A method for exploiting coalbed methane by underpressure type coal reservoir horizontal well group

Abstract

The application discloses a method for exploiting coalbed methane by using a horizontal well group in a coal reservoir with an underpressure type, which comprises the following steps: arranging a horizontal well group, the horizontal well group comprising a predetermined number of horizontal wells, each horizontal well comprising a horizontal well section, and each horizontal well section being located in a coal reservoir of the same layer system; dividing each horizontal well section into a plurality of fracturing sections with a predetermined length, perforating a perforation position in each fracturing section to form a fracturing point; performing fracturing construction on each fracturing section, the fracturing construction comprising a liquid injection and energy storage fracturing stage and a main fracturing stage, in the liquid injection and energy storage fracturing stage, a first part of liquid is injected into the coal reservoir through the fracturing point at a predetermined discharge capacity and a predetermined construction pressure to fill natural fractures and cleats of the coal reservoir, in the main fracturing stage, a second part of liquid is injected into the coal reservoir through the fracturing point to form artificial fractures in the coal reservoir, and the predetermined construction pressure is not higher than a formation fracture pressure of a region where the horizontal well group is located; and after all the fracturing sections complete fracturing, coalbed methane exploitation is performed.

Description

Technical Field

[0001] This invention relates to the technical field of coalbed methane extraction, and more particularly to a method for extracting coalbed methane from horizontal well groups in under-pressured coal reservoirs. Background Technology

[0002] The proportion of under-pressured coal reservoirs currently used for coalbed methane extraction is increasing. Under-pressured coal reservoirs refer to coalbed methane blocks in a certain area with a burial depth of 1000-1200m, and the overall coal reservoir pressure range of the gas block is 0.7-0.9MPa / 100m (coal reservoir pressure at 100 meters above the ground). The overall coal reservoir pressure range of this gas block is less than the normal pressure value of the reservoir (1MPa / 100m).

[0003] Currently, the main method for developing coal reservoirs follows conventional oil and gas fracturing enhancement methods. However, this method is insufficiently targeted at coal reservoirs with characteristics such as underpressure, undersaturation, well-developed fracture systems, and high compressibility. In other words, this method does not fully consider the impact or damage to coal reservoirs with these characteristics. For example, while enhancing permeability, it easily damages the natural fractures and cleavage systems of the coal reservoir. Therefore, the current method fails to adequately protect and utilize the natural fracture and cleavage systems of the coal reservoir. Furthermore, current coal reservoir development still focuses on single-well fracturing design and construction, lacking design for the overall transformation of well groups. The overall utilization rate is low, and it cannot fully meet the requirements for overall fracturing construction of horizontal well groups in current coalbed methane extraction. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a method for extracting coalbed methane from horizontal well groups in under-pressured coal reservoirs. This method solves the technical problem that existing conventional fracturing methods fail to adequately protect and utilize the natural fractures and cleavage systems of coal reservoirs.

[0005] This invention discloses a method for extracting coalbed methane from horizontal well groups in under-pressured coal reservoirs, comprising the following steps:

[0006] Arrange a horizontal well group, the horizontal well group including a predetermined number of horizontal wells, each of the horizontal wells including a horizontal well section, and each of the horizontal well sections being located in the same coal reservoir stratum;

[0007] Each horizontal well section is divided into several fracturing sections, and perforation is performed at selected perforation locations within each fracturing section to form fracturing points;

[0008] Each fracturing section is subjected to fracturing operations, which include a fluid injection and energy-charging fracturing stage and a main fracturing stage. In the fluid injection and energy-charging fracturing stage, a first portion of liquid is injected into the coal reservoir through the fracturing point at a predetermined flow rate and a predetermined construction pressure to fill the natural fractures and cleavages of 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.

[0009] After all the fracturing sections have been completed, coalbed methane extraction will commence.

[0010] According to one embodiment of the present invention, each of the horizontal well sections is arranged in parallel, and the orientation of the horizontal well section is perpendicular to the direction of the maximum principal stress in the horizontal plane of the coal reservoir or 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 horizontal well section is as follows: the order of fracturing construction for each fracturing segment is as follows: first, each fracturing segment in the two horizontal well sections located on both sides of the horizontal well group is fracturing simultaneously, and then each fracturing segment in the horizontal well section located in the middle of the horizontal well group is fracturing sequentially, until the fracturing construction of all the horizontal well sections is completed.

[0012] According to one embodiment of the present invention, when performing fracturing operations on each of the fracturing sections, the pump is stopped and pressure is increased after the injection and energizing fracturing stage, and then the main fracturing stage continues.

[0013] According to one embodiment of the present invention, the step of selecting a perforation location to form a fracturing point includes: selecting a primary structure coal seam within each fracturing section to form a fracturing point, wherein the fracturing points in two adjacent horizontal well sections are arranged alternately.

[0014] According to one embodiment of the present invention, the gas measurement value of the original structure coal seam is >50% and the gamma value is <35 API.

[0015] According to one embodiment of the present invention, the perforation is performed by directional perforation at a predetermined offset angle relative to the horizontal plane of the coal reservoir.

