Method for testing flow conductivity of support fracture under action of closed stress

Abstract

The invention relates to the technical field of hydraulic fracturing of unconventional oil and gas reservoirs, and particularly discloses a method for testing the flow conductivity of a propping fracture under the action of closed stress, which comprises the following steps of: firstly, reducing the paving state of a propping agent in a propping agent migration paving experiment on a rock plate of a flow guide experiment in an equal proportion, and paving the propping agent; then applying closed stress to the rock plate to obtain the real sanding concentration of the support crack, and finally testing the flow conductivity of the support crack based on the real sanding concentration. The method provided by the invention overcomes the defect that the existing method cannot fully consider the extrusion deformation and re-spreading of the proppant laying layer under the action of the closed stress, can accurately test the flow conductivity of the proppant in the hydraulic fracture in a real laying state under the action of the closed stress, and provides theoretical guidance for the fracturing design of unconventional oil and gas reservoirs.

Description

Technical Field

[0001] The invention relates to the technical field of hydraulic fracturing of unconventional oil and gas reservoirs, and in particular to a method for testing the conductivity of propped fractures under the action of closure stress. Background Art

[0002] The development of unconventional oil and gas reservoirs requires transformation through hydraulic fracturing technology to form propped fractures with high conductivity in the formation. Hydraulic fracturing is to use a high-pressure pump on the ground to squeeze a fracturing fluid with high viscosity into the oil layer through the wellbore. When the injection speed of the fracturing fluid exceeds the absorption capacity of the oil layer, a very high pressure is formed on the oil layer at the bottom of the well. When this pressure exceeds the fracture pressure of the oil layer rock near the bottom of the well, the oil layer will be squeezed open and cracks will be generated. At this time, the fracturing fluid is continuously squeezed into the oil layer, and the cracks will continue to expand into the oil layer. In order to keep the cracks opened in an open state, a sand-carrying fluid with a proppant is then squeezed into the oil layer. After the sand-carrying fluid enters the crack, on the one hand, it can make the crack continue to extend forward, and on the other hand, it can support the cracks that have been opened so that they will not close. Then, a displacement fluid is injected to displace all the sand-carrying fluid in the wellbore into the cracks, and the cracks are propped up with proppant. Finally, the injected high-viscosity fracturing fluid will automatically degrade and be discharged from the wellbore, leaving one or more cracks of varying lengths, widths, and heights in the oil layer, establishing a new fluid channel between the oil layer and the wellbore. After fracturing, the production of oil and gas wells generally increases significantly.

[0003] Propped fractures refer to fractures that are filled and kept open by proppants during the fracturing process. Accurately predicting the conductivity of propped fractures is of great significance for evaluating the post-fracturing transformation effect and production increase capacity.

[0004] Chinese patent publication CN116451300B discloses a method for laying corresponding proppant states in rock slabs based on a sand bank state distribution map, which simulates the unlaid proppant area in the hydraulic fracture by not laying proppant in the local area of ​​the rock slab. However, the real formation is more deformable, and the fracture area without proppant will be completely closed under the closing stress. In the diversion experiment, the rock slab will not be completely closed, so that the unlaid sand area forms a high diversion channel. Therefore, this method is difficult to reflect the real diversion capacity of the propped fracture under the action of closing stress.

[0005] Chinese patent publication CN104358554A discloses a method for evaluating the flow conductivity of closed cracks. This method assumes that the area not filled with proppant is a closed crack and uses dense material instead of filling. Although this method takes into account the influence of crack closure, the dense material also limits the extrusion deformation of the proppant paving layer under the action of closing stress, resulting in the sand-paved area and sand-paved concentration in the rock plate not being consistent with the actual situation, so it is difficult to accurately reflect the actual flow conductivity of the propped crack under the action of closing stress.

