On-line variable-viscosity fracturing method for high-order coal reservoir

Abstract

The invention discloses an online variable-viscosity fracturing method for a high-order coal reservoir, which comprises the following steps: sequentially carrying out a perforation stage, a prepad fluid stage, a sand-carrying fluid stage and a displacing fluid stage by adopting a bridge-shooting combined fracturing process; in the early stage of the prepad fluid stage, clear water is adopted as fracturing fluid, and the displacement is increased in a stepped mode according to the first incremental quantity; in the later stage of the prepad fluid stage, the fracturing fluid system with the first concentration is adopted, and the displacement is continuously increased to the maximum displacement in a stepped mode with the second increasing amount larger than the first increasing amount; in the early stage of the sand-carrying fluid stage, a fracturing fluid system with the first concentration is adopted, and the maximum displacement is kept; in the middle stage of the sand-carrying fluid stage, the fracturing fluid system with the second concentration is adopted, and the maximum displacement is kept; in the later stage of the sand-carrying fluid stage, a fracturing fluid system with the third concentration is adopted, and the maximum displacement is kept; the first concentration, the second concentration and the third concentration are sequentially increased or decreased. According to the method, the fracture length and the fracture network complexity can be effectively increased, and then the coal reservoir transformation volume is enlarged.

Description

Technical Field

[0001] The present invention relates to the technical field of coalbed methane mining, and in particular to an online variable viscosity fracturing method for high-rank coal reservoirs. Background Art

[0002] High-rank coal reservoirs refer to lean coal and anthracite with a vitrinite reflectance greater than 1.9%. They are distributed in a relatively limited area in my country, mainly in Shanxi, Guizhou and southern Sichuan, represented by the Qinshui Basin. High-rank coal has the characteristics of high degree of thermal evolution, high gas content, strong reservoir heterogeneity, and extremely low coal seam permeability. In order to achieve efficient development of high-rank coal reservoirs, reservoir transformation is necessary, and hydraulic fracturing is the most important means of transformation.

[0003] The high-rank coal in the Qinshui coalbed methane field has entered the middle stage of development. The main development well type is horizontal wells. The transformation method has been determined to be mainly bridge-shot joint fracturing, supplemented by ordinary tubing drag fracturing and continuous tubing drag fracturing. The fracturing fluid is mainly active water fracturing fluid. With the advancement of the development process, during the fracturing of high-rank coal in the middle and deep layers in the southern Qinshui Basin, there have been difficulties in increasing the displacement and adding sand. The actual displacement during the construction process is quite different from the designed displacement, which means that the current transformation process cannot adapt to the high-stress coal reservoirs in the middle and deep layers, and the fracturing process needs to be optimized. The active water fracturing fluid currently used in fracturing construction has low viscosity, high friction during fracturing, and a single fracture morphology. It is difficult to achieve the goal of forming a large-scale complex fracture network by relying solely on active water fracturing fluid. Summary of the invention

[0004] The main purpose of the present invention is to provide an online variable viscosity fracturing method for high-rank coal reservoirs to solve the problems of difficulty in increasing displacement, easy sand plugging and small scale of transformation during the fracturing of medium-deep high-rank coal reservoirs.

[0005] According to one aspect of the present invention, an online variable viscosity fracturing method for high-rank coal reservoirs is proposed, comprising: adopting a bridge-shot combined fracturing process, sequentially performing a perforation stage, a pre-fluid stage, a sand-carrying fluid stage, and a displacement fluid stage; wherein

[0006] In the early stage of the pre-fluid stage, clean water is used as the fracturing fluid and the displacement is increased stepwise in a first increment;

[0007] At the end of the pre-fluid stage, a fracturing fluid system having a first concentration is used and the displacement is continuously increased stepwise to a maximum displacement with a second increment greater than the first increment;

[0008] In the early stage of the sand-carrying fluid stage, a fracturing fluid system having the first concentration is used and the maximum displacement is maintained;

[0009] In the middle of the sand-carrying fluid stage, a fracturing fluid system having a second concentration is used and the maximum displacement is maintained;

[0010] In the later stage of the sand-carrying fluid stage, a fracturing fluid system having a third concentration is used and the maximum displacement is maintained;

[0011] Among them, when the well spacing is greater than the predetermined spacing value, the first concentration, the second concentration and the third concentration increase successively; when the well spacing is less than the predetermined spacing value, the first concentration, the second concentration and the third concentration decrease successively.

[0012] According to one embodiment of the present invention, the first increment is 1 cubic meter / minute, the second increment is 2 cubic meters / minute, the maximum displacement is 15 cubic meters / minute; and / or the predetermined spacing value is 250m; and / or the first concentration, the second concentration and the third concentration are between 0.05% and 0.2%.

