A horizontal well fracturing experimental device and method considering inter-cluster stress differences

Abstract

The present invention discloses a horizontal well fracturing experimental method that takes into account inter-cluster stress differences, including the following steps: determining a rock sample partition casting scheme, a stress loading method and size of a true triaxial fracturing device; using finite element software to conduct static stress analysis, and determining the rock mechanical parameters corresponding to each area of ​​the partitioned cast rock sample; configuring concrete slurries of different proportions to make artificial rock samples; loading the artificial rock samples on the true triaxial fracturing device, applying triaxial stress, turning on the injection system, and completing the fracturing experiment. The present invention also discloses a horizontal well fracturing experimental device that takes into account inter-cluster stress differences. The present invention adopts the above-mentioned horizontal well fracturing experimental device and method that takes into account inter-cluster stress differences, and realizes differentiated transmission of inter-cluster stress during triaxial stress loading by preparing rock samples with different mechanical parameters in the lateral direction, which can meet the requirements of horizontal well multi-cluster fracture synchronous expansion experiments for heterogeneous loading of inter-cluster stress.

Description

Technical Field

[0001] The present invention relates to the technical field of hydraulic fracturing, and in particular to a horizontal well fracturing experimental device and method taking into account stress differences among clusters. Background Art

[0002] In the long geological history, due to the combined influence of sedimentation, diagenesis and later tectonic action, the stress heterogeneity of oil and gas reservoirs is serious. As one of the main technical methods for the efficient development of unconventional low-permeability oil and gas reservoirs at this stage, the horizontal well segmented multi-cluster fracturing technology may have large differences in the minimum horizontal stress between each perforation cluster in a single fracturing segment, resulting in some perforation clusters in the segment not being opened or difficult to start and expand, the development of multiple clusters of fractures is poorly balanced, and the geological reserves cannot be effectively mobilized. Therefore, the study of the effect of inter-cluster stress heterogeneity on the balanced expansion law of multiple clusters of fractures in horizontal wells has important guiding significance for the efficient development of unconventional oil and gas.

[0003] At present, large-scale true triaxial fracturing physical simulation experiments are one of the most effective technical means to study the expansion law of hydraulic fractures. Based on the existing conventional true triaxial fracturing equipment specifications, if the influence of inter-cluster stress differences is to be considered, multiple sets of triaxial stress loading components need to be added, which greatly increases the experimental cost and complexity. Therefore, at this stage, no relevant information has been retrieved on the physical simulation experimental research on the influence of inter-cluster stress differences on the balanced expansion law of multi-cluster fractures.

[0004] The existing true triaxial hydraulic fracturing experimental device and method fail to realize the study of the influence of inter-cluster stress difference on the balanced expansion law of multi-cluster fractures. Therefore, it is urgent to propose a horizontal well fracturing experimental device and method considering inter-cluster stress difference to meet the demand of hydraulic fracturing experiment for considering inter-cluster stress difference. Summary of the invention

[0005] The purpose of the present invention is to provide a horizontal well fracturing experimental device and method that takes into account the stress differences between clusters. By preparing rock samples with different mechanical parameters in the lateral direction, the differentiated transfer of stress between clusters during triaxial stress loading is realized, which can meet the needs of horizontal well multi-cluster fracture synchronous expansion experiments for heterogeneous loading of stress between clusters.

[0006] To achieve the above object, the present invention provides a horizontal well fracturing experimental method taking into account the stress difference between clusters, comprising the following steps:

[0007] Step 1: Determine the rock sample zoning pouring plan, the stress loading method and size of the true triaxial fracturing equipment;

[0008] Step 2: Use finite element software to conduct static stress analysis and determine the rock mechanics parameters corresponding to each area of ​​the partitioned cast rock sample;

[0009] Step 3: Based on the rock mechanics parameters in step 2, concrete slurries with different proportions are prepared, and the concrete slurries are poured into the experimental device to prepare artificial rock samples;

[0010] Step 4: Load the artificial rock sample on the true triaxial fracturing equipment, apply the minimum horizontal geostress X, the maximum horizontal geostress Y, and the vertical geostress Z three-dimensional stress, start the injection system, and complete the fracturing experiment.