[0006] In summary, the existing propped fracture conductivity test methods mainly do not lay proppant in the local rock plate to equivalently simulate the unlaid proppant area of ​​the hydraulic fracture, or use dense materials to fill the unlaid proppant area, which fails to fully consider the extrusion deformation and re-spreading of the proppant layer under the action of closure stress, and it is difficult to accurately characterize the true conductivity of the hydraulic fracture. Therefore, it is urgent to establish a set of propped fracture conductivity test methods that can comprehensively consider the action of closure stress to provide guidance for the optimization design of hydraulic fracturing in unconventional oil and gas reservoirs. Summary of the invention

[0007] In view of the above-mentioned problems in the prior art, the present invention provides a method for testing the conductivity of propped fractures under closed stress, which overcomes the problem that the prior art is difficult to accurately characterize the true conductivity of hydraulic fractures. The method can accurately test the conductivity of the proppant in the true laying state of the hydraulic fracture under closed stress, and provide theoretical guidance for the fracturing design of unconventional oil and gas reservoirs.

[0008] The technical solution adopted by the present invention is as follows:

[0009] A method for testing the conductivity of a propped crack under a closed stress, the steps are as follows:

[0010] S1. Proppane laying state obtained in proppant transport and laying experiment is proportionally reduced to proppant laying state of diversion experiment, and proppant is laid on the diversion experiment rock plate;

[0011] S2, using image numerical processing technology to obtain the area of ​​the sand-covered area on the rock slab in step S1;

[0012] S3, applying closing stress to the rock plate to obtain the real paving state of the proppant paving layer after extrusion deformation and re-spreading under the closing stress;

[0013] S4, digitally process the image of the actual paving state of the proppant on the rock plate in step S3, obtain the area size of the actual paving area, and calculate the actual paving concentration of the propped crack in combination with the original paving area size in step S2;

[0014] S5. Based on the actual sand concentration, conduct diversion experiments to test the diversion capacity of the propped cracks.

[0015] Preferably, in S1, the specific operation process of proportionally reducing the proppant laying state obtained in the proppant migration and laying experiment to the proppant laying state of the diversion experiment is:

[0016] S11. Soaking the proppant with fracturing fluid is equivalent to simulating the wet state of the proppant in the real hydraulic fracture, and the soaking time is the real fracturing construction time.

[0017] S12. According to the sand-laying state of the rock slab in the diversion experiment, the soaked proppant is laid on the rock slab, and the laying concentration is consistent with the proppant laying concentration in the migration experiment.

[0018] Preferably, in S2, the sandstone slab is subjected to image digitization processing based on MATLAB software.

[0019] Preferably, in S2, the area of ​​the rock slab sanding region is obtained by calculating the area percentage of each color system.

[0020] Preferably, in S3, an API flow guide chamber is used to apply closing stress to the rock plate.

[0021] Preferably, the specific steps of S3 for obtaining the real laying state of the proppant laying layer after extrusion deformation and re-spreading under the action of closing stress are: applying closing stress, and after the width of the supporting crack is stable, unloading the closing stress and removing the rock plate.

[0022] Preferably, in S4, based on the principle of mass conservation, the actual sand concentration of the propped crack is calculated, and the formula is: Where C1 is the initial proppant concentration without closure stress, kg / m 2 ; C2 is the actual proppant laying concentration under closure stress, kg / m 2 ; S1 is the initial diversion test rock plate proppant paving area, m 2 ; S2 is the paving area of ​​the rock plate proppant in the diversion experiment under the action of closed stress, m 2 .

[0023] Preferably, in S5, according to the actual proppant laying concentration under the closure stress measured in S4, the proppant is evenly spread on the guide rock plate, and a diversion experiment is carried out to test the diversion capacity of the propped fracture.

[0024] The present invention has the following characteristics and advantages:

[0025] The present invention overcomes the shortcomings of the existing methods that the extrusion deformation and re-spreading of the proppant laying layer under the action of closing stress cannot be fully considered, and can accurately test the conductivity of the proppant in the actual laying state in the hydraulic fracture under the action of closing stress.

