully utilize the natural fracture and cleat system and effectively avoid the damage to the natural fractures and cleats of the coal reservoir by conventional fracturing, avoid the compression and closure of the natural fractures and cleats around the artificial fractures caused by conventional large-scale direct fracturing, and help to reduce the space where the injected liquid may be lost in the subsequent main fracturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0035] Figure 1 A schematic flow chart of a method for exploiting coalbed methane from a horizontal well group in an underpressure coal reservoir disclosed in one embodiment of the present invention;

[0036] Figure 2 A schematic diagram of the development of a horizontal well group disclosed in one embodiment of the present invention. DETAILED DESCRIPTION

[0037] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in combination with specific embodiments and with reference to the accompanying drawings.

[0038] It should be noted that all expressions using "first" and "second" in the embodiments of the present invention are for distinguishing two non-identical entities with the same name or non-identical parameters. It can be seen that "first" and "second" are only for the convenience of expression and should not be understood as limitations on the embodiments of the present invention. The subsequent embodiments will not explain this one by one.

[0039] like Figure 1 As shown, an embodiment of the present invention discloses a method for exploiting coalbed methane from a horizontal well group in an underpressure coal reservoir, comprising the following steps:

[0040] S10: Arranging a horizontal well group, the horizontal well group includes a predetermined number of horizontal wells, each horizontal well includes a horizontal well section, and each horizontal well section is located in a coal reservoir of the same formation;

[0041] S20: Divide each horizontal well section into a plurality of fracturing sections of predetermined lengths, and select a perforation position in each fracturing section to perform perforation to form a fracturing point;

[0042] S30: performing fracturing construction on each fracturing stage, the fracturing construction includes a liquid injection and energy fracturing stage and a main fracturing stage. In the liquid injection and energy fracturing stage, a first portion of liquid is injected into the coal reservoir through the fracturing point at a predetermined displacement and a predetermined construction pressure to fill natural fractures and cleats in the coal reservoir. In the main fracturing stage, a second portion of liquid is injected into the coal reservoir through the fracturing point to form artificial fractures in the coal reservoir.

[0043] S40: After all the fracturing stages are completed, coalbed methane extraction is carried out.

[0044] In the above embodiment, by first injecting the first part of liquid into the coal reservoir to fill the natural fractures and cleats of the coal reservoir, the initial underpressure state of the coal reservoir of the coalbed methane well group is effectively changed, which can supplement the formation energy within the control range of the horizontal well group, greatly improving the overall compressive strength of the coal reservoir in the horizontal well group, and making full use of the natural fracture and cleat system to effectively avoid the damage of conventional fracturing to the natural fractures and cleats of the coal reservoir, and reduce the compression and closure of the natural fractures and cleats around the artificial fractures caused by conventional large-scale direct fracturing, which is beneficial to reduce the space where the injected liquid may be lost in the subsequent main fracturing.

[0045] In some embodiments, each horizontal well section is arranged in parallel, and the direction of the horizontal well section is perpendicular to the direction of the maximum principal stress in the horizontal plane of the coal reservoir or is oblique to the direction of the maximum principal stress at an angle of 60° to 90°.

[0046] Figure 2 The development schematic diagram of a horizontal well group according to an embodiment of the present invention is shown. The horizontal well group includes three horizontal wells (horizontal well 1, horizontal well 2 and horizontal well 3), each of which includes a vertical well section ( Figure 2 and a horizontal well section connected to the vertical well section. Figure 2 The horizontal well sections of three horizontal wells are shown, and the three horizontal well sections are located in the same horizontal plane of the coal reservoir. The horizontal well section of each horizontal well in the horizontal well group is divided into a plurality of predetermined lengths of fracturing sections 4. a represents the distance between the horizontal well section of horizontal well 1 and the horizontal well section of horizontal well 2, and L represents the length of the horizontal well section of the horizontal well.