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

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

[0018] According to one embodiment of the present invention, the predetermined displacement control range is 0.5 to 4 m³. 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-absorbing liquids.

[0020] According to one 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 injection and charging fracturing stage, 70-100 mesh quartz sand proppant 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, during the main fracturing stage, 40-70 mesh quartz sand proppant and 20-40 mesh quartz sand proppant are sequentially added to the second part of the liquid during the injection of the second part of the liquid.

[0023] According to one embodiment of the present invention, after all the fracturing sections have been completed and before coalbed methane extraction, the method further includes: detecting the wellhead pressure of the horizontal well group; if the wellhead pressure is lower than the fracture closure pressure, then pressure control and venting are performed.

[0024] According to one embodiment of the present invention, if the wellhead pressure is 20-10 MPa, a 6 mm nozzle is used for venting; if the wellhead pressure is 10-5 MPa, a 10 mm nozzle is used for venting; if the wellhead pressure is <5 MPa, the wellhead is left open for venting.

[0025] According to one embodiment of the present invention, the method further includes: when deploying a horizontal well group, acquiring regional geological data to establish a geological model of the horizontal well group, the geological model including data from 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 location of the fracturing section based on the data in the geological model.

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

[0027] According to one embodiment of the present invention, the method further includes: calculating the geological reserve volume utilized within the horizontal well group based on data in the geological model; optimizing the length of the fracturing section and the fracturing operation parameters of each fracturing section based on the geological reserve volume and the well spacing using regional fracture monitoring data and simulation methods such as FracproPT or Meyer software; wherein the fracturing operation parameters include the maximum injection rate P of the main fracturing operation, and the total fluid volume Q injected for fracturing operation of each fracturing section and the total amount N of proppant added; wherein the total fluid volume Q includes the fluid volume Q1 injected in the injection-charging fracturing stage and the fluid volume Q2 injected in the main fracturing stage, and the total amount N of proppant includes the proppant dosage N1 added in the injection-charging fracturing stage and the proppant dosage N2 added in the main fracturing stage.

[0028] According to one embodiment of the present invention, the formula for calculating 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 of all fracturing sections; h is the average effective thickness of the coal reservoir, m.

[0029] According to one embodiment of the present invention, the formula for calculating the fluid volume Q1 in the fluid injection and charging 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 section, in m 3 The controlled reservoir volume is the product of the expected length of the area occupied by natural fractures on both sides of each fracturing section, the expected width of the area occupied by natural fractures on both sides of each fracturing section, and the average effective thickness of the coal reservoir; φ is the average porosity of the coal and rock, %; C f The average comprehensive compressibility coefficient of coal and rock, in MPa -1 ΔP is the increase in coal reservoir pressure after the injection and fracturing, in MPa.

[0030] According to one embodiment of the present invention, the injection displacement of the main fracturing is controlled within the range of 4–20 m³. 3 / min.

[0031] According to one embodiment of the present invention, the injection displacement of the main fracturing is set to start from an initial displacement of 4m³. 3 After a certain number of minutes, gradually increase the flow rate to the maximum injection flow rate.

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

[0033] The method for extracting coalbed methane from under-pressured coal reservoirs provided by this invention involves injecting a first portion of liquid into the coal reservoir to fill the natural fractures and cleavages, effectively changing the initial under-pressure state of the coal reservoir in the coalbed methane well group. This replenishes the formation energy within the controlled storage area of ​​the horizontal well group, significantly improving the overall compressive strength of the coal reservoir within the horizontal well group. It also clears and maintains the openness of the natural fractures and cleavages in the coal reservoir, making full use of the natural fracture and cleavage system while effectively avoiding the damage to the natural fractures and cleavages caused by conventional fracturing. It also avoids the compression and closure of the natural fractures and cleavages around the artificial fractures caused by conventional large-scale direct fracturing, which helps to reduce the space for possible fluid loss during subsequent main fracturing. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This is a schematic flowchart of a method for extracting coalbed methane from a horizontal well group in an under-pressured coal reservoir, as disclosed in an embodiment of the present invention.

[0036] Figure 2 This is a schematic diagram illustrating the development of a horizontal well group according to an embodiment of the present invention. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to specific examples and the accompanying drawings.

[0038] It should be noted that all uses of "first" and "second" in the embodiments of the present invention are for the purpose of distinguishing two entities or parameters with the same name but different names. It is clear that "first" and "second" are only for the convenience of expression and should not be construed as limiting the embodiments of the present invention. Subsequent embodiments will not explain this in detail.

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

[0040] S10: Arrange horizontal well groups, which include a predetermined number of horizontal wells, each horizontal well including a horizontal well section, and each horizontal well section located in the same coal reservoir stratum;

[0041] S20: Divide each horizontal well section into several fracturing sections of predetermined length, and select perforation locations within each fracturing section to form fracturing points;

[0042] S30: Perform fracturing operations on each fracturing section. The fracturing operations include the fluid injection and energy-charging fracturing stage and the main fracturing stage. In the fluid injection and energy-charging fracturing stage, the first part of the liquid is injected into the coal reservoir through the fracturing point at a predetermined discharge rate and predetermined construction pressure to fill the natural fractures and cleavages of the coal reservoir. In the main fracturing stage, the second part of the liquid is injected into the coal reservoir through the fracturing point to form artificial fractures in the coal reservoir.