[0011] Preferably, in step 1, the rock sample partition casting plan includes the number and size of the rock sample partitions, and the number and size of the rock sample partitions are determined according to the number of clusters and cluster spacing required by the experiment.

[0012] Preferably, in step one, the stress loading method of the true triaxial fracturing equipment is to clarify the minimum horizontal geostress X, the maximum horizontal geostress Y, and the vertical geostress Z, and the loading method adopted is one-way piston cylinder loading or opposing piston cylinder loading.

[0013] Preferably, in step 2, the specific process of using finite element software to carry out static stress analysis is as follows:

[0014] S2-1, establish equal-size models based on the rock sample partition casting plan;

[0015] S2-2, set the same stress loading mode and magnitude according to the stress loading conditions of the true triaxial equipment;

[0016] S2-3, perform meshing, using high-quality meshing method;

[0017] S2-4. Run the calculation example to calculate the stress conditions of different rock mechanics parameter combinations in each area of ​​the rock sample;

[0018] S2-5. Select the best combination of rock mechanics parameters that meets the experimental requirements and has the best inter-cluster stress difference.

[0019] A horizontal well fracturing experimental device considering inter-cluster stress differences comprises a true triaxial fracturing device and a sample preparation device, wherein the sample preparation device comprises a bottom plate, a front cover plate, a rear cover plate, a left cover plate, a right cover plate, fixing bolts, and a wellbore, wherein one end of the front cover plate and the right cover plate are fixedly connected, and one end of the rear cover plate and the left cover plate are fixedly connected, and the front cover plate, the right cover plate, the rear cover plate, and the left cover plate are respectively connected and fixed to the base by fixing bolts to form a casting cavity, and a through hole is provided in the middle of the bottom plate, and the wellbore is pre-buried and enters the casting cavity through the through hole.

[0020] Preferably, the diameter of the through hole is larger than the inner diameter of the wellbore and smaller than the outer diameter of the wellbore.

[0021] Preferably, the fixing bolt cooperates with the wellbore through the through hole so that the end face of the wellbore is in close contact with the inner side of the bottom plate.

[0022] Preferably, the outer diameter of the front end of the fixing bolt is consistent with the inner diameter of the wellbore, and the outer diameter of the rear end of the fixing bolt is larger than the diameter of the through hole.

[0023] The advantages and positive effects of the horizontal well fracturing experimental device and method according to the present invention that take into account the stress difference between clusters are:

[0024] Finite element software is used to determine the rock mechanical parameters of each area of ​​the rock sample and prepare the corresponding artificial rock samples. This can achieve differentiated stress loading between clusters under the stress loading conditions of existing fracturing equipment. By adjusting the combination of rock mechanical parameters in each area of ​​the rock sample, the size of the stress difference between clusters can be changed, which fills the gap in existing research methods, avoids the modification of true triaxial fracturing equipment, and reduces experimental costs.

[0025] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the structure of the sample preparation device in the experimental device of the present invention;

[0027] Figure 2 It is a cross-sectional schematic diagram of the cooperation between the wellbore, the bottom plate and the fixing bolts in the sample preparation device of the present invention;

[0028] Figure 3 A schematic diagram of a stress loading method during stress analysis simulation in an embodiment of the present invention;

[0029] Figure 4 The stress simulation results of the rock sample at three perforation clusters under the experimental pressure in the embodiment of the present invention;

[0030] Figure 5 It is a flow chart of the experimental method of the present invention;

[0031] Figure 6 This is a flow chart of using software to carry out static stress analysis and calculate Young's modulus in different areas of a rock sample in the experimental method of the present invention;

[0032] Figure 7 The injection pressure curves and flow rate curves of each cluster in the three-cluster fracture fracturing experiment considering the stress difference between clusters in the embodiment of the present invention;

[0033] Figure 8 The injection pressure curves of the three-cluster fracture fracturing experiment and the flow rate curves of each cluster are shown without considering the stress difference between clusters.