[0026] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a proppant paving state diagram of the proppant paving and migration experiment of the present invention;

[0028] Figure 2 This is a diagram of the initial paving state of the proppant of the present invention;

[0029] Figure 3 is a diagram showing the variation of the width of the supporting crack of the present invention;

[0030] Figure 4 It is the real laying state of the proppant under the closure stress of the present invention, wherein 1 is a high conductivity channel;

[0031] Figure 5 This is a diagram showing a state in which the proppant is evenly spread on the rock plate in the diversion experiment of the present invention. DETAILED DESCRIPTION

[0032] The following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] The present invention proposes a method for testing the conductivity of a propped crack under a closed stress, and the specific steps are as follows:

[0034] S1. Proppane laying state obtained in proppant transport and laying experiment is proportionally reduced to proppant laying state of diversion experiment, and proppant is laid on the diversion experiment rock plate;

[0035] S2, using image numerical processing technology to obtain the area of ​​the sand-covered area on the rock slab in step S1;

[0036] S3, applying closing stress to the rock plate to obtain the real paving state of the proppant paving layer after extrusion deformation and re-spreading under the closing stress;

[0037] S4, digitally process the image of the actual paving state of the proppant on the rock plate in step S3, obtain the area size of the actual paving area, and calculate the actual paving concentration of the propped crack in combination with the original paving area size in step S2;

[0038] S5. Based on the actual sand concentration, conduct diversion experiments to test the diversion capacity of the propped cracks.

[0039] In S1, the specific operation process of proportionally reducing the proppant laying state obtained in the proppant transport and laying experiment to the proppant laying state of the diversion experiment is as follows:

[0040] S11, soaking the proppant with fracturing fluid to simulate the wet state of the proppant in the real hydraulic fracture, and the soaking time is the real fracturing construction time;

[0041] S12. According to the sand-laying state of the rock slab in the diversion experiment, the soaked proppant is laid on the rock slab, and the laying concentration is consistent with the proppant laying concentration in the migration experiment.

[0042] It is now necessary to evaluate the effectiveness of the fracture network under high closure stress in the deep shale of a certain block. This embodiment uses the deep shale reservoir rock of the Longmaxi Formation as the experimental material, uses API diversion equipment, and based on the test method of the conductivity of propped fractures under closure stress, conducts a deep shale propped fracture conductivity experiment to test the conductivity of propped fractures under closure stress. The following steps are included:

[0043] Step 1: The proppant paving state obtained in the proppant transport and paving experiment is as follows: Figure 1 As shown in the figure, the proppant laying state of the diversion experiment is reduced in proportion to the Figure 2 As shown; soak the proppant with fracturing fluid for 2 hours; and evenly spread the soaked proppant on the diversion test slab according to the sand-laying state of the diversion test slab. The laying concentration is consistent with the proppant laying concentration in the proppant migration experiment, which is 5kg / m in this embodiment. 2 .

[0044] Step 2: Use MATLAB software to perform image digitization on the sandstone slab and calculate the area percentage of each color system to obtain the sandstone slab area of ​​0.0043m 2 .

[0045] Step 3: If Figure 3 As shown in the figure, the API diversion chamber is used to apply closing stress to the rock plate. Taking 30MPa as an example, after the width of the propped crack is stable, the closing stress is unloaded and the rock plate is removed to obtain the real paving state of the proppant paving layer after extrusion deformation and re-spreading under the action of closing stress, as shown in Figure 4 shown.

[0046] Step 4: Digitally process the image of the actual sand-laying state of the rock slab obtained in step S3 using MATLAB software, and determine that the paving area of ​​the proppant on the rock slab under the action of closure stress is 0.0058m by calculating the area percentage of each color system. 2 .

[0047] According to the proppant concentration calculation formula, the proppant concentration under closure stress is 3.71 kg / m 2 , and its calculation formula is: Where C1 is the initial proppant concentration without closure stress, kg / m 2 ; C2 is the actual proppant laying concentration under closure stress, kg / m 2 ; S1 is the initial diversion test rock plate proppant paving area, m 2 ; S2 is the paving area of ​​the rock plate proppant in the diversion experiment under the action of closed stress, m 2 .

[0048] In addition, by Figure 4 It can be seen that even after the closure stress is applied, the proppant paving layer still has a high conductivity channel 1, which indicates that the method disclosed in CN116451300B is difficult to measure the accurate propped fracture conductivity.