[0043] S40: After all fracturing sections are completed, coalbed methane extraction will commence.

[0044] In the above embodiments, by injecting a first portion of liquid into the coal reservoir to fill the natural fractures and cleavages, the initial underpressure state of the coal reservoir in the coalbed methane well group is effectively changed. This can replenish the formation energy within the controlled storage range of the horizontal well group, greatly improving the overall compressive strength of the coal reservoir within the horizontal well group. It fully utilizes the natural fracture and cleavage system to effectively avoid the damage to the natural fractures and cleavages of the coal reservoir caused by conventional fracturing, and reduces the compression and closure of the natural fractures and cleavages around the artificial fractures caused by conventional large-scale direct fracturing. This is beneficial to reduce the space that the liquid injected in the subsequent main fracturing may be lost.

[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 oblique to the direction of the maximum principal stress at an angle of 60° to 90°.

[0046] Figure 2 This diagram illustrates the development of a horizontal well group according to an embodiment of the present invention. The horizontal well group includes three horizontal wells (horizontal well 1, horizontal well 2, and horizontal well 3), and each horizontal well includes a vertical well section. Figure 2 (not shown in the image) and the horizontal well section connected to the vertical well section, Figure 2 The diagram shows the horizontal well sections of three horizontal wells, all located within the same horizontal plane of the coal reservoir. Each horizontal well in this group is divided into several fracturing sections 4 of predetermined lengths. 'a' represents the distance between the horizontal well sections of horizontal well 1 and horizontal well 2, and 'L' represents the length of the horizontal well section.

[0047] In some embodiments, the fracturing sequence for each fracturing segment is as follows: first, each fracturing segment in two horizontal well segments located on either side of the horizontal well group is simultaneously fracturing; then, each fracturing segment in the horizontal well segment located in the middle of the horizontal well group is fracturing sequentially, until fracturing of all horizontal well segments is completed. For example, with... Figure 2Taking the development diagram of the horizontal well group as an example, each fractured segment in the two outermost horizontal wells 1 and 3 is fractured simultaneously first, followed by each fractured segment in the middle horizontal well 2. In another embodiment, if the horizontal well group includes four horizontal wells, each fractured segment in the two outermost horizontal wells is fractured first, followed by each fractured segment in the two middle horizontal wells. In another embodiment, if the horizontal well group includes five horizontal wells, each fractured segment in the two outermost horizontal wells is fractured first, followed by each fractured segment in the two horizontal wells adjacent to the two outermost horizontal wells, and finally each fractured segment in the middle horizontal well. If the horizontal well group includes six or more horizontal wells, the fracturing sequence is similar to the above embodiments and will not be repeated here.

[0048] In the above embodiment, the fracturing section in the outermost horizontal well of the horizontal well group is fracturing first. After the outer horizontal wells are fracturing, a high-pressure formation boundary can be constructed. This allows the fracturing of the outer horizontal wells to disrupt the in-situ stress in the horizontal plane of the coal reservoir in the middle area, thereby reducing the difference between the maximum and minimum principal stresses in the horizontal plane of the coal reservoir within the horizontal well group. Then, the horizontal wells in the middle of the horizontal well group are fracturing in sequence. This promotes the formation of an overall complex fracture network in the coal reservoir and prevents certain areas within the coal reservoir from forming blank zones due to the absence of fractures. This achieves the design of overall well group modification and greatly improves the overall utilization rate of the coal reservoir.

[0049] In some embodiments, the fracturing sequence for each fracturing section within a horizontal well segment is as follows: each fracturing segment is fracturing sequentially from the end furthest from the wellhead of the horizontal well to the other end closest to the wellhead of the horizontal well.

[0050] In some embodiments, during fracturing operations on each fracturing segment, a pump-stopping diffusion phase is performed after the fluid injection and charging fracturing stage, followed by the main fracturing stage. Pump-stopping diffusion refers to pausing for a period after fluid injection and charging fracturing to allow for proper closure of natural fractures and cleavages. Specifically, during pump-stopping diffusion, the diameter of natural fractures during fluid injection and charging is appropriately reduced, thereby facilitating the generation of artificial main fractures in subsequent main fracturing. In some embodiments, after injecting displacement fluid at a volume of 1 to 1.5 times the total wellbore volume of a horizontal well in the later stages of the fluid injection and charging fracturing stage, pump-stopping diffusion is performed until the wellhead pressure of the horizontal well is less than 1 MPa.

[0051] In some embodiments, the step of selecting perforation locations to form fracturing points includes: selecting native coal seams within each fracturing section for perforation to form fracturing points, with fracturing points in adjacent horizontal well sections being staggered. In this embodiment, compared to tectonic coal, native coal seams are more prone to fracture formation during fracturing, thereby facilitating the formation of complex fracture networks in the coal reservoir.