[0034] Reference numerals

[0035] 1. Base plate; 2. Front cover plate; 3. Left cover plate; 4. Rear cover plate; 5. Right cover plate; 6. Fixing bolts; 7. Shaft; 8. Through hole. DETAILED DESCRIPTION

[0036] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0037] Unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0038] Example

[0039] like Figure 1 and Figure 2 As shown, a horizontal well fracturing experimental device considering inter-cluster stress differences includes a true triaxial fracturing device and a sample preparation device. The true triaxial fracturing device (not shown in the figure) is a conventional true triaxial fracturing device in the prior art. The sample preparation device includes a bottom plate 1, a front cover plate 2, a rear cover plate 4, a left cover plate 3, a right cover plate 5, a fixing bolt 6, and a wellbore 7. The front cover plate 2 and the right cover plate 5 are fixedly connected at one end, and the rear cover plate 4 and the left cover plate 3 are fixedly connected at one end. The front cover plate 2, the right cover plate 5, the rear cover plate 4, and the left cover plate 3 are respectively connected and fixed with the base through fixing bolts 6 to form a casting cavity. A through hole 8 is provided in the middle of the bottom plate 1, and the wellbore 7 is pre-buried and enters the casting cavity through the through hole 8. The diameter of the through hole 8 is larger than the inner diameter of the wellbore 7 and smaller than the outer diameter of the wellbore 7. The fixing bolt 6 cooperates with the wellbore 7 through the through hole 8, so that the end face of the wellbore 7 is close to the inner side of the bottom plate 1. The front end outer diameter of the fixing bolt 6 is consistent with the inner diameter of the shaft 7, and the rear end outer diameter of the fixing bolt 6 is larger than the diameter of the through hole 8. When in use, the front cover plate 2, the right cover plate 5, the rear cover plate 4, the left cover plate 3, and the shaft 7 are respectively assembled with the base to form a casting cavity, and the mixed and configured concrete slurry is poured into the casting cavity. After the concrete slurry solidifies, the fixing bolts are removed, and the front cover plate 2, the right cover plate 5, the rear cover plate 4, the left cover plate 3, and the shaft 7 are removed. The solidified concrete slurry is the artificial rock sample, which is taken out for subsequent use.

[0040] A horizontal well fracturing experimental method considering inter-cluster stress differences comprises the following steps:

[0041] Step 1: Determine the rock sample zoning pouring plan, the stress loading method and size of the true triaxial fracturing equipment;

[0042] Step 2: Use finite element software to conduct static stress analysis and determine the rock mechanics parameters corresponding to each area of ​​the partitioned cast rock sample;

[0043] Step 3: Based on the rock mechanics parameters in step 2, concrete slurries of different proportions are configured, and the concrete slurries are poured into the pouring cavity of the sample preparation device of the experimental device to prepare artificial rock samples;

[0044] Step 4: Load the artificial rock sample on the true triaxial fracturing equipment, apply the minimum horizontal geostress X, the maximum horizontal geostress Y, and the vertical geostress Z three-dimensional stress, start the injection system, and complete the fracturing experiment.

[0045] In step 1, the rock sample partition casting plan includes the number and size of the rock sample partitions, and the number and size of the rock sample partitions are determined according to the number of clusters and cluster spacing required by the experiment.

[0046] In step 1, the stress loading method of the true triaxial fracturing equipment is to clarify the minimum horizontal geostress X, the maximum horizontal geostress Y, and the vertical geostress Z. The loading method adopted is one-way piston cylinder loading or opposing piston cylinder loading.

[0047] In step 2, the specific process of using finite element software to carry out static stress analysis is as follows:

[0048] S2-1, establish equal-size models based on the rock sample partition casting plan;

[0049] S2-2, set the same stress loading mode and magnitude according to the stress loading conditions of the true triaxial equipment;

[0050] S2-3, perform meshing, using high-quality meshing method;

[0051] S2-4. Run the calculation example to calculate the stress conditions of different rock mechanics parameter combinations in each area of ​​the rock sample;

[0052] S2-5. Select the best combination of rock mechanics parameters that meets the experimental requirements and has the best inter-cluster stress difference.

[0053] The rock sample size is 300×300×600mm, the number of clusters is 3, the cluster spacing is 20cm, the maximum horizontal geostress Y and vertical geostress Z are 20MPa, the minimum horizontal geostress X is 10MPa, and the stress difference between clusters is set to 1.3MPa. The equipment used in the experiment is a conventional large-scale true triaxial fracturing experimental equipment, and the three-dimensional stress loading method is: there is only one loading cylinder on one side of the X (minimum horizontal geostress) direction, and the other side is fixed; there are loading cylinders on both sides of the Y and Z (maximum horizontal geostress, vertical geostress) directions.