[0047] In some embodiments, the order of fracturing each fracturing segment is: first, simultaneously fracturing each fracturing segment in the two horizontal well segments on both sides of the horizontal well group, and then fracturing each fracturing segment in the horizontal well segment in the middle of the horizontal well group in sequence, until the fracturing of all horizontal well segments is completed. Figure 2 Taking the development schematic diagram of the horizontal well group shown as an example, each fracturing section in the two outermost horizontal wells 1 and horizontal well 3 of the horizontal well group is fractured simultaneously first, and then each fracturing section in the middle horizontal well 2 is fractured. In another embodiment, if the horizontal well group includes 4 horizontal wells, each fracturing section in the two outermost horizontal wells is fractured first, and then each fracturing section in the two middle horizontal wells is fractured. In another embodiment, if the horizontal well group includes 5 horizontal wells, each fracturing section in the two outermost horizontal wells is fractured first, and then each fracturing section in the two horizontal wells adjacent to the two outermost horizontal wells is fractured, and finally each fracturing section in the middle horizontal well is fractured. If the horizontal well group includes more than 6 horizontal wells, the fracturing sequence is similar to the above embodiment and will not be repeated here.

[0048] In the above embodiment, the fracturing sections in the outermost horizontal wells of the horizontal well group are firstly fractured, and after the fracturing of the outer horizontal wells, a high-pressure formation boundary can be constructed, so that the ground stress in the horizontal plane of the coal reservoir in the middle area is diverted through the fracturing of the outer horizontal wells, and the difference between the maximum principal stress and the minimum principal stress in the horizontal plane of the coal reservoir in the horizontal well group is reduced; and then the horizontal wells in the middle of the horizontal well group are fractured in sequence, which is conducive to promoting the formation of an overall complex fracture network in the coal reservoir, preventing the formation of blank zones in certain areas of the coal reservoir due to the lack of fractures, realizing the design of the overall transformation of the well group, and greatly improving the overall utilization rate of the coal reservoir.

[0049] In some embodiments, the order of fracturing construction of the fracturing stages in each horizontal well section is: fracturing each fracturing stage in turn from one end far from the wellhead of the horizontal well to the other end close to the wellhead of the horizontal well.

[0050] In some embodiments, when performing fracturing construction on each of the fracturing sections, the pump is stopped and the pressure is expanded after the injection and energy fracturing stage, and then the main fracturing stage is continued. Stopping the pump and expanding the pressure means: pausing for a period of time after the injection and energy fracturing to allow the natural fractures and cleats to close properly, that is, during the process of stopping the pump and expanding the pressure, the diameter of the natural fractures during the injection and energy is appropriately reduced, so as to facilitate the subsequent main fracturing to produce artificial main fractures. In some embodiments, after injecting a displacement fluid with a volume of 1 to 1.5 times the total volume of a horizontal well in the late stage of the injection and energy fracturing, the pump is stopped and the pressure is expanded, and the pump is stopped and the pressure is expanded until the wellhead pressure of the horizontal well is less than 1MPa.

[0051] In some embodiments, the step of selecting a perforation position to perform perforation to form a fracturing point includes: selecting a primary structural coal seam in each fracturing section to perform perforation to form a fracturing point, and the fracturing points in two adjacent horizontal well sections are arranged in an interlaced manner. In this embodiment, compared with tectonic coal, primary structural coal seams are more likely to form cracks during fracturing, which is conducive to forming a complex fracture network in the coal reservoir.

[0052] Figure 2 The development schematic diagram of the horizontal well group in one embodiment shown shows that "the fracturing sections in the horizontal well sections of adjacent horizontal wells are arranged in an interlaced manner", so that the fracturing points in the horizontal well sections of adjacent horizontal wells can be arranged in an interlaced manner. The fracturing points in the horizontal well section of horizontal well 1 and the fracturing points in the horizontal well section of horizontal well 2 are arranged in an offset manner, so that the artificial fractures formed by injecting liquid into the coal reservoir through the fracturing points in the horizontal well section of horizontal well 1 and the artificial fractures formed by injecting liquid into the coal reservoir through the fracturing points in the horizontal well section of horizontal well 2 can be interlaced with each other, so that the artificial fractures formed by injecting liquid into the coal reservoir through the fracturing points in the horizontal well section can extend to the vicinity of the adjacent horizontal well section, thereby improving the permeability of the coal reservoir near the adjacent horizontal well section. At the same time, the interlaced arrangement of the fracturing points between two adjacent wells is conducive to the mutual disturbance of fractures in synchronous or zipper fracturing to form a complex fracture network, thereby improving the fracture formation effect of the well group.

[0053] In some embodiments, the gas measurement value of the original structure coal seam is greater than 50% and the gamma value is less than 35 API. The original structure coal seam that meets the above conditions is selected to easily form cracks during the fracturing process, which is conducive to the extension of the fracturing cracks, thereby facilitating the formation of a complex fracture network in the coal reservoir.