[0052] Figure 2 The schematic diagram of a horizontal well group development in one embodiment shows that "the fracturing sections within the horizontal sections of adjacent horizontal wells are arranged in an alternating manner," thereby enabling the fracturing points within the horizontal sections of adjacent horizontal wells to be arranged in an alternating manner. The fracturing points in the horizontal sections of horizontal well 1 and horizontal well 2 are staggered, allowing the artificial fractures formed by injecting fluid into the coal reservoir through the fracturing points in the horizontal section of horizontal well 1 to intersect with those formed through the fracturing points in the horizontal section of horizontal well 2. This allows the artificial fractures formed by injecting fluid into the coal reservoir through the fracturing points in the horizontal sections to extend to the vicinity of adjacent horizontal well sections, increasing the permeability of the coal reservoir near those adjacent horizontal sections. Simultaneously, the alternating arrangement of fracturing points between adjacent wells facilitates the mutual disturbance of fractures during synchronous or zipper-type fracturing, forming a complex fracture network and improving the fracture-creating effect of the well group.

[0053] In some embodiments, the gas measurement value of the native coal seam is >50% and the gamma value is <35 API. Selecting a native coal seam that meets the above conditions makes it easier to form fractures during the fracturing process, which is conducive to the extension of the fracturing fractures and thus to the formation of a complex fracture network in the coal reservoir.

[0054] In some embodiments, perforation is performed by directional perforation at a predetermined offset angle relative to the horizontal plane of the coal reservoir, either obliquely upwards and / or downwards. This facilitates the extension of the artificial fracture network within the cleavage fractures of the coal reservoir, reducing the negative impact of tectonic coal on the extension of the artificial fracture network.

[0055] In some embodiments, the predetermined misalignment angle is 0 to 15°, which is beneficial for the extension of the artificial fracture network in the cleavage fractures of the coal reservoir.

[0056] In some embodiments, the predetermined construction pressure does not exceed the formation fracturing pressure in the area where the horizontal well group is located. The formation fracturing pressure in the area where the horizontal well group is located is obtained by injection / pressure drop testing combined with fracturing curves from adjacent wells. The construction pressure during the fluid injection and energy charging fracturing stage does not exceed the formation fracturing pressure in the area where the horizontal well group is located, thus avoiding the generation of artificial fractures in the coal reservoir during this stage.

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

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

[0059] In some embodiments, the first liquid portion and the second liquid portion 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 fluid injection and fracturing stage, 70-100 mesh quartz sand proppant is added to the first portion of the fluid during injection, thereby facilitating the support of connected natural fractures and cleavages. For example, in some embodiments, during the fluid injection and fracturing stage, after injecting half the required fluid volume, an appropriate amount of 70 / 100 mesh quartz sand is added at a low sand ratio of approximately 5% by volume.

[0061] In some embodiments, during the fluid injection and fracturing stage, 40-70 mesh quartz sand proppant (fine-grained quartz sand proppant) and 20-40 mesh quartz sand proppant (medium-grained quartz sand proppant) are sequentially added to the second portion of the fluid during injection. This combination of fine- and medium-grained quartz sand proppant is used to achieve better filling of the artificial fractures.

[0062] In some embodiments, the method for extracting coalbed methane from horizontal well groups in under-pressured coal reservoirs includes, after all fracturing stages are completed and before coalbed methane extraction, monitoring the wellhead pressure of the horizontal well group. If the wellhead pressure is lower than the fracture closure pressure, controlled pressure venting is performed. After venting, the well group is pumped into production. Controlled pressure venting helps reduce the backflow of pulverized coal and silica sand added during fracturing.

[0063] In some embodiments, if the wellhead pressure is 20–10 MPa, a 6 mm nozzle is used for venting; if the wellhead pressure is 10 MPa–5 MPa, a 10 mm nozzle is used for venting; if the wellhead pressure is <5 MPa, the wellhead is left open for venting. This significantly reduces the backflow of pulverized coal and quartz sand added during fracturing.

[0064] In some embodiments, the method for developing coalbed methane from horizontal well groups in under-pressured coal reservoirs further includes: acquiring regional geological data to establish a geological model of the horizontal well group when deploying the horizontal well group, the geological model including data from the regional geological data; setting the well spacing between two adjacent horizontal well sections, setting the length of the horizontal well sections, and selecting the perforation location of the fracturing section based on the data in the geological model. Figure 2 Taking the development diagram of the horizontal well group shown as an example, the distance 'a' between the horizontal well sections of horizontal well 1 and horizontal well 2 is the well spacing between the horizontal well sections, and L is the length of the horizontal well section. In this embodiment, the geological model allows for direct visualization of the acquired regional geological data, which can be used to set the required data. It also facilitates data optimization using data from the geological model in simulations conducted by FracproPT or Meyer software.

