A method for testing the conductivity of supported cracks under closing stress

Abstract

The present invention relates to the technical field of hydraulic fracturing for unconventional oil and gas reservoirs, and specifically discloses a method for testing the conductivity of propped fractures under closure stress. The method comprises: firstly, scaling down the proppant laying state of a proppant transport and laying experiment on a diversion test rock plate in proportion and laying the proppant; then, applying closure stress to the rock plate to obtain the true sand concentration of the propped fracture; and finally, testing the conductivity of the propped fracture based on the true sand concentration. The method proposed by the present invention overcomes the shortcomings of existing methods that fail to fully consider the extrusion deformation and re-spreading of the proppant laying layer under closure stress. It can accurately test the conductivity of the proppant in the true laying state of the hydraulic fracture under closure stress, and provides theoretical guidance for the design of hydraulic fracturing for unconventional oil and gas reservoirs.

Description

Technical Field

[0001] The present 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 closure stress. Background Art

[0002] The development of unconventional oil and gas reservoirs requires hydraulic fracturing to create highly conductive, propped fractures in the formation. Hydraulic fracturing involves injecting a highly viscous fracturing fluid into the reservoir through a wellbore using a high-pressure pump. When the injection rate of the fracturing fluid exceeds the reservoir's absorption capacity, high pressure builds up in the reservoir at the bottom of the well. When this pressure exceeds the fracture pressure of the rock near the bottom of the wellbore, the reservoir is forced open and fractures form. Continued injection of fracturing fluid will cause the fractures to expand further into the reservoir. To maintain the fractures, a proppant-laden fluid is then injected into the reservoir. This fluid not only propels the fractures further but also supports them, preventing them from closing. Displacement fluid is then injected to displace all the fluid from the wellbore into the fractures, propping them up with the proppant. Finally, the injected high-viscosity fracturing fluid automatically degrades and drains out of the wellbore, leaving one or more cracks of varying length, width, and height in the oil layer, establishing a new fluid pathway between the oil layer and the wellbore. After fracturing, oil and gas well production typically 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 stimulation effect and production increase capacity.

[0004] Chinese patent publication CN116451300B discloses a method for placing proppant in a rock slab based on a sand bank distribution map. This method simulates unpropped areas within a hydraulic fracture by omitting proppant in localized areas of the slab. However, real formations are more prone to deformation, and unpropped fracture areas will completely close under closure stress. However, in diversion experiments, the slab does not completely close, resulting in highly conductive channels in the unpropped areas. Therefore, this method is difficult to reflect the true conductivity of propped fractures under closure stress.

[0005] Chinese patent publication CN104358554A discloses a method for evaluating conductivity that considers closed fractures. This method assumes that areas not filled with proppant are closed fractures and uses dense material instead. Although this method accounts for the effect of fracture closure, the dense material also limits the extrusion deformation of the proppant layer under closure stress. This results in the sand-filled areas and concentrations within the rock slab being inconsistent with the actual situation, making it difficult to accurately reflect the true conductivity of propped fractures under closure stress.

[0006] In summary, existing propped fracture conductivity testing methods primarily rely on omitting proppant in localized areas of the slab to simulate unpropped areas within the hydraulic fracture, or filling these areas with dense materials. These methods fail to fully account for the extrusion and re-expansion of the proppant layer under closure stress, making it difficult to accurately characterize the true conductivity of the hydraulic fracture. Therefore, it is urgent to establish a comprehensive test method for propped fracture conductivity under closure stress to provide guidance for optimized hydraulic fracturing design in unconventional oil and gas reservoirs. Summary of the Invention

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

[0008] The technical solutions adopted in the present invention are as follows:

[0009] A method for testing the conductivity of a propped crack under closure stress, comprising the following steps:

[0010] S1. Proppand the proppant placement state obtained in the proppant transport and placement experiment in proportion to the proppant placement state of the diversion experiment, and place the proppant on the diversion experiment slab;

[0011] S2. Using image numerical processing technology, obtain the area of ​​the sand-paved area on the rock slab in step S1;

[0012] S3. Apply closing stress to the rock plate to obtain the actual paving state of the proppant paving layer after extrusion deformation and re-spreading under the action of closing stress;

[0013] S4. Perform digital image processing on the actual paving state of the proppant on the rock slab in step S3 to obtain the area of ​​the actual paving area. Combined with the original paving area size in step S2, the actual paving concentration of the propped crack is calculated.

[0014] S5. Based on the actual sand concentration, conduct diversion experiments to test the diversion capacity of the support 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. Soak the proppant with fracturing fluid to simulate the wet state of the proppant in a real hydraulic fracture. The soaking time is the actual 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 sand paving area 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 actual laying state of the proppant laying layer after extrusion deformation and re-spreading under the action of closing stress are: applying closing stress, unloading closing stress after the width of the support crack is stable, and removing the rock plate.