[0054] In some embodiments, the perforation is performed in a directional manner by performing upward and / or downward perforation at a predetermined offset angle relative to the horizontal plane of the coal reservoir, which is conducive to the extension of the formed artificial fracture network in the cleat fissures of the coal reservoir and reduces the negative impact of tectonic coal on the extension of the artificial fracture network.

[0055] In some embodiments, the predetermined misalignment angle is 0-15°, which is beneficial for the artificial fracture network to extend in the cleat fissures of the coal reservoir.

[0056] In some embodiments, the predetermined construction pressure does not exceed the formation fracture pressure of the area where the horizontal well group is located, and the formation fracture pressure of the area where the horizontal well group is located is obtained by injection / pressure drop test combined with the fracturing curve of the adjacent well. The construction pressure of the liquid injection and energy fracturing stage does not exceed the formation fracture pressure of the area where the horizontal well group is located, thereby avoiding the generation of artificial fractures in the coal reservoir at this stage.

[0057] In some embodiments, the control range of the predetermined displacement of the first portion of liquid is 0.5 to 4 m 3 / min. That is, in the stage of liquid injection and charging, injecting the first part of liquid at a low displacement can reduce the damage to the micropores of the coal reservoir during fracturing and improve the connectivity of the natural fractures and cleats in the coal reservoir.

[0058] In some embodiments, both the first portion of liquid and the second portion of liquid are coal-repellent liquids, which can reduce damage to micropores in the coal reservoir.

[0059] In some embodiments, the first portion of liquid and the second portion of liquid are a mixture of KCl and water, wherein the mass fraction of KCl in the mixture is 0.3% to 1%, preferably 0.5%.

[0060] In some embodiments, during the liquid injection and energy fracturing stage, a quartz sand proppant of 70-100 mesh is added to the first part of the liquid during the injection of the first part of the liquid, so as to facilitate the support of the connected natural fractures and cleats. For example, in some embodiments, during the liquid injection and energy fracturing stage, after half of the required liquid volume is injected, an appropriate amount of quartz sand of 70 / 100 mesh is added at a low sand ratio of about 5% by volume.

[0061] In some embodiments, during the liquid injection and energy fracturing stage, quartz sand proppants of 40-70 mesh (fine-grained quartz sand proppants) and quartz sand proppants of 20-40 mesh (medium-grained quartz sand proppants) are sequentially added to the second portion of liquid during the injection of the second portion of liquid. That is, the artificial fractures are better filled by injecting a combination of fine-grained and medium-grained quartz sand proppants.

[0062] In some embodiments, in some embodiments, the method for exploiting coalbed methane in a horizontal well group of an underpressure coal reservoir further includes detecting the wellhead pressure of the horizontal well group after all the fracturing stages are completed and before the coalbed methane is exploited, and if the wellhead pressure is lower than the fracture closure pressure, pressure-controlled blowout is performed. After the blowout is completed, the well group is pumped down and put into production. Pressure-controlled blowout is beneficial to reducing the return of coal powder and quartz sand added during fracturing.

[0063] In some embodiments, if the wellhead pressure is 20-10MPa, a 6mm nozzle is used for spraying; if the wellhead pressure is 10MPa-5MPa, a 10mm nozzle is used for spraying; if the wellhead pressure is less than 5MPa, the wellhead is opened for spraying. This greatly reduces the spitting back of coal powder and quartz sand added during fracturing.

[0064] In some embodiments, the method for exploiting coalbed methane in a horizontal well group of an underpressured coal reservoir further includes: when arranging the horizontal well group, obtaining regional geological data to establish a geological model of the horizontal well group, wherein the geological model includes data of the regional geological data; and setting the well spacing between two adjacent horizontal well sections, setting the length of the horizontal well section, and selecting the perforation position of the fracturing section according to the data in the geological model. Figure 2Taking the development schematic diagram of the horizontal well group as an example, the distance a between the horizontal well section of horizontal well 1 and the horizontal well section of horizontal well 2 is the well spacing between the horizontal well sections, and L is the length of the horizontal well section of the horizontal well. In this embodiment, the geological model can directly visualize the data of the acquired regional geological data so as to be used to set the required data, and at the same time facilitate FracproPT or Meyer software simulation to optimize the data using the data in the geological model.