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

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

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

[0068] In some embodiments, the well spacing between two adjacent horizontal well sections ranges from 200 to 300 meters, preferably 250 meters, thereby enabling the efficient extraction of coalbed methane between two adjacent horizontal well sections in a coal reservoir. The length of the horizontal well section is 1000 meters. Increasing the length of the horizontal well section, while meeting drilling process requirements, increases the volume of coalbed methane that can be extracted, thus improving the efficiency of coalbed methane extraction. The spacing between two adjacent fracturing sections within each horizontal well is 60 to 80 meters, maximizing the permeability of coalbed methane and thus maximizing the efficiency of coalbed methane extraction.

[0069] In an embodiment of the present invention, a method for extracting coalbed methane from a horizontal well group in an under-pressured coal reservoir is characterized by further comprising: calculating the volume of geological reserves available within the horizontal well group based on data from a geological model; optimizing the length of each fracturing segment and the fracturing operation parameters for each fracturing segment based on the geological reserve volume and well spacing using regional fracture monitoring data and simulation methods with FracproPT or Meyer software. The fracturing operation parameters include the maximum injection volume P for the main fracturing operation and the total volume Q of fluid injected for each fracturing segment and the total amount N of proppant added. The total volume Q includes the volume Q1 injected during the injection-charging fracturing stage and the volume Q2 injected during the main fracturing stage. The total amount N of proppant includes the proppant dosage N1 added during the injection-charging fracturing stage and the proppant dosage N2 added during the main fracturing stage. The optimized fracturing operation parameters must meet the requirement that artificial fractures cover more than 90% of the area between horizontal well segments and between each fracturing segment.

[0070] In some embodiments, when fracturing each fracturing segment in one or two horizontal wells within the middle of the horizontal well group, the injection rate, total fluid volume, and total proppant volume of the main fracturing process can be slightly greater than those of each fracturing segment in the two horizontal wells on the outer sides of the horizontal well group. This increases the complexity of the artificial fractures formed in each fracturing segment of the middle horizontal well, enabling effective communication between these artificial fractures and those formed in the two horizontal wells on either side of the horizontal well group.

[0071] In some embodiments, the formula for calculating the volume of geological reserves is V. 总 =LWh, where L is the length of the horizontal section of the horizontal well, in meters; W is the overall controlled length of the artificial fractures formed by the horizontal well group, in meters, 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 of all fracturing sections; h is the average effective thickness of the coal reservoir, in meters. The artificial fractures formed after fracturing of the fracturing sections extend to both sides of the horizontal well section, and this 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 of all fracturing sections on both sides of the entire horizontal well section. Figure 2 Taking the development schematic diagram of the horizontal well group as an example, the overall length of the artificial fractures formed by this horizontal well group is taken as three times the distance between wells. The average effective thickness of the coal reservoir is the average effective thickness of the coal seam after removing interbedded rock and tectonic coal.

[0072] In some embodiments, the formula for calculating the fluid volume Q1 during the injection and charging fracturing stage is Q1=V1×φ×C f ×ΔP, where V1 is the reservoir volume controlled by the natural fractures on both sides of each fracturing section, in m 3 The controlled reservoir volume is the product of the expected length of the area occupied by natural fractures on both sides of each fractured section, the expected width of the area occupied by natural fractures on both sides of each fractured section, and the average effective thickness of the coal reservoir; φ is the average porosity of the coal and rock, in %; C f The average comprehensive compressibility coefficient of coal and rock, in MPa -1 ΔP represents the pressure increase in the coal reservoir after fluid injection and energy fracturing, in MPa. In some embodiments, the expected length of the area occupied by natural fractures on both sides of each fracturing section can be taken as the distance between two adjacent horizontal well sections, and the expected width of the area occupied by natural fractures on both sides of each fracturing section can be taken as the length of each fracturing section.

[0073] In some embodiments, the injection rate of the main fracturing is controlled within the range of 4–20 m³. 3 / min. This allows artificial fractures to cover more than 90% of the area between horizontal well sections and between fractured sections.

[0074] In some embodiments, the injection flow rate for main fracturing is set to start from an initial flow rate of 4m³. 3 After a certain number of minutes, the flow rate is gradually increased to the maximum injection flow rate P. This step-by-step fracturing method aims to meet the requirement that artificial fractures cover more than 90% of the area between horizontal well sections and between fracturing sections.

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

[0076] Example 1

[0077] In a coalbed methane block in the southern Qinshui Basin, the burial depth is 1000-1200m, the overall coal seam pressure is 0.7-0.9MPa / 100m, the main coal seam in the proposed mining area is No. 3 coal, the average effective thickness of the coal reservoir is 5m, the average porosity of the coal and rock is 4%, the coal reservoir pressure is 0.85MPa / 100m, and the direction of the maximum principal stress in the horizontal plane of the coal reservoir is 30° north of east. The design adopts the method of horizontal well group for mining coalbed methane in under-pressured coal reservoirs according to the embodiment of the present invention. The construction steps are as follows:

[0078] (1) Collect regional geological data for the block, including coal reservoir pressure of 0.85 MPa / 100m, average coal porosity of 4%, maximum principal stress in the horizontal plane of the coal reservoir of 28 MPa and direction of maximum principal stress of 30° north of east, difference between maximum and minimum principal stress in the horizontal plane of the coal reservoir of 8 MPa, no faults in the well area, coal permeability of approximately 0.03 md, roof and floor lithology of mudstone with good sealing properties, regional coal structure dominated by primary coal, and regional average comprehensive compressibility coefficient of coal and rock of 0.05 MPa. -1 A geological model of the horizontal well group was established, including regional geological data. Based on the geological model, the direction of maximum principal stress (approximately 60° north-west) was set within the horizontal plane perpendicular to the coal reservoir. Figure 2 The horizontal well group shown contains three parallel horizontal wells. The spacing between the horizontal well sections of this horizontal well group is set to 250m, and the length of the horizontal well sections of the three horizontal wells is set to 1000m.