[0013] According to one embodiment of the present invention, the fracturing fluid system comprises a non-damaging gemini hydrophobic association fracturing fluid, a gel breaker and an anti-swelling agent.

[0014] According to one embodiment of the present invention, in the early and late stages of the prefluid stage, the slugs are made of quartz sand of 40-70 mesh; in the early stages of the sand-carrying fluid stage, the proppant is made of quartz sand of 40-70 mesh; in the middle and late stages of the sand-carrying fluid stage, the proppant is made of quartz sand of 20-40 mesh.

[0015] According to one embodiment of the present invention, in the early and late stages of the pre-fluid stage, 1 cubic meter of sand is added to each segment plug, and the sand ratio is 5%; in the early stage of the sand-carrying fluid stage, the sand ratio is increased stepwise from 8% to 12%, and 80 cubic meters of quartz sand is added; in the middle stage of the sand-carrying fluid stage, the sand ratio is increased stepwise from 10% to 15%, and 110 cubic meters of quartz sand is added; in the late stage of the sand-carrying fluid stage, the sand ratio is increased to 18%, and 10 cubic meters of quartz sand is added.

[0016] According to one embodiment of the present invention, the amount of pre-liquid accounts for 30%-35% of the total liquid volume.

[0017] According to one embodiment of the present invention, it also includes:

[0018] Conduct fracturing simulation based on reservoir basic data and requirements for fracture geometry and conductivity;

[0019] Based on the reservoir basic data and the fracturing simulation results, determine whether to cluster and the number of clusters and the cluster spacing under clustering;

[0020] Determine relevant parameters of the perforation stage based on the fracturing simulation results, logging data, and drilling and completion data;

[0021] The relevant parameters of the pre-fluid stage, the sand-carrying fluid stage and the displacement fluid stage are determined based on the fracturing simulation results.

[0022] According to one embodiment of the present invention, when the horizontal stress difference of the well section is less than 5MPa, the well section is not clustered; when the horizontal stress difference of the well section is greater than or equal to 5MPa, the well section is constructed in 2-3 clusters with a cluster spacing of 10-20m.

[0023] According to one embodiment of the present invention, during the perforation stage, directional perforation is performed, the perforation section length is 1-2m, and a combination of 102 guns and 127 bullets is used; when the wellbore is located at the lower part of the coal seam, perforation is performed at 30° upward; when the wellbore is located in the middle of the coal seam, perforation is performed horizontally; when the wellbore is located at the upper part of the coal seam, perforation is performed downward at 30°.

[0024] According to one embodiment of the present invention, it also includes: after all the layers are fractured and the well is shut down, the wellhead pressure is detected after a predetermined time; when the wellhead pressure is greater than 10MPa, a 3mm oil nozzle is used for blowout; when the wellhead pressure is 10-5MPa, a 5mm oil nozzle is used for blowout; when the wellhead pressure is 5-1MPa, an 8mm oil nozzle is used for blowout; when the wellhead pressure is less than 1MPa, the oil pipe is opened for blowout.

[0025] In the technical solution of the present invention, by adopting different fracturing fluid systems and displacements at different stages, the length of the fractures and the complexity of the fracture network can be effectively increased, thereby expanding the volume of the coal reservoir transformation. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] 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.

[0027] Figure 1 A flow chart showing an online viscosity-changing fracturing method for a high-rank coal reservoir according to an embodiment of the present invention;

[0028] Figure 2 A construction schematic diagram of an online variable viscosity fracturing method for a high-rank coal reservoir according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0029] 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.

[0030] 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.

[0031] The present invention proposes an online variable viscosity fracturing method for high-rank coal reservoirs, comprising: adopting a bridge-shot combined fracturing process, sequentially performing a perforation stage, a pre-fluid stage, a sand-carrying fluid stage, and a displacement fluid stage; wherein

[0032] In the early stage of the pre-fluid stage, clean water is used as the fracturing fluid and the displacement is increased stepwise in a first increment;

[0033] At the end of the pre-fluid stage, a fracturing fluid system having a first concentration is used and the displacement is continuously increased stepwise to a maximum displacement with a second increment greater than the first increment;

[0034] In the early stage of the sand-carrying fluid stage, a fracturing fluid system having the first concentration is used and the maximum displacement is maintained;

[0035] In the middle of the sand-carrying fluid stage, a fracturing fluid system having a second concentration is used and the maximum displacement is maintained;

[0036] In the later stage of the sand-carrying fluid stage, a fracturing fluid system having a third concentration is used and the maximum displacement is maintained;

[0037] Among them, when the well spacing is greater than the predetermined spacing value, the first concentration, the second concentration and the third concentration increase successively; when the well spacing is less than the predetermined spacing value, the first concentration, the second concentration and the third concentration decrease successively.