[0065] In some embodiments, regional geological data include coal reservoir pressure, average porosity of coal rock, maximum principal stress and direction in the horizontal plane of the coal reservoir, fault and fracture development characteristics, coal body permeability, roof and floor lithology, regional coal body structure distribution, and average comprehensive compression coefficient of coal rock.

[0066] In some embodiments, well spacing is optimized based on regional fracture detection data and FracproPT or Meyer software simulation methods based on data in the geological model.

[0067] In the embodiments of the present invention, the regional crack detection data and the FracproPT or Meyer software simulation method are well known to those skilled in the art, and therefore are not described in detail in the present invention.

[0068] In some embodiments, the well spacing between the horizontal well sections of two adjacent horizontal wells ranges from 200 to 300 meters, and the well spacing is preferably 250 meters, thereby achieving that the two adjacent horizontal well sections in the coal reservoir can exploit the coalbed methane between them with higher efficiency. The length of the horizontal well section of the horizontal well is 1000 meters. While meeting the drilling process requirements, the length of the horizontal well section is increased, thereby increasing the volume of coalbed methane that can be exploited and improving the efficiency of coalbed methane exploitation. The segment spacing between two adjacent fracturing sections in each horizontal well is 60 to 80 meters, which can maximize the permeability of coalbed methane and thereby maximize the efficiency of coalbed methane exploitation.

[0069] In an embodiment of the present invention, a method for exploiting coalbed methane by a horizontal well group in an underpressured coal reservoir is characterized in that it also includes: calculating the geological reserve volume mobilized within the horizontal well group according to the data in the geological model; optimizing the segment length of the fracturing section (the length of each fracturing section) and the fracturing construction parameters of each fracturing section according to the geological reserve volume and the well spacing by using the regional fracture monitoring data and the FracproPT or Meyer software simulation method, wherein the fracturing construction parameters include the maximum injection displacement P of the main fracturing and the total liquid volume Q injected into each fracturing section for fracturing construction and the total amount N of proppant added; wherein the total liquid volume Q includes the liquid volume Q injected in the injection and charging fracturing stage 1 and the amount of liquid injected during the main fracturing stage Q 2The total amount of proppant N includes the amount of proppant added during the fluid injection and energy fracturing stage N 1 and the amount of proppant added in the main fracturing stage N 2 The above optimized fracturing construction parameters must meet the requirement that the artificial fractures cover more than 90% of the area between horizontal well sections and between each fracturing section.

[0070] In some embodiments, when each fracturing stage in two or one horizontal well in the middle of the horizontal well group is fracturing, the injection displacement, total liquid volume and total proppant of the main fracturing can be slightly greater than the injection displacement, total liquid volume and total proppant of the main fracturing in each fracturing stage in the two horizontal wells outside the horizontal well group. This can increase the complexity of the artificial fractures formed in each fracturing stage in the horizontal well in the middle of the fracturing, so as to achieve effective communication between the artificial fractures and the artificial fractures formed in the two horizontal wells on both sides of the horizontal well group.

[0071] In some embodiments, the calculation formula for geological reserve volume is V 总 =LWh, where L is the length of the horizontal well section of the horizontal well, m; W is the overall controlled length of the artificial fractures formed by the horizontal well group, m. The overall controlled length of the artificial fractures formed by the horizontal well group is the total length of the area occupied by the artificial fractures formed after fracturing in all the fracturing sections; h is the average effective thickness of the coal reservoir, m. The artificial fractures formed after fracturing in the fracturing section extend in both directions of the horizontal well section, which direction is oblique or perpendicular to the direction of the horizontal well section. The overall controlled length of the artificial fractures formed by the horizontal well group is the total length of the artificial fractures formed after fracturing in all the fracturing sections on both sides of all the horizontal well sections. Figure 2 Taking the development schematic diagram of the horizontal well group as an example, the value of the overall controlled length of the artificial fracture formed by the horizontal well group is 3 times the value of the well spacing. The average effective thickness of the coal reservoir is the average effective thickness of the coal seam excluding interlayer gangue and structural coal.