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

[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, in meters; W is the width controlled by the entire horizontal well group, in meters, and this width value is taken as 3 times the distance between wells; h is the average effective thickness of the coal reservoir, in meters.

[0082] Based on regional fracture monitoring data and Meyer software simulations, the optimal length of the fracturing section in the horizontal well section was selected as 60–80 m. Then, simulations were conducted with the expected length of the artificial fracture area generated after each fracturing section being 250 m (based on the well spacing) and the expected width of the artificial fracture area being 80 m (based on the section length of each fracturing section). The maximum injection rate P for the main fracturing stage was optimized to be 12 m³ / s. 3 / min, the total fluid volume Q for fracturing each fracturing section is 1500m³. 3 / section, the total amount of proppant N used for fracturing each fracturing section is 100m³. 3 / section, and the maximum injection displacement P, total fluid volume Q and total proppant volume N of the above main fracturing meet the theoretical design requirement that artificial fractures cover more than 90% of the area between horizontal well sections and between fracturing sections.

[0083] (3) Using the geological model of the horizontal well group and logging data while drilling, in the range of 60-80m interval between the fracturing sections, the primary structure coal with high gas content greater than 50% and gamma value <35API is preferred for perforation, and the fracturing points in the horizontal well sections of adjacent horizontal wells are arranged in an alternating manner.

[0084] (4) Inject fluid into the horizontal well group at a low flow rate to replenish the energy of the coal reservoir cleavage fractures. The expected length of the area occupied by the natural fractures on both sides of each fractured section is 250m (this value is taken as the distance between wells) and the expected width of the area occupied by the natural fractures on both sides of each fractured section is 80m (this value is taken as the length of each fractured section), and the formation compressibility coefficient is 0.05MPa. -1 Calculated based on the coal reservoir pressure increasing from 8.5 MPa to 10 MPa, the total fluid volume Q1 required to be injected during the fluid injection and charging fracturing stage in each fracturing section is: The construction pressure shall not exceed 30 MPa of the average formation fracturing pressure calculated from the fracturing of adjacent wells, and the injection rate during this stage shall be controlled at 0.5 m³ / s under pressure-limited conditions. 3 / min~4m 3 The injection rate was [ratio] / min, using a mixture of KCl and water (where KCl accounted for 0.5% by mass). At this stage, when injecting 150m... 3 After the liquid is injected, a low sand ratio of approximately 5% (quartz sand to injected liquid volume ratio) is added to the proppant, with a dosage of N1 of 5-10 mg / L. 370 / 100 mesh silt is used to support the cleavage and fractures. During the later stage of the injection and charging fracturing stage, 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 and the pressure is increased until the pressure at the wellhead of the horizontal well is less than 1 MPa.

[0085] (5) Following the method of fracturing the horizontal wells on both sides of the horizontal well group first, and then fracturing the horizontal well in the middle of the horizontal well group, such as... Figure 2 As shown, simultaneous fracturing is performed first. Figure 2 Horizontal wells 1 and 3 were fractured first, followed by horizontal well 2. Each fracturing stage was performed sequentially, starting from the end furthest from the horizontal wellhead and moving towards the end closest to it. The total fluid injection volume Q2 required for each fracturing stage during the main fracturing phase was 1200 m³. 3 (Q2 = Q - Q1). The total liquid volume Q2 includes the pre-fluid volume of 240m³. 3 (The proportion of the pre-fluid to the total fluid volume Q2 is designed to be 20%), and the sand-carrying fluid volume with added quartz sand proppant is 935m³. 3 and displacement fluid volume 25m 3 Each fracturing section initially uses an injection flow rate of 4m³ during the main fracturing stage. 3 / min, then gradually increase the injection rate to 12m 3 / min. The proppant dosage N2 (N2 = N - N1) added during this main fracturing stage includes 60m 3 40 / 70 mesh fine quartz sand and 30m 3 20 / 40 mesh medium quartz sand.

[0086] (6) After all fracturing sections in the horizontal well group are completed, the wellhead pressure of the horizontal well group is monitored. If the wellhead pressure is lower than the fracture closure pressure, controlled pressure release is performed. If the wellhead pressure is 20-10 MPa, a 6 mm nozzle is used for release; if the wellhead pressure is 10-5 MPa, a 10 mm nozzle is used; if the wellhead pressure is <5 MPa, the wellhead is left open for release.