[0038] As mentioned in the above background technology, in the prior art, high-order coalbed methane fracturing fluid usually adopts active water fracturing fluid, which ensures low damage to coal reservoirs, but cannot adapt to the needs of coal reservoir transformation in high ground stress areas. In order to fully transform the coal reservoir, it is necessary to increase the displacement. However, in the process of increasing the displacement, the friction resistance continues to increase, and there will be difficulties in increasing the displacement, and the pump will stop due to overpressure, and the large-scale transformation of the reservoir cannot be achieved. Therefore, the present invention introduces a harmless fracturing fluid system in the later stage of the pre-fluid, which ensures that there is no damage to the reservoir, reduces the friction resistance in the seam making process, and quickly increases the fracturing construction displacement, reaching the maximum construction displacement in the pre-fluid stage. In the sand-carrying fluid stage, a large displacement is maintained to add sand, and there is no need to configure harmless fracturing fluid in advance. The concentration of the fracturing fluid polymer is adjusted on site to achieve online viscosity, increase the sand ratio, increase the concentration, and increase the viscosity, ensuring the transformation of the coal reservoir with increased displacement, increased sand volume, and increased liquid volume, realizing three-lift transformation, expanding the transformation volume, improving the reservoir permeability, and establishing a good gas production channel. Compared with active water bridge shot fracturing, the present invention can form a complex fracture network, increase the fracture length by 50%, increase the transformation control volume by 31%, and realize large-scale transformation of coal reservoirs.

[0039] In an embodiment of the present invention, clean water is used as the fracturing fluid in the early stage of the prefluid stage. This is because: compared with other fracturing fluids, clean water can reduce the damage to the coal reservoir, and in the early stage of the prefluid, the displacement is low and no sand is added, and there is no need to add liquid that can reduce friction, and clean water can meet the requirements.

[0040] In the embodiment of the present invention, a larger incremental step-wise displacement is adopted in the later stage of the pre-fluid stage, that is, the displacement is increased faster, which has the effect of increasing the static pressure in the seam, improving the seam-making capacity and the in-situ fracturing effect.

[0041] In the embodiments of the present invention, fracturing fluid positive mixing (the first concentration, the second concentration and the third concentration increase in sequence) and fracturing fluid reverse mixing (the first concentration, the second concentration and the third concentration decrease in sequence) are respectively adopted for the cases where the well spacing is large and small. The reason is that: when the well spacing is large, in order to increase the fracture length, the displacement and sand ratio must be continuously increased. Increasing the displacement and sand ratio will lead to increased friction resistance during the sand addition process, which is prone to sand blockage and pump stop. Therefore, it is necessary to increase the fracturing fluid concentration, reduce friction resistance, and ensure the scale of transformation; when the well spacing is small, in order to avoid connection with the fractures of adjacent wells, a high concentration is initially used to quickly increase the fracture length, and then the viscosity of the fracturing fluid is reduced, the friction resistance increases, the ability to create long fractures is weakened, and the fracture network expands laterally.

[0042] In some embodiments, the first incremental amount is 1 cubic meter / minute, the second incremental amount is 2 cubic meter / minute, and the maximum displacement is 15 cubic meter / minute. In the early stage of the pre-liquid stage, the displacement starts with a low displacement of 2 cubic meter / minute, and the displacement is increased step by step to 6-7 cubic meter / minute; in the late stage of the pre-liquid stage, the displacement is increased step by step from 7-9 cubic meter / minute to 15 cubic meter / minute. The sand volume designed in the present invention is 200 cubic meters. Small displacement will cause sand blockage in the sand-carrying liquid stage, affecting the amount of sand added, and further affecting the scope of transformation. Therefore, compared with the prior art, the present invention increases the maximum displacement to 15 cubic meter / minute, increases the scale of sand addition, and expands the transformation volume.

[0043] In some embodiments, the predetermined spacing value is 250m. In some embodiments, the first concentration, the second concentration and the third concentration are between 0.05% and 0.2%. For example, when the well spacing is greater than 250m, the first concentration is 0.05%, the second concentration is 0.1%, and the third concentration is 0.2%; when the well spacing is less than 250m, the first concentration is 0.2%, the second concentration is 0.15%, and the third concentration is 0.1%. The concentration of the fracturing fluid system may refer to the percentage of the mass of the concentrated fracturing fluid used to prepare the fracturing fluid system to the total mass of the fracturing fluid system.