[0054] like Figure 5As shown in the figure, first, according to the number of clusters and cluster spacing required by the experiment, the rock sample partition casting scheme is determined as follows: the artificial rock sample is divided into 3 areas horizontally, and the width of each area is 20 cm. The stress loading method of the true triaxial fracturing equipment is: the X (minimum horizontal ground stress) direction is a one-way piston cylinder loading, the size is 10MPa, and the other side is a fixed boundary; the Y and Z (maximum horizontal ground stress, vertical ground stress) directions are opposite piston cylinder loading, the size is 20MPa (such as Figure 3 shown).

[0055] Secondly, SolidWorks software was selected to carry out static stress analysis to determine the rock mechanical parameters of each area of ​​the rock sample. The specific process is as follows (e.g. Figure 6 shown):

[0056] (1) Establish a rock sample model of equal size, with a size of 300×300×600 mm; the model is divided into three areas, and the width of each area is 200 mm;

[0057] (2) According to the three-dimensional stress loading method and the magnitude of the three-dimensional stress of the experimental equipment, it is determined that a uniform pressure of 10 MPa is applied to the left end of the rock sample in the X direction, and no pressure is applied to the right end, using a fixed displacement method; a uniform pressure of 20 MPa is applied to both ends of the Y and Z directions (such as Figure 3 shown);

[0058] (3) Meshing, using high-quality meshes and maximum density meshing methods;

[0059] (4) Run the example to calculate the stress conditions in different regions of the rock sample under different rock mechanics parameter combinations;

[0060] (5) Combined with the simulation results, according to the experimental requirement of the inter-cluster stress difference of 1.3 MPa, the rock mechanics parameter combination that meets the experimental requirements of the inter-cluster stress difference is selected as the best solution, that is, the rock mechanics parameters (Young's modulus) of each region are 20 GPa, 16 GPa, and 24 GPa, respectively, which can achieve differentiated loading of the minimum horizontal stress at the location of the three clusters of fracture surfaces. The minimum horizontal stresses on the main parts of the three clusters of fracture surfaces are 6.5 MPa, 7.8 MPa, and 9.1 MPa, respectively (e.g. Figure 4 shown).

[0061] Then, concrete slurries with different proportions were prepared and poured into the pouring cavity of the sample preparation device of the experimental device to prepare artificial rock samples with Young's modulus of 20 GPa, 16 GPa and 24 GPa in each region, respectively.

[0062] Finally, the artificial rock sample was loaded on the true triaxial fracturing equipment, and triaxial stress was applied (10 MPa in the X direction, 20 MPa in the Y and Z directions) to achieve differentiated stress loading between clusters. The injection system was turned on to complete the hydraulic fracturing experiment.

[0063] Figure 7 The results of a set of true triaxial horizontal well segmented multi-cluster fracturing experiments completed using the above experimental methods and steps, taking into account the influence of stress differences between clusters, are presented. Figure 7 There are differences in the order of fluid injection time for each perforation cluster. The stress in the area where the first, second, and third clusters of perforations are located gradually increases, and the time of fluid injection gradually lags behind, indicating that there is a difference in stress between clusters, which leads to the inability of cracks in each perforation cluster to initiate and expand at the same time. The perforation cluster in the low stress area initiates and expands first. For the convenience of comparison, Figure 8 The results of a set of conventional true triaxial horizontal well multi-cluster fracturing experiments without considering the stress difference between clusters are presented. Figure 7 Except for the differences in the corresponding experimental schemes, other experimental parameters were the same as Figure 7 Consistent in. Figure 8 The characteristics of the fluid inflow curves of each perforation cluster and Figure 7 There are significant differences. In the early stage of fracturing, the three clusters of perforations are filled with fluid at the same time, indicating that when the stress between clusters is the same, the three perforation clusters have the ability to initiate and expand fractures at the same time. In summary, the experimental device and method provided by the present invention can effectively realize the differentiated loading of stress between clusters, and provide an effective means for accurately revealing the influence of stress differences between clusters on the balanced expansion behavior of multi-cluster hydraulic fractures.