[0087] (7) After the blowout is completed, the well group is pumped into production.

[0088] In summary, the method for developing coalbed methane in horizontal well groups of under-pressured coal reservoirs provided by the embodiments of the present invention effectively changes the initial under-pressure state of the coal reservoir in the coalbed methane well group by injecting a first portion of liquid into the coal reservoir to fill the natural fractures and cleavages of the coal reservoir. This can replenish the formation energy within the controlled storage range of the horizontal well group, greatly improve the overall compressive strength of the coal reservoir within the horizontal well group, clear the natural fractures and cleavages of the coal reservoir and keep them open. This method not only makes full use of the natural fracture and cleavage system but also effectively avoids the damage to the natural fractures and cleavages of the coal reservoir caused by conventional fracturing. It also avoids the compression and closure of the natural fractures and cleavages around the artificial fractures caused by conventional large-scale direct fracturing, which helps to reduce the space that the injected liquid may be lost in the subsequent main fracturing.

[0089] Furthermore, the method for extracting coalbed methane from horizontal well groups in under-pressured coal reservoirs provided in this embodiment of the invention, taking into account the characteristics of under-pressured coal reservoirs, changes the previous single-well fracturing construction mode by optimizing the layout of horizontal well groups, optimizing fracturing parameters, and synchronous or zipper-style fracturing of well groups. It also redesigns and modifies the entire well group to achieve effective connection and communication between fractures within the well group, ultimately forming a coordinated volumetric fracture network for depressurization and drainage, improving the degree of fracture control in the well group, and greatly improving the overall recovery rate of the well group.

[0090] It should be noted that the components or steps in the above embodiments can be interchanged, substituted, added, or deleted. Therefore, the combinations formed by these reasonable permutations and transformations should also fall within the protection scope of this invention, and the protection scope of this invention should not be limited to the above embodiments.

[0091] The above are exemplary embodiments disclosed in this invention. The order of the disclosed embodiments is merely for descriptive purposes and does not represent the superiority or inferiority of the embodiments. However, it should be noted that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the disclosed embodiments of this invention (including the claims) is limited to these examples. Various changes and modifications can be made without departing from the scope defined by the claims. The functions, steps, and / or actions of the methods according to the disclosed embodiments described herein do not need to be performed in any particular order. Furthermore, although the elements disclosed in the embodiments of this invention may be described or claimed individually, they may be understood as multiple unless explicitly limited to a singular.

[0092] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of the different aspects of the invention as described above exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A method for extracting coalbed methane from a horizontal well group in an under-pressured coal reservoir, characterized in that, Includes the following steps: Arrange a horizontal well group, the horizontal well group including a predetermined number of horizontal wells, each of the horizontal wells including a horizontal well section, and each of the horizontal well sections being located in the same coal reservoir stratum; Each horizontal well section is divided into several fracturing sections, and perforation is performed at selected perforation locations within each fracturing section to form fracturing points; Each fracturing segment is subjected to fracturing operations, which include a fluid injection and charging fracturing stage and a main fracturing stage. In the fluid injection and charging fracturing stage, a first portion of fluid is injected into the coal reservoir through the fracturing point at a predetermined flow rate and pressure to fill natural fractures and cleavages. In the main fracturing stage, a second portion of fluid is injected into the coal reservoir through the fracturing point to create artificial fractures. The fracturing operation sequence for each fracturing segment is as follows: first, each fracturing segment in two horizontal well sections located on either side of the horizontal well group is simultaneously fracturing; then, each fracturing segment in the horizontal well section located in the middle of the horizontal well group is sequentially fracturing until all horizontal well sections are completed. During the fracturing operation of each fracturing segment, pump shutdown and pressure enhancement are performed after the fluid injection and charging fracturing stage, followed by the main fracturing stage. Both the first and second portions of fluid are coal-draining fluids. After all the fracturing sections have been completed, coalbed methane extraction will commence.

2. The method for extracting coalbed methane from under-pressured coal reservoirs using horizontal well groups 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 oblique to the direction of the maximum principal stress at an angle of 60° to 90°.

3. The method for extracting coalbed methane from under-pressured coal reservoirs using horizontal well groups according to claim 1, characterized in that, The steps of selecting perforation locations to form fracturing points include: selecting primary structural coal seams within each fracturing section to form fracturing points, with the fracturing points in two adjacent horizontal well sections being arranged alternately.

4. The method for extracting coalbed methane from under-pressured coal reservoirs using horizontal well groups according to claim 3, characterized in that, The gas measurement value of the original structure coal seam is >50%, and the gamma value is <35 API.

5. The method for extracting coalbed methane from under-pressured coal reservoirs using horizontal well groups according to claim 1, characterized in that, The perforation is performed by directional perforation at a predetermined offset angle relative to the horizontal plane of the coal reservoir.

6. The method for extracting coalbed methane from under-pressured coal reservoirs using horizontal well groups according to claim 5, characterized in that, The predetermined misalignment angle is 0 to 15°.