[0044] In some embodiments, the fracturing fluid system comprises a non-harmful Gemini hydrophobic association fracturing fluid (which comprises a Gemini hydrophobic monomer, acrylamide and acrylic acid), a gel breaker and an anti-swelling agent. For example, the fracturing fluid system may comprise 0.05%-0.2% (as described above, the concentration value is adjusted between 0.05%-0.2% according to different fracturing stages) of a non-harmful Gemini hydrophobic association fracturing fluid, 0.03% of a gel breaker and 0.5% of an anti-swelling agent, and the remainder is water.

[0045] In some embodiments, in the early and late stages of the pre-fluid phase, the slug uses quartz sand of 40-70 mesh; in the early stages of the sand-carrying fluid phase, the proppant uses quartz sand of 40-70 mesh; in the middle and late stages of the sand-carrying fluid phase, the proppant uses quartz sand of 20-40 mesh. During the slug phase, the displacement is low, and a small-size proppant is used to polish the blasthole. In the late stage of the sand-carrying fluid, medium sand is used to seal the seam opening and improve the conductivity of the main fracture.

[0046] In some embodiments, in the early and late stages of the pre-fluid stage, each slug is added with 1 cubic meter of sand, and the sand ratio is 5%; in the early stage of the sand-carrying fluid stage, the sand ratio is increased from 8% to 12% in a stepwise manner, and 80 cubic meters of quartz sand are added; in the middle stage of the sand-carrying fluid stage, the sand ratio is increased from 10% to 15% in a stepwise manner, and 110 cubic meters of quartz sand are added; in the late stage of the sand-carrying fluid stage, the sand ratio is increased to 18%, and 10 cubic meters of quartz sand are added. The slug is added with sand on a small scale, and the perforation holes are polished, so that the subsequent proppant can enter the cracks smoothly. Proppants are formally added at the beginning of the sand-carrying fluid stage, and the sand ratio starts from 8%. If the sand ratio is too small, the support effect is not good. The larger the sand ratio, the larger the extension range of the cracks. However, if the sand ratio is increased to too high, the cracks have not yet expanded on a large scale. The addition of sand with a large sand ratio will cause quartz sand to accumulate in the near-well area and the seam mouth, resulting in too high construction pressure to continue construction. The step-by-step increase in sand ratio can increase the sand ratio while the cracks gradually expand, and the cracks continue to extend forward.

[0047] In an embodiment of the present invention, in the early stage of the sand-carrying fluid, about 80 cubic meters of 40-70 mesh quartz sand are added, and the quartz sand has a small particle size, which ensures that nearly half of the proppant is continuously transported forward to increase the effective support seam length. In the middle stage of the sand-carrying fluid, the proppant is changed to 20-40 mesh quartz sand, and 120 cubic meters of proppant are added. The proppant particle size increases, and while extending forward, the conductivity is increased, further ensuring the transformation effect. In the middle stage of the sand-carrying fluid, the sand ratio is slightly reduced after replacing the large-particle proppant to prevent sand plugging. The predetermined well spacing (e.g., 250m) and the sand amount and sand ratio combination when the fracturing half-slit length reaches a predetermined value (e.g., about 200m) can be obtained by simulation of, for example, FrSmart software.

[0048] In some embodiments, the pre-pad fluid accounts for 30%-35% of the total fluid volume (wherein the fluid volume is measured by volume). A larger pre-pad fluid volume can form a longer fracture, which is beneficial for the entry and transportation of proppants in the later stage.

[0049] In some embodiments, the method further comprises:

[0050] Conduct fracturing simulation based on reservoir basic data and requirements for fracture geometry and conductivity;

[0051] Based on the reservoir basic data and the fracturing simulation results, determine whether to cluster and the number of clusters and the cluster spacing under clustering;

[0052] Determine relevant parameters of the perforation stage based on the fracturing simulation results, logging data, and drilling and completion data;

[0053] The relevant parameters of the pre-fluid stage, the sand-carrying fluid stage and the displacement fluid stage are determined based on the fracturing simulation results.

[0054] In some embodiments, when the horizontal stress difference of the well section is less than 5 MPa, the well section is not clustered; when the horizontal stress difference of the well section is greater than or equal to 5 MPa, the well section is constructed in 2-3 clusters, with a cluster spacing of 10-20 m.

[0055] In some embodiments, during the perforation stage, directional perforation is performed with a perforation section length of 1-2 m, using a combination of 102 guns and 127 bullets; when the wellbore is located at the lower part of the coal seam, perforation is performed at 30° upward; when the wellbore is located in the middle of the coal seam, perforation is performed horizontally; when the wellbore is located at the upper part of the coal seam, perforation is performed downward at 30°.