[0064] Therefore, the present invention adopts the above-mentioned horizontal well fracturing experimental device and method considering the stress difference between clusters, uses finite element software to simulate and calculate the rock mechanics parameters of each region of the rock sample, and prepares the corresponding artificial rock sample. It can realize the differentiated stress loading between clusters under the stress loading conditions of the existing fracturing equipment, and can change the size of the stress difference between clusters by adjusting the combination of rock mechanics parameters in each region of the rock sample, so as to better study the influence of the stress difference between clusters on the balanced expansion of multiple cluster cracks. The device and method are suitable for horizontal well fracturing experimental simulation considering the stress difference between clusters under different cluster numbers, which fills the gap in the existing research methods, avoids the modification of the true triaxial fracturing equipment, and greatly reduces the experimental cost. The device and method have a perfect theoretical basis and simple operation steps, making the experimental results more real and reliable.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A horizontal well fracturing experimental method considering inter-cluster stress differences, characterized in that: The following steps are involved: Step 1: Determine the rock sample zoning pouring plan, the stress loading method and size of the true triaxial fracturing equipment; Step 2: Use finite element software to conduct static stress analysis and determine the rock mechanics parameters corresponding to each area of ​​the partitioned cast rock sample; Step 3: Based on the rock mechanics parameters in step 2, concrete slurries with different proportions are prepared, and the concrete slurries are poured into the experimental device to prepare artificial rock samples; Step 4: Load the artificial rock sample on the true triaxial fracturing equipment, apply the minimum horizontal ground stress X, the maximum horizontal ground stress Y, and the vertical ground stress Z three-dimensional stress, start the injection system, and complete the fracturing experiment; In step 1, the rock sample partition casting scheme includes the number and size of rock sample partitions, which are determined according to the number of clusters and cluster spacing required by the experiment; the rock sample partition casting scheme determined by the number of clusters and cluster spacing required by the experiment is: the artificial rock sample is horizontally divided into 3 areas, and the width of each area is 20 cm; In step 1, the stress loading method of the true triaxial fracturing equipment is to clarify the minimum horizontal geostress X, the maximum horizontal geostress Y, and the vertical geostress Z. The loading method adopted is one-way piston cylinder loading or opposing piston cylinder loading; the stress loading method of the true triaxial fracturing equipment is: the minimum horizontal geostress X direction is one-way piston cylinder loading, and the other side is a fixed boundary, and the maximum horizontal geostress Y and vertical geostress Z directions are opposing piston cylinder loading; In step 2, the specific process of using finite element software to carry out static stress analysis is as follows: S2-1, establish equal-size models based on the rock sample partition casting plan; S2-2. Set the same stress loading mode and magnitude according to the stress loading conditions of the true triaxial equipment. According to the three-dimensional stress loading mode and three-dimensional stress magnitude of the experimental equipment, determine to apply uniform pressure on the left end of the rock sample in the X direction, and no pressure on the right end. Use a fixed displacement method and apply uniform pressure on both ends of the Y and Z directions; S2-3, perform meshing, using high-quality meshing method; S2-4. Run the calculation example to calculate the stress conditions of different rock mechanics parameter combinations in each area of ​​the rock sample; S2-5, select the rock mechanics parameter combination of inter-cluster stress difference that meets the experimental requirements as the best solution; The experimental device comprises a true triaxial fracturing device and a sample preparation device, wherein the sample preparation device comprises a bottom plate, a front cover plate, a rear cover plate, a left cover plate, a right cover plate, fixing bolts, and a wellbore, wherein one end of the front cover plate and the right cover plate are fixedly connected, and one end of the rear cover plate and the left cover plate are fixedly connected, and the front cover plate, the right cover plate, the rear cover plate, and the left cover plate are respectively connected and fixed to the base by fixing bolts to form a casting cavity, and a through hole is provided in the middle of the bottom plate, and the wellbore is pre-buried and enters the casting cavity through the through hole; The diameter of the through hole is larger than the inner diameter of the wellbore and smaller than the outer diameter of the wellbore; The fixing bolts are matched with the wellbore through the through holes so that the end surface of the wellbore is closely attached to the inner side of the bottom plate; The outer diameter of the front end of the fixing bolt is consistent with the inner diameter of the wellbore, and the outer diameter of the rear end of the fixing bolt is larger than the diameter of the through hole.

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