[0072] In some embodiments, the amount of liquid Q in the fluid injection charging fracturing stage is 1 The calculation formula is Q 1 =V 1 ×φ×C f ×ΔP, where V 1 is the controlled reservoir volume of the natural fractures on both sides of each fracturing stage, m 3 , the controlled reservoir volume is the product of the length of the area expected to be occupied by the natural fractures on both sides of each fracturing stage, the width of the area expected to be occupied by the natural fractures on both sides of each fracturing stage and the average effective thickness of the coal reservoir; φ is the average porosity of the coal rock, %; C f is the average comprehensive compression coefficient of coal rock, MPa -1; ΔP is the increase in coal reservoir pressure after fluid injection and energy fracturing, MPa. In some embodiments, the length of the area that the natural fractures on both sides of each fracturing section are expected to occupy can be the value of the well spacing between two adjacent horizontal well sections, and the width of the area that the natural fractures on both sides of each fracturing section are expected to occupy can be the value of the section length of each fracturing section.

[0073] In some embodiments, the control range of the injection displacement of the main fracturing is 4 to 20 m 3 / min. Thus, the artificial fractures cover more than 90% of the area between horizontal well sections and fracturing sections.

[0074] In some embodiments, the injection rate of the main fracturing is set to be 4m 3 / min, and then gradually increase the displacement to the maximum injection displacement P, that is, through the fracturing method of step-by-step increase of displacement, to meet the requirement that the artificial fractures cover more than 90% of the area between horizontal well sections and fracturing sections.

[0075] The present invention is described below with reference to specific embodiments. It should be noted that these embodiments are merely illustrative and do not limit the present invention in any way.

[0076] Example 1

[0077] A coalbed methane block in the southern part of the Qinshui Basin has a burial depth of 1000-1200m, an overall coal seam pressure of 0.7-0.9MPa / 100m, the main coal seam in the planned mining area is 3# coal, the average effective thickness of the coal reservoir is 5m, the average porosity of the coal rock is 4%, the coal reservoir pressure is 0.85MPa / 100m, and the maximum principal stress direction in the horizontal plane of the coal reservoir is 30° north-east. The method for mining coalbed methane using the horizontal well group of the underpressure coal reservoir according to the embodiment of the present invention is designed to be used for mining, and the construction steps are as follows:

[0078] (1) Collect regional geological data of the block, including coal reservoir pressure of 0.85MPa / 100m, average coal porosity of 4%, maximum principal stress of 28MPa in the horizontal plane of the coal reservoir and the direction of the maximum principal stress of 30° north-east, the difference between the maximum principal stress and the minimum principal stress in the horizontal plane of the coal reservoir of 8MPa, no faults in the well area, coal permeability of about 0.03md, top and bottom lithology of mudstone with good plugging properties, regional coal structure dominated by primary coal, and regional coal rock average comprehensive compression coefficient of 0.05MPa -1 , establish a geological model of the horizontal well group, which includes the data of regional geological data. Combined with the geological model, set the maximum principal stress direction in the horizontal plane perpendicular to the coal reservoir (about 60° north-west) Figure 2The horizontal well group shown contains three parallel horizontal wells. The well spacing of the horizontal well sections of this horizontal well group is set to 250m, and the lengths of the horizontal well sections of the three horizontal wells are all set to 1000m.

[0079] (2) Calculate the volume of geological reserves within the well group:

[0080] V 总 =LWh=1000×750×5=375×10 4 m 3

[0081] In the above formula, L is the length of the horizontal section of the horizontal well, m; W is the width of the overall control of the horizontal well group, m, and the width value is 3 times the well spacing value; h is the average effective thickness of the coal reservoir, m.

[0082] Combined with the regional fracture monitoring data and Meyer software simulation, the length of the fracturing section on the horizontal well section is preferably 60-80m. Then, according to the length of the area occupied by the artificial fractures generated after fracturing in each fracturing section of 250m (this value is taken from the value of the well spacing) and the width of the area occupied by the artificial fractures generated after fracturing in each fracturing section of 80m (this value is taken from the value of the length of each fracturing section), the simulation is performed to optimize the maximum injection displacement P of the main fracturing stage to 12m 3 / min, the total liquid volume Q for each fracturing stage is 1500m 3 / stage, the total amount of proppant N used for fracturing construction in each fracturing stage is 100m 3 / stage, and the maximum injection displacement P, total liquid volume Q and total proppant N of the above-mentioned main fracturing meet the requirements of theoretical design that the artificial fractures cover more than 90% of the area between horizontal well sections and fracturing sections.