[0056] In some embodiments, the method also includes: after all layers are fractured and the well is shut down, the wellhead pressure is detected after a predetermined time; when the wellhead pressure is greater than 10MPa, a 3mm oil nozzle is used for blowout; when the wellhead pressure is 10-5MPa, a 5mm oil nozzle is used for blowout; when the wellhead pressure is 5-1MPa, an 8mm oil nozzle is used for blowout; when the wellhead pressure is less than 1MPa, the oil pipe is opened for blowout.

[0057] refer to Figure 1 and Figure 2 In some embodiments, the technical solution adopted by the present application includes the following specific steps:

[0058] (A) Evaluation of basic reservoir characteristics of target layer:

[0059] Analyze basic data such as the development of natural cleats and cracks in the target layer, the magnitude of coal rock in-situ stress, physical properties of coal reservoirs, mechanical properties of the target layer and top and bottom plates, and in-situ stress characteristics of the top and bottom plates.

[0060] (1) Analyze the maximum and minimum horizontal principal stress differences. When the stress difference is less than 5 MPa, consider fracturing to form a fracture network, reduce the number of clusters in each section or do not divide the clusters; when the stress difference is greater than 5 MPa, consider fracturing to form a single principal fracture, and increase the number of clusters in each section;

[0061] (2) Analyze the physical properties of coal reservoirs and select areas with high gas content and low gamma value as fracturing points;

[0062] (3) Analyze the magnitude of the top and bottom plate ground stresses to determine the longitudinal extension of the cracks. If the target layer is thin and the top and bottom plate ground stresses are similar to those of the target layer, control the displacement increase speed to ensure that the cracks extend in the target layer.

[0063] (B) Fracturing design software simulation:

[0064] According to the above analysis results, taking into account the influence of adjacent wells and based on the requirements of fracture geometry and conductivity after fracturing, a series of fracturing-related parameters such as perforation position, displacement, liquid volume, sand ratio in the pumping stage, construction time, fracturing fluid parameters and proppant parameters are determined through fracturing design software under the condition of optimal fracture length and conductivity.

[0065] Specifically, fracturing and development simulation software such as FrSmart, Meyer and Eclipse are used to establish a model based on the basic reservoir parameters in step (A), simulate the fracturing of the entire horizontal well section, consider the location of adjacent wells and the fracturing situation, perform capacity simulation analysis, and optimize the fracturing construction parameters under the maximum capacity conditions.

[0066] (C) Optimization of the number of horizontal well clusters and the distance between clusters:

[0067] According to the magnitude of ground stress and fracturing simulation results, clustered and non-clustered fracturing construction are selected, and the number of clusters and the distance between clusters are determined.

[0068] Specifically, according to the stress difference analysis results of step (A) and the simulation results of step (B), it is determined that the well section with a horizontal stress difference less than 5 MPa is not clustered; the well section with a horizontal stress difference greater than or equal to 5 MPa is constructed in 2-3 clusters, with a cluster spacing of 10-20 m.

[0069] (D) Determination of perforation plan:

[0070] According to the fracturing simulation results, combined with logging data and drilling and completion data, appropriate perforation plan parameters such as perforation location, perforation method, perforation section length and perforation parameters are selected.

[0071] According to the result of step (B), the number of horizontal sections to be transformed under reasonable production capacity conditions is determined. Combined with the position of the horizontal section suitable for transformation selected in step (A), the specific position of the perforation section is determined, and the cable transmission casing perforation method is selected. The perforation parameters are adjusted according to the position of the wellbore in the target layer and the thickness of the coal seam.

[0072] Specifically, the horizontal well adopts directional perforation, with a perforation section length of 1-2m / section, and a combination of 102 guns and 127 bullets. The directional perforation orientation is determined according to the position of the wellbore in the target layer. When the wellbore is located in the lower part of the coal seam, the perforation is performed at 30° upward; when the wellbore is located in the middle of the coal seam, the perforation is performed horizontally; when the wellbore is located in the upper part of the coal seam, the perforation is performed at 30° downward.