[0022] Preferably, in S4, based on the principle of conservation of mass, the actual concentration of sand in the support crack is calculated using the formula: Where C1 is the initial proppant concentration without closure stress, kg / m 2 ; C2 is the actual proppant placement 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 diversion 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 existing methods that fail to fully consider the extrusion deformation and re-spreading of the proppant laying layer under the action of closing stress, and can accurately test the conductivity of the proppant in the actual laying state within 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 This is a diagram showing the variation of the width of the support crack of the present invention;

[0030] Figure 4 It is the actual 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 the 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 claimed invention, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall 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 closure stress, and the specific steps are as follows:

[0034] S1. Proppand the proppant placement state obtained in the proppant transport and placement experiment in proportion to the proppant placement state of the diversion experiment, and place the proppant on the diversion experiment slab;

[0035] S2. Using image numerical processing technology, obtain the area of ​​the sand-paved area on the rock slab in step S1;

[0036] S3. Apply closing stress to the rock plate to obtain the actual paving state of the proppant paving layer after extrusion deformation and re-spreading under the action of closing stress;

[0037] S4. Perform digital image processing on the actual paving state of the proppant on the rock slab in step S3 to obtain the area of ​​the actual paving area. Combined with the original paving area size in step S2, the actual paving concentration of the propped crack is calculated.

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

[0039] In S1, the specific operation process of proportionally reducing the proppant placement state obtained in the proppant transport and placement experiment to the proppant placement 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 a real hydraulic fracture, and the soaking time is the actual fracturing operation 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 necessary to evaluate the effectiveness of the fracture network under high closure stress in deep shale in a certain block. This example uses the Longmaxi Formation deep shale reservoir rock as the experimental material, uses API diversion equipment, and based on the test method for propped fracture conductivity under closure stress, conducts a deep shale propped fracture conductivity experiment to test the propped fracture conductivity under closure stress. The following steps are included:

[0043] Step 1: Place the proppant in the proppant placement state obtained in the proppant placement experiment. Figure 1 As shown, the proppant laying state of the diversion experiment is scaled down to the same scale as shown in Figure 2 As shown; soak the proppant with fracturing fluid for 2 hours; and evenly spread the soaked proppant on the diversion test rock plate according to the sand-laying state of the diversion test rock plate. 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: Perform numerical image processing of the sandstone slab using MATLAB software, and calculate the area percentage of each color system to obtain the sand paving area of ​​the slab as 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 actual paving state of the proppant paving layer after extrusion deformation and re-spreading under the action of closing stress, as shown in the figure. Figure 4 shown.

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

[0047] According to the proppant paving concentration calculation formula, the proppant paving 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 placement 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 layer still has a high conductivity channel 1, which indicates that the method disclosed in CN116451300B is difficult to accurately measure the conductivity of the propped fracture.

[0049] Step 5: Figure 5 As shown, following the proppant placement concentration under the closure stress measured in Step 4, the proppant was evenly spread on the diversion rock plate, and a diversion experiment was conducted to test the conductivity of the propped fracture. Repeated experiments following this procedure were conducted to measure the actual conductivity of the propped hydraulic fracture under different closure stresses. The test results are shown in Table 1.

[0050] Table 1 Propped crack 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 present invention proposes a method for testing the conductivity of propped fractures under closure stress, which can accurately test the conductivity of the proppant in the actual laying state of the hydraulic fracture under closure 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 solutions of the present invention rather than to limit the same. 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 solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions 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. Proppand the proppant placement state obtained in the proppant transport and placement experiment in proportion to the proppant placement state of the diversion experiment, and place the proppant on the diversion experiment slab; S2. Using image numerical processing technology, obtain the area of ​​the sand-paved area on the rock slab in step S1; S3. Apply closing stress to the rock plate to obtain the actual paving state of the proppant paving layer after extrusion deformation and re-spreading under the action of closing stress; S4. Perform digital image processing on the actual paving state of the proppant on the rock slab in step S3 to obtain the area of ​​the actual paving area. Combined with the original paving area size in step S2, the actual paving concentration of the propped crack is calculated. 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 placement 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 ; S5. Based on the actual sand concentration, conduct diversion experiments to test the diversion capacity of the support cracks.

2. The method for testing the conductivity of a propped crack under closed stress according to claim 1, characterized in that: In S1, the specific operation process of proportionally reducing the proppant placement state obtained in the proppant transport and placement experiment to the proppant placement state of the diversion experiment is as follows: S11, soaking the proppant with fracturing fluid to simulate the wet state of the proppant in a real hydraulic fracture, and the soaking time is the actual fracturing operation 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 a propped crack under closed stress according to claim 1, characterized in that: In S2, the image of the sandstone slab was numerically processed based on MATLAB software.

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

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

6. The method for testing the conductivity of a propped crack under closed stress according to claim 1, characterized in that: The specific steps of S3 to obtain the actual 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 support crack is stable, unloading the closing stress and removing the rock plate.

7. The method for testing the conductivity of a propped crack 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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