[0083] (3) Using the geological model of the horizontal well group and the logging data while drilling, the primary structural coal with high gas logging greater than 50% and gamma value less than 35API is preferably perforated within the interval range of 60 to 80 m between the fracturing sections, and the fracturing points in the horizontal sections of adjacent horizontal wells are arranged in an interlaced manner.

[0084] (4) Inject fluid into the horizontal well group at a low displacement rate to supplement the energy of the coal reservoir cleat fissures. The length of the natural fractures on both sides of each fracturing section is expected to be 250 m (the value is taken from the well spacing value) and the width of the natural fractures on both sides of each fracturing section is expected to be 80 m (the value is taken from the length of each fracturing section), and the formation compression coefficient is 0.05 MPa. -1 , the coal reservoir pressure is increased from 8.5MPa to 10MPa. The total amount of liquid Q required to be injected into each fracturing stage during the fluid injection and energy fracturing stage 1 for The construction pressure shall not exceed the average formation fracture pressure of 30MPa calculated from the adjacent well fracturing. The injection displacement at this stage shall be controlled at 0.5m 3 / min~4m 3 / min, the injection design uses a mixture of KCl and water (the mass fraction of KCl in the mixture is 0.5%). 3 After the liquid is added, the support dosage N is added at a low sand ratio of about 5% (volume ratio of quartz sand to injected liquid). 1 5~10m 3 70 / 100 mesh silt sand is used to support the communicating cleats and fissures. In the later stage of fluid injection and energy fracturing, a displacement fluid with a volume of 1 to 1.5 times the total volume of a horizontal well is injected, and then the pump is stopped to expand the pressure, and the pump is stopped to expand the pressure until the wellhead pressure of the horizontal well is less than 1MPa.

[0085] (5) First, the horizontal wells on both sides of the horizontal well group are fractured, and then the horizontal wells in the middle of the horizontal well group are fractured. Figure 2 As shown, first fracturing Figure 2 Horizontal well 1 and horizontal well 3 are fractured in the middle, and horizontal well 2 is fractured last. Each fracturing stage is fractured in turn from one end far away from the wellhead of the horizontal well to the other end close to the wellhead of the horizontal well. The total amount of fluid that needs to be injected into each fracturing stage during the main fracturing stage is Q 2 1200m 3 (Q 2 =QQ 1 ). The total liquid volume Q 2 Liquid volume including pre-fluid 240m 3 (The amount of liquid in the pre-fluid accounts for the total liquid volume Q 2 The proportion is designed to be 20%), the amount of sand-carrying fluid added with quartz sand proppant is 935m 3 and displacement fluid volume 25m 3 Each fracturing stage initially uses a fluid injection rate of 4m 3 / min, and then gradually increase the injection displacement to 12m 3 / min. The amount of proppant added in the main fracturing stage is N 2 (N 2 =NN 1 ) including 60m 3 40 / 70 mesh fine quartz sand and 30m 3 20 / 40 mesh medium quartz sand.

[0086] (6) After all the fracturing stages in the horizontal well group have been completed, the wellhead pressure of the horizontal well group is tested. If the wellhead pressure is lower than the fracture closure pressure, pressure control and blowout are performed. If the wellhead pressure is 20-10MPa, a 6mm nozzle is used for blowout; if the wellhead pressure is 10MPa-5MPa, a 10mm nozzle is used; if the wellhead pressure is less than 5MPa, the wellhead is opened for blowout.

[0087] (7) After the blowdown is completed, the well group is pumped down and put into production.

[0088] In summary, the method for exploiting coalbed methane in a horizontal well group of an underpressured coal reservoir provided in an embodiment of the present invention effectively changes the initial underpressure state of the coal reservoir of the coalbed methane well group by first injecting a first portion of liquid into the coal reservoir to fill the natural fractures and cleats of the coal reservoir. The method can replenish the formation energy within the control range of the horizontal well group, greatly improve the overall compressive strength of the coal reservoir in the horizontal well group, dredge the natural fractures and cleats of the coal reservoir and keep the natural fractures and cleats of the coal reservoir open, fully utilize the natural fracture and cleat system and effectively avoid the damage to the natural fractures and cleats of the coal reservoir by conventional fracturing, avoid the compression and closure of the natural fractures and cleats around the artificial fractures caused by conventional large-scale direct fracturing, and help to reduce the space where the injected liquid may be lost in the subsequent main fracturing.