[0073] (E) Optimization of pumping parameters design for variable viscosity fracturing process:

[0074] According to the result of step (B), the online variable viscosity fracturing fluid system is preferred, and the harmless concentrated fracturing fluid is added online with the clean water, and the concentration of the fracturing fluid is adjusted at any time. The fracturing method of increasing the displacement, liquid volume and sand volume is adopted to determine the pumping parameters (see Table 1 and Table 2). The design ideas of various parameters in the fracturing process are as follows:

[0075] (1) In order to achieve the purpose of expanding the volume of coal reservoir transformation, the fracturing fluid adopts an online variable viscosity fracturing fluid system, and different fracturing fluid systems and displacements are used in different construction stages:

[0076] (1.1) In the early stage of pre-fluid, a clean water fracturing fluid system is used, starting with a low displacement and increasing the displacement in a step-by-step manner to form the main fracture;

[0077] (1.2.1) Positive mixing construction: low-concentration fracturing fluid system is used in the later stage of pre-fluid to quickly increase the displacement and expand the fracture network;

[0078] (1.2.2) In reverse mixing construction, a higher concentration fracturing fluid system is used in the later stage of the pre-fluid to quickly increase the displacement and quickly create long fractures;

[0079] (1.3) In both forward and reverse mixing construction, the sand-carrying fluid should use a medium-concentration fracturing fluid system in the early stage, with a large displacement to improve the remote sand-carrying capacity;

[0080] (1.4.1) In positive mixing construction, a higher concentration fracturing fluid system is used in the later stage of sand-carrying fluid to maintain a large displacement and improve the shape of the sand bank near the wellbore;

[0081] (1.4.2) In reverse mixing construction, a low-concentration fracturing fluid system is used in the later stage of sand-carrying fluid to maintain a large displacement and increase the lateral range of the fracture;

[0082] (1.5) Inject displacement fluid;

[0083] Among them, in the positive mixing construction plan, (1.1), (1.2.1), (1.3), (1.4.1), and (1.5) are adopted; in the reverse mixing construction plan, (1.1), (1.2.2), (1.3), (1.4.2), and (1.5) are adopted.

[0084] (2) The proppant is selected with different particle size combinations according to the depth of the target layer, and the amount of sand is determined according to the thickness of the coal seam.

[0085] (F) Perforation operation:

[0086] The perforating operation is completed according to step (D), and the perforating position avoids the casing collar.

[0087] (G) Fracturing operations:

[0088] Complete the fracturing operation according to step (E) (see Figure 2 ), adjust the fracturing construction parameters on site according to the construction dynamics.

[0089] Specifically, observe the wellhead pressure. When the pressure increase rate is less than 2MPa / min, maintain the current displacement and sand ratio, do not change the liquid-sand ratio, and continue construction according to the pumping procedure after the pressure stabilizes. When the wellhead pressure rise rate is greater than 2MPa / min, increase the fracturing fluid concentration, or inject isolation fluid, or reduce the sand ratio. After the pressure stabilizes, continue construction according to the pumping procedure.

[0090] (H) The remaining fracturing stages of the horizontal well are constructed.

[0091] (I) After all the layers were fractured, the well was shut down for 5 days. A pressure gauge was installed at the wellhead to observe the pressure drop. After 5 days, a nozzle was selected for blowout according to the wellhead pressure. When the wellhead pressure was greater than 10MPa, a 3mm nozzle was used for blowout; when the wellhead pressure was 10-5MPa, a 5mm nozzle was used for blowout; when the wellhead pressure was 5-1MPa, an 8mm nozzle was used for blowout; when the wellhead pressure was less than 1MPa, the tubing was opened for blowout.

[0092] The specific parameters of the fracturing construction process in a specific embodiment are described below.

[0093] (1) When the well spacing is greater than 250m, online fracturing fluid mixing is used. The specific parameters are shown in Table 1:

[0094] Table 1 Positive Mixing Variable Viscosity Fracturing Pumping Program

[0095]

[0096] (1.1) In the early stage of pre-fluid, clean water is used as the fracturing fluid, and the displacement is increased in a step-by-step manner, with the displacement increased by 1 cubic meter per minute each time until the displacement is increased to 7 cubic meters per minute. 2-3 segment plugs are added, and the segment plugs are made of 40-70 mesh quartz sand. 1 cubic meter of sand is added to each segment plug, and the sand ratio is 5%.

[0097] (1.2) At the later stage of pre-fluid, the fracturing fluid is replaced with a harmless fracturing fluid system. Concentrated fracturing fluid is added to clean water on site, and a harmless fracturing fluid system with a concentration of 0.05% is configured online and injected into the formation. The displacement is increased step by step, with the displacement increased by 2 / minute each time until the displacement is increased to 15 cubic meters / minute. 1-2 segment plugs are added. The segment plugs are made of 40-70 mesh quartz sand. 1 cubic meter of sand is added to each segment plug, and the sand ratio is 5%. The overall proportion of pre-fluid accounts for 30%-35% of the total liquid volume.