7. The method for extracting coalbed methane from under-pressure coal reservoirs using horizontal well groups according to claim 1, characterized in that, The predetermined construction pressure shall not exceed the formation fracturing pressure in the area where the horizontal well group is located. The formation fracturing pressure in the area where the horizontal well group is located is obtained by injection and / or pressure drop testing combined with fracturing curves of adjacent wells.

8. The method for extracting coalbed methane from under-pressure coal reservoirs using horizontal well groups according to claim 1, characterized in that, The predetermined displacement control range is 0.5~4m. 3 / min.

9. The method for extracting coalbed methane from a horizontal well group in an under-pressured coal reservoir according to claim 1, characterized in that, The first part of the liquid and the second part of the liquid are a mixture of KCl and water, wherein the mass fraction of KCl in the mixture is 0.3% to 1%.

10. The method for extracting coalbed methane from a horizontal well group in an under-pressured coal reservoir according to claim 1, characterized in that, During the injection and charging fracturing stage, 70-100 mesh quartz sand proppant is added to the first portion of liquid during the injection process.

11. The method for extracting coalbed methane from under-pressured coal reservoirs using horizontal well groups according to claim 1, characterized in that, During the main fracturing stage, 40-70 mesh quartz sand proppant and 20-40 mesh quartz sand proppant are added sequentially to the second part of the liquid during the injection process.

12. The method for extracting coalbed methane from under-pressured coal reservoirs using horizontal well groups according to claim 1, characterized in that, After all the fracturing sections are completed and before coalbed methane extraction, the following steps are also included: detecting the wellhead pressure of the horizontal well group; if the wellhead pressure is lower than the fracture closure pressure, pressure control and venting are carried out.

13. The method for extracting coalbed methane from under-pressured coal reservoirs using horizontal well groups according to claim 12, characterized in that, If the wellhead pressure is 20-10 MPa, a 6mm nozzle is used for venting; if the wellhead pressure is 10-5 MPa, a 10mm nozzle is used for venting; if the wellhead pressure is <5 MPa, the wellhead is left open for venting.

14. The method for extracting coalbed methane from under-pressured coal reservoirs using horizontal well groups according to claim 1, characterized in that, Also includes: When deploying horizontal well groups, regional geological data is acquired to establish a geological model of the horizontal well groups, and the geological model includes data from the regional geological data. Based on the data in the geological model, the well spacing between two adjacent horizontal well sections is set, the length of the horizontal well section is set, and the perforation location of the fracturing section is selected.

15. The method for extracting coalbed methane from under-pressured coal reservoirs using horizontal well groups according to claim 14, characterized in that, The regional geological data include coal reservoir pressure, average porosity of coal and rock, maximum principal stress and direction in the horizontal plane of the coal reservoir, fault and fracture development characteristics, coal permeability, lithology of the roof and floor, regional coal structure distribution, and average comprehensive compression coefficient of coal and rock.

16. The method for extracting coalbed methane from under-pressured coal reservoirs using horizontal well groups according to claim 14, characterized in that, Also includes: The geological reserve volume utilized within the horizontal well group is calculated based on the data in the geological model. Based on the geological reserve volume and the well spacing, the length of the fracturing section and the fracturing operation parameters for each fracturing section are optimized using regional fracture monitoring data and simulation methods with FracproPT or Meyer software. These fracturing operation parameters include the maximum injection rate P for the main fracturing operation and the total fluid volume Q injected for each fracturing section and the total amount of proppant N added. The total fluid volume Q includes the fluid volume Q1 injected during the injection-energized fracturing stage and the fluid volume Q2 injected during the main fracturing stage. The total amount of proppant N includes the proppant dosage N1 added during the injection-energized fracturing stage and the proppant dosage N2 added during the main fracturing stage.

17. The method for extracting coalbed methane from under-pressured coal reservoirs using horizontal well groups according to claim 16, characterized in that, The formula for calculating the volume of the geological reserves 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 of all fracturing sections; h is the average effective thickness of the coal reservoir, m.

18. The method for extracting coalbed methane from a horizontal well group in an under-pressured coal reservoir according to claim 16, characterized in that, The formula for calculating the fluid volume Q1 in the fluid injection and charging 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 section, in m 3 The controlled reservoir volume is the product of the expected length of the area occupied by natural fractures on both sides of each fracturing section, the expected width of the area occupied by natural fractures on both sides of each fracturing section, and the average effective thickness of the coal reservoir; φ is the average porosity of the coal and rock, %; C f The average comprehensive compressibility coefficient of coal and rock, in MPa -1 ΔP is the increase in coal reservoir pressure after the injection and fracturing, in MPa.

19. The method for extracting coalbed methane from a horizontal well group in an under-pressured coal reservoir according to claim 16, characterized in that, The injection rate for the main fracturing operation is controlled within the range of 4~20m. 3 / min.

20. The method for extracting coalbed methane from a horizontal well group in an under-pressured coal reservoir according to claim 16, characterized in that, The injection rate for the main fracturing operation is set to start from an initial rate of 4 m³ / s. 3 After a certain number of minutes, gradually increase the flow rate to the maximum injection flow rate P.

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