[0049] Step 5: Figure 5 As shown, according to the proppant laying concentration under the closure stress measured in step 4, the proppant is evenly spread on the diversion rock plate, and a diversion experiment is carried out to test the diversion capacity of the propped fracture. According to the above steps, the experiment is carried out multiple times to measure the actual diversion capacity of the propped hydraulic fracture under different closure stresses. The test results are shown in Table 1.

[0050] Table 1 Prop fracture conductivity

[0051] Closing stress / MPa Displacement / (ml / min) Fluid viscosity / (mPa·s) Pressure difference / kPa Conductivity / (D·cm) 10 5 0.916 0.457 55.671 20 5 0.916 0.617 41.234 30 5 0.916 0.869 29.277 40 5 0.916 1.393 18.264 50 5 0.916 2.711 9.388 60 5 0.916 5.362 4.744 70 5 0.916 6.596 3.857 80 5 0.916 8.152 3.121

[0052] Therefore, the method for testing the conductivity of propped fractures under closed stress proposed in the present invention can accurately test the conductivity of the proppant in the actual laying state of the hydraulic fracture under closed stress, and provide theoretical guidance for the fracturing design of unconventional oil and gas reservoirs.

[0053] 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 method for testing the conductivity of a propped crack under closed stress, characterized in that: Here are the steps: S1. Proppane laying state obtained in proppant transport and laying experiment is proportionally reduced to proppant laying state of diversion experiment, and proppant is laid on the diversion experiment rock plate; S2, using image numerical processing technology to obtain the area of ​​the sand-covered area on the rock slab in step S1; S3, applying closing stress to the rock plate to obtain the real paving state of the proppant paving layer after extrusion deformation and re-spreading under the closing stress; S4, digitally process the image of the actual paving state of the proppant on the rock plate in step S3, obtain the area size of the actual paving area, and calculate the actual paving concentration of the propped crack in combination with the original paving area size in step S2; S5. Based on the actual sand concentration, conduct diversion experiments to test the diversion capacity of the propped cracks.

2. A method for testing the conductivity of propped cracks under closed stress according to claim 1, characterized in that: In S1, the specific operation process of proportionally reducing the proppant laying state obtained in the proppant transport and laying experiment to the proppant laying state of the diversion experiment is as follows: S11, soaking the proppant with fracturing fluid to simulate the wet state of the proppant in the real hydraulic fracture, and the soaking time is the real fracturing construction time; S12. According to the sand-laying state of the rock slab in the diversion experiment, the soaked proppant is laid on the rock slab, and the laying concentration is consistent with the proppant laying concentration in the migration experiment.

3. The method for testing the conductivity of propped cracks under closed stress according to claim 1, characterized in that: In S2, the image of the sandstone slab was digitized based on MATLAB software.

4. The method for testing the conductivity of propped cracks under closed stress according to claim 1, characterized in that: In S2, the area of ​​the rock slab paved with sand is obtained by calculating the area percentage of each color system.

5. The method for testing the conductivity of propped cracks under closed stress according to claim 1, characterized in that: In S3, an API flow chamber is used to apply closing stress to the rock plate.

6. A method for testing the conductivity of propped cracks under closed stress according to claim 1, characterized in that: The specific steps of S3 to obtain the real paving state of the proppant paving layer after extrusion deformation and re-spreading under the action of closing stress are: applying closing stress, and after the width of the propped crack is stable, unloading the closing stress and removing the rock plate.

7. A method for testing the conductivity of propped cracks under closed stress according to claim 1, characterized in that: In S4, based on the principle of mass conservation, the actual sand concentration of the support crack is calculated as follows: Where C1 is the initial proppant concentration without closure stress, kg / m 2 ; C2 is the actual proppant concentration under closure stress, kg / m 2 ; S1 is the initial diversion test rock plate proppant paving area, m 2 ; S2 is the paving area of ​​the rock slab proppant under closed stress, m 2 .

8. The method for testing the conductivity of propped cracks under closed stress according to claim 1, characterized in that: In S5, according to the actual proppant laying concentration under the closure stress measured in S4, the proppant is evenly spread on the diversion rock plate, and a diversion experiment is carried out to test the diversion capacity of the propped fracture.

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