[0098] (1.3) In the early stage of the sand-carrying fluid, the displacement was maintained at 15 cubic meters per minute, the proppant used was 40-70 mesh quartz sand, the sand ratio was increased from 8% to 10%, and then to 12%, 80 cubic meters of quartz sand was added, and the fracturing fluid used was 0.05% concentration of non-destructive fracturing fluid.

[0099] (1.4) In the middle stage of sand-carrying fluid, the displacement is maintained at 15 cubic meters per minute, the proppant is replaced with 20-40 mesh quartz sand, the sand ratio is increased from 10% to 12%, and then to 15%, 110 cubic meters of quartz sand is added, and the fracturing fluid is adjusted online to a non-destructive fracturing fluid with a concentration of 0.1%.

[0100] (1.5) In the later stage of the sand-carrying fluid, the displacement is maintained at 15 cubic meters / minute, the proppant is maintained at 20-40 mesh quartz sand, the sand ratio is increased to 18%, 10 cubic meters of quartz sand is added, and the fracturing fluid is adjusted online to a 0.2% concentration of non-destructive fracturing fluid.

[0101] (1.6) Inject displacement fluid, the displacement fluid volume is the wellbore volume plus 2 cubic meters. The total fracturing fluid is about 2500 cubic meters, the total sand volume is 200 cubic meters, the maximum displacement is 15 cubic meters / minute, and the maximum sand ratio is 20%.

[0102] (2) The well network is dense, the well spacing is less than 250m, and the fracturing fluid is mixed online to control the crack length and increase the lateral sweep range of the fracture network. The specific parameters are shown in Table 2:

[0103] Table 2 Inverse Mixing Viscosity Variable Fracturing Pumping Program

[0104]

[0105] (2.1) In the early stage of pre-fluid, clean water is used as the fracturing fluid, and the displacement is increased in a step-by-step manner, with the displacement increased by 1 cubic meter per minute each time until the displacement is increased to 6 cubic meters per minute. Two segment plugs are added, and the segment plugs are made of 40-70 mesh quartz sand. 1 cubic meter of sand is added to each segment plug, and the sand ratio is 5%.

[0106] (2.2) At the later stage of pre-fluid, the fracturing fluid is replaced with a harmless fracturing fluid system. Concentrated fracturing fluid is added to clean water on site, and a 0.2% concentration of harmless fracturing fluid system is configured online and injected into the formation. The displacement is increased step by step, and the displacement is increased by 2 / minute each time until the displacement is increased to 15 cubic meters / minute. 2-3 segment plugs are added. The segment plugs are made of 40-70 mesh quartz sand. Each segment plug is added with 1 cubic meter of sand, and the sand ratio is 5%. The overall proportion of pre-fluid accounts for 30%-35% of the total liquid volume.

[0107] (2.3) In the early stage of the sand-carrying fluid, the displacement was maintained at 15 cubic meters per minute, the proppant used was 40-70 mesh quartz sand, the sand ratio was increased from 8% to 10%, and then to 12%, 80 cubic meters of quartz sand was added, and the fracturing fluid used was 0.2% concentration of non-destructive fracturing fluid.

[0108] (2.4) In the middle stage of the sand-carrying fluid, the displacement was maintained at 15 cubic meters per minute, the proppant was replaced with 20-40 mesh quartz sand, the sand ratio was increased from 10% to 12%, and then to 15%, 110 cubic meters of quartz sand was added, and the fracturing fluid was adjusted online to a non-destructive fracturing fluid with a concentration of 0.15%.

[0109] (2.5) In the later stage of the sand-carrying fluid, the displacement is maintained at 15 cubic meters / minute, the proppant is maintained at 20-40 mesh quartz sand, the sand ratio is increased to 18%, 10 cubic meters of quartz sand is added, and the fracturing fluid is adjusted online to a non-destructive fracturing fluid with a concentration of 0.1%.

[0110] (2.6) Inject displacement fluid, the displacement fluid volume is the wellbore volume plus 2 cubic meters. The total fracturing fluid is about 2500 cubic meters, the total sand volume is 200 cubic meters, the maximum displacement is 15 cubic meters / minute, and the maximum sand ratio is 20%.

[0111] In summary, the present invention innovatively proposes an online variable viscosity fracturing process. The new process is the first to apply a non-damaging fracturing fluid system in the transformation of medium-deep high-stress high-rank coal reservoirs. The viscosity changes online, and with the increase of displacement and sand ratio, the concentration of the fracturing fluid is increased, and the friction resistance is reduced, forming a three-increase transformation method of increasing displacement, increasing sand volume, and increasing fluid volume. At the same time, the non-damaging fracturing fluid does not harm the reservoir, realizing non-damaging large-scale transformation of high-rank coal reservoirs.