[0089] Furthermore, the method for exploiting coalbed methane in a horizontal well group in an underpressure coal reservoir provided in an embodiment of the present invention, based on the characteristics of the underpressure coal reservoir, changes the previous single-well fracturing construction mode by optimizing the horizontal well group layout, optimizing fracturing parameters, and performing well group synchronization or zipper disturbance fracturing, and redesigns the well group as a whole to achieve effective series connection and communication between fractures in the well group, ultimately forming a volume fracture network that coordinates pressure reduction and drainage, thereby improving the degree of fracture control of the well group and greatly improving the overall recovery rate of the well group.

[0090] It should be particularly pointed out that the various components or steps in the above-mentioned embodiments can be cross-linked, replaced, added, or deleted with each other. Therefore, the combinations formed by these reasonable arrangements and combinations should also fall within the scope of protection of the present invention, and the scope of protection of the present invention should not be limited to the embodiments.

[0091] The above are exemplary embodiments disclosed in the present invention. The order disclosed in the above embodiments of the present invention is only for description and does not represent the advantages and disadvantages of the embodiments. However, it should be noted that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope (including claims) disclosed in the embodiments of the present invention is limited to these examples. Various changes and modifications may be made without departing from the scope defined in the claims. The functions, steps and / or actions of the method claims according to the disclosed embodiments described herein do not need to be performed in any particular order. In addition, although the elements disclosed in the embodiments of the present invention may be described or required in individual form, they may also be understood as multiple unless explicitly limited to the singular.

[0092] A person skilled in the art should understand that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of the disclosure of the embodiments of the present invention (including the claims) is limited to these examples; under the concept of the embodiments of the present invention, the technical features in the above embodiments or different embodiments can also be combined, and there are many other changes in different aspects of the embodiments of the present invention as described above, which are not provided in detail for the sake of simplicity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present invention should be included in the protection scope of the embodiments of the present invention.

Claims

1. A method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir, characterized in that: The steps include: Arranging a horizontal well group, wherein the horizontal well group includes a predetermined number of horizontal wells, each of the horizontal wells includes a horizontal well section, and each of the horizontal well sections is located in a coal reservoir of the same formation; Dividing each of the horizontal well sections into a plurality of fracturing sections, selecting a perforation position in each of the fracturing sections for perforation to form a fracturing point; Performing fracturing construction on each of the fracturing sections, the fracturing construction includes a liquid injection and energy fracturing stage and a main fracturing stage, in which a first portion of liquid is injected into the coal reservoir through the fracturing point at a predetermined displacement and a predetermined construction pressure to fill natural fractures and cleats in the coal reservoir, and in the main fracturing stage, a second portion of liquid is injected into the coal reservoir through the fracturing point to form artificial fractures in the coal reservoir; After all the fracturing stages are completed, coalbed methane extraction is carried out.

2. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: Each of the horizontal well sections is arranged in parallel, and the direction of the horizontal well section is perpendicular to the direction of the maximum principal stress in the horizontal plane of the coal reservoir or is oblique to the direction of the maximum principal stress at an angle of 60° to 90°.

3. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1 or 2, characterized in that: The order of fracturing construction for each of the fracturing sections is: first, simultaneously fracturing each of the fracturing sections in the two horizontal well sections on both sides of the horizontal well group, and then sequentially fracturing each of the fracturing sections in the horizontal well section in the middle of the horizontal well group until the fracturing construction of all the horizontal well sections is completed.

4. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: When fracturing construction is carried out on each of the fracturing sections, the pump is stopped to expand the pressure after the fluid injection and energy charging fracturing stage, and then the main fracturing stage is continued.

5. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: The step of selecting perforation positions for perforation to form fracturing points includes: selecting native structural coal seams in each fracturing section for perforation to form fracturing points, and the fracturing points in two adjacent horizontal well sections are arranged alternately.

6. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: The gas measurement value of the native structural coal seam is >50% and the gamma value is <35API.

7. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: The perforating method is to perform directional perforation at a predetermined offset angle relative to the horizontal plane of the coal reservoir, obliquely upward and / or obliquely downward.

8. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 7, characterized in that: The predetermined misalignment angle is 0-15°.

9. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: The predetermined construction pressure does not exceed the formation fracture pressure of the area where the horizontal well group is located, and the formation fracture pressure of the area where the horizontal well group is located is obtained by injection and / or pressure drop test combined with adjacent well fracturing curves.

10. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: The control range of the predetermined displacement is 0.5 to 4 m 3 / min.

11. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: The first portion of liquid and the second portion of liquid are both coal-phobic liquids.

12. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 11, characterized in that: The first portion of liquid and the second portion of liquid are a mixture of KCl and water, wherein the mass fraction of KCl in the mixture is 0.3% to 1%.

13. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: In the fluid injection and energy fracturing stage, a quartz sand proppant of 70-100 mesh is added to the first portion of liquid during the injection of the first portion of liquid.

14. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: In the main fracturing stage, during the injection of the second portion of liquid, a quartz sand proppant of 40-70 mesh and a quartz sand proppant of 20-40 mesh are sequentially added to the second portion of liquid.

15. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: After all the fracturing stages are completed and before coalbed methane extraction, the method further includes: detecting the wellhead pressure of the horizontal well group, and performing pressure-controlled blowout if the wellhead pressure is lower than the fracture closure pressure.

16. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: If the wellhead pressure is 20-10MPa, a 6mm nozzle is used for blowout; if the wellhead pressure is 10MPa-5MPa, a 10mm nozzle is used for blowout; if the wellhead pressure is less than 5MPa, the wellhead is opened for blowout.

17. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 1, characterized in that: Also includes: When arranging a horizontal well group, obtaining regional geological data to establish a geological model of the horizontal well group, wherein the geological model includes data of the regional geological data; The well spacing between two adjacent horizontal well sections is set according to the data in the geological model, the length of the horizontal well section is set, and the perforation position of the fracturing section is selected.

18. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 17, characterized in that: The regional geological data include coal reservoir pressure, average porosity of coal rock, maximum principal stress and direction in the horizontal plane of coal reservoir, fault and fracture development characteristics, coal body permeability, roof and floor lithology, regional coal body structure distribution, and average comprehensive compression coefficient of coal rock.

19. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 17, characterized in that: Also includes: The volume of geological reserves mobilized within the horizontal well group is calculated based on the data in the geological model. According to the volume of geological reserves and the well spacing, the length of the fracturing section and the fracturing construction parameters of each fracturing section are optimized and set by regional fracture monitoring data and FracproPT or Meyer software simulation method. The fracturing construction parameters include the maximum injection displacement P of the main fracturing and the total amount of liquid Q injected for fracturing construction of each fracturing section and the total amount of proppant N added; wherein the total amount of liquid Q includes the amount of liquid Q1 injected in the injection and energizing fracturing stage and the amount of liquid Q2 injected in the main fracturing stage, and the total amount of proppant N includes the amount of proppant N1 added in the injection and energizing fracturing stage and the amount of proppant N2 added in the main fracturing stage.

20. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 19, characterized in that: The calculation formula of the geological reserve volume is V 总 =LWh, where L is the length of the horizontal well section, m; W is the overall controlled length of the artificial fractures formed by the horizontal well group, m, and the overall controlled length of the artificial fractures formed by the horizontal well group is the total length of the area occupied by the artificial fractures formed after fracturing in all the fracturing sections; h is the average effective thickness of the coal reservoir, m.

21. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 19, characterized in that: The calculation formula of the liquid volume Q1 in the injection and energy fracturing stage is Q1 = V1 × φ × C f ×ΔP, where V1 is the controlled reservoir volume of the natural fractures on both sides of each fracturing stage, m 3 , the controlled reservoir volume is the product of the length of the area expected to be occupied by the natural fractures on both sides of each fracturing stage, the width of the area expected to be occupied by the natural fractures on both sides of each fracturing stage and the average effective thickness of the coal reservoir; φ is the average porosity of the coal rock, %; C f is the average comprehensive compression coefficient of coal rock, MPa -1 ; ΔP is the increase in coal reservoir pressure after the fluid injection and energy fracturing, MPa.

22. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 19, characterized in that: The control range of the main fracturing fluid injection rate is 4 to 20 m 3 / min.

23. The method for exploiting coalbed methane from a horizontal well group in an underpressured coal reservoir according to claim 19, characterized in that: The main fracturing fluid injection rate is set to an initial rate of 4m 3 / min and then gradually increase the displacement to the maximum injection displacement P.

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