[0112] 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. An online viscosity-changing fracturing method for high-rank coal reservoirs, characterized in that: include: The bridge-shot fracturing process is adopted, and the perforation stage, the pre-fluid stage, the sand-carrying fluid stage and the displacement fluid stage are carried out in sequence; in In the early stage of the pre-fluid stage, clean water is used as the fracturing fluid and the displacement is increased stepwise in a first increment; At the end of the pre-fluid stage, a fracturing fluid system having a first concentration is used and the displacement is continuously increased stepwise to a maximum displacement with a second increment greater than the first increment; In the early stage of the sand-carrying fluid stage, a fracturing fluid system having the first concentration is used and the maximum displacement is maintained; In the middle of the sand-carrying fluid stage, a fracturing fluid system having a second concentration is used and the maximum displacement is maintained; In the later stage of the sand-carrying fluid stage, a fracturing fluid system having a third concentration is used and the maximum displacement is maintained; Among them, when the well spacing is greater than the predetermined spacing value, the first concentration, the second concentration and the third concentration increase successively; when the well spacing is less than the predetermined spacing value, the first concentration, the second concentration and the third concentration decrease successively.

2. The method according to claim 1, characterized in that The first increment is 1 cubic meter / minute, the second increment is 2 cubic meters / minute, the maximum displacement is 15 cubic meters / minute; and / or the predetermined spacing value is 250m; and / or the first concentration, the second concentration and the third concentration are between 0.05% and 0.2%.

3. The method according to claim 1, characterized in that The fracturing fluid system comprises a non-harmful gemini hydrophobic associating fracturing fluid, a gel breaker and an anti-swelling agent.

4. The method according to claim 1, characterized in that: In the early and late stages of the pre-fluid phase, the segment plugs are made of quartz sand of 40-70 meshes; in the early stages of the sand-carrying fluid phase, the proppant is made of quartz sand of 40-70 meshes; in the middle and late stages of the sand-carrying fluid phase, the proppant is made of quartz sand of 20-40 meshes.

5. The method according to claim 1, characterized in that In the early and late stages of the pre-fluid stage, 1 cubic meter of sand was added to each segment plug, and the sand ratio was 5%; in the early stage of the sand-carrying fluid stage, the sand ratio was increased stepwise from 8% to 12%, and 80 cubic meters of quartz sand were added; in the middle stage of the sand-carrying fluid stage, the sand ratio was increased stepwise from 10% to 15%, and 110 cubic meters of quartz sand were added; in the late stage of the sand-carrying fluid stage, the sand ratio was increased to 18%, and 10 cubic meters of quartz sand were added.

6. The method according to claim 1, characterized in that The amount of pre-fluid accounts for 30%-35% of the total fluid volume.

7. The method according to claim 1, characterized in that Also includes: Conduct fracturing simulation based on reservoir basic data and requirements for fracture geometry and conductivity; Based on the reservoir basic data and the fracturing simulation results, determine whether to cluster and the number of clusters and the cluster spacing under clustering; Determine relevant parameters of the perforation stage based on the fracturing simulation results, logging data, and drilling and completion data; The relevant parameters of the pre-fluid stage, the sand-carrying fluid stage and the displacement fluid stage are determined based on the fracturing simulation results.

8. The method according to claim 1, characterized in that When the horizontal stress difference of the well section is less than 5MPa, the well section is not divided into clusters; when the horizontal stress difference of the well section is greater than or equal to 5MPa, the well section is constructed in 2-3 clusters with a cluster spacing of 10-20m.

9. The method according to claim 1, characterized in that: During the perforating stage, directional perforating is carried out with a perforating section length of 1-2m, using a combination of 102 guns and 127 bullets; when the wellbore is located at the lower part of the coal seam, perforating is performed at 30° upward; when the wellbore is located in the middle of the coal seam, perforating is performed horizontally; when the wellbore is located at the upper part of the coal seam, perforating is performed downward at 30°.

10. The method according to claim 1, characterized in that Also includes: After all the layers are fractured, the well is shut down and the wellhead pressure is tested after a predetermined period of time. When the wellhead pressure is greater than 10MPa, a 3mm nozzle is used for blowout. When the wellhead pressure is 10-5MPa, a 5mm nozzle is used for blowout. When the wellhead pressure is 5-1MPa, an 8mm nozzle is used for blowout. When the wellhead pressure is less than 1MPa, the oil pipe is opened for blowout.

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