Method for proppant reflux test

Abstract

The invention relates to a method for a proppant reflux test, which is characterized in that on one hand, a rock sample forms a split surface in a splitting manner, so that the roughness of a fractured surface formed by fracturing underground rocks is simulated more truly; on the other hand, the fractures are in a vertical state, and a horizontal loading mode is adopted, so that the real state of the fractures in the stratum can be simulated more accurately. In the testing process, the flowing process of fluid in the pumping injection and later flow-back period of hydraulic fracturing construction in actual production is simulated, and some testing parameters in the testing process are changed, so that a proppant critical flow-back speed layout is drawn conveniently. Therefore, the influence of factors such as the magnitude of stress acting on the proppant, the use amount of the proppant and the type of the proppant on backflow of the proppant can be obtained, so that test data are more referential, and comprehensive and effective guidance is conveniently provided for on-site actual flow-back work.

Description

Technical Field

[0001] This invention relates to the field of fluid drainage testing technology after fracturing in oil and gas wells, and specifically to a method for proppant backflow testing. Background Technology

[0002] my country's abundant tight oil and gas resources have become the main battleground for increasing oil and gas reserves and production, and volumetric fracturing is a key technology for the efficient development of these resources. However, after oil and gas wells are stimulated with large volumes of fluids and proppant, the drainage cycle becomes long, and proppant backflow occurs frequently. On the one hand, the longer the drainage cycle, the longer the contact time between the fracturing fluid and the formation. This increases the adsorption of polymer molecules in the fracturing fluid on the formation rock and the water absorption and swelling of clay minerals in the formation rock, thereby damaging the formation structure, reducing formation permeability, and ultimately affecting the production capacity of the oil and gas well. On the other hand, proppant backflow in the hydraulic fractures during drainage may cause wellbore blockage, increasing the workload of unblocking operations.

[0003] To minimize formation damage and wellbore blockage, fracturing fluid needs to be flowed back as quickly as possible while preventing proppant backflow from the fractures. Therefore, it is necessary to provide an apparatus and method for testing proppant backflow. Summary of the Invention

[0004] In view of the above-mentioned problems in the existing technology, the present invention provides a method for proppant backflow testing, which can realistically simulate the influence of underground rock fracture state on proppant backflow.

[0005] The technical solution adopted by this invention to solve its technical problem is: to provide a method for proppant reflux testing, comprising the following steps,

[0006] Step S1: Collect a rock sample, then split the rock sample into two rock slabs with split surfaces that can fit together. Install the two rock slabs on the testing device with their split surfaces touching and in a vertical position, and apply initial stress to the two rock slabs in the horizontal direction through the testing device.

[0007] Step S2: Pump fracturing fluid containing proppant into the space between the two rock slabs along the first direction using the testing device, wherein the amount of proppant used is the product of the projected area of ​​the rock slab's fracture surface and the average proppant concentration.

[0008] Step S3: Gradually increase the initial stress to the target stress using the testing device;

[0009] Step S4: Pump backflow fluid between the two rock slabs through the testing device in a second direction opposite to the first direction until the proppant return reaches a stable level;

[0010] Step S5: Change the flow rate of the backflow liquid and repeat step S4 to obtain the cumulative proppant return curve under different flow rates, and then obtain the critical flow rate of proppant backflow under the current target stress.

[0011] Step S6: Change the value of the target stress and repeat steps S3 to S4 to obtain the critical flow rate of proppant recirculation under different target stresses;

[0012] Step S7: Change the amount of proppant and repeat steps S2 to S4 to obtain the critical flow rate of proppant recirculation under different proppant concentrations.

[0013] Step S8: Change the proppant type and repeat steps S2 to S4 to obtain the critical flow rate of proppant reflux under different proppant types;

[0014] Step S9: Based on the critical flow rates obtained in steps S6 to S8, obtain the critical backflow velocity chart for the proppant. Further, the target stress is derived using the following formula:

[0015] σ=σ c -ρgh0-p0

[0016] Where σ is the target stress acting on the proppant, in MPa; σ c ρ is the closure stress, MPa; ρ is the actual density of the fluid inside the wellbore, g / cm³. 3 g is the acceleration due to gravity; h0 is the vertical depth at the center of the fracture, in meters; p0 is the wellhead pressure at the initial stage of fluid drainage after fracturing, in MPa.

[0017] Furthermore, the average proppant concentration is obtained by the following formula:

[0018]

[0019] Where n is the average proppant concentration, kg / m³ 2 Q represents the total amount of proppant used in a single well, in kg; c represents the number of fracture clusters opened; l f To support the crack length, m; h f To support the crack height, m.

[0020] Furthermore, in step S1, the magnitude of the initial stress is in the range of 1.0 MPa to 5.0 MPa.

[0021] Furthermore, in step 2, the duration of pumping fracturing fluid and proppant between the two rock plates by the testing device is between 15 min and 45 min.

[0022] Furthermore, the testing apparatus includes:

[0023] A hydraulic press for installing the first and second rock plates required for testing; and

[0024] A fluid supply assembly for injecting proppant-containing fracturing fluid or flowback fluid into the split fracture formed between the first rock plate and the second rock plate.

[0025] Furthermore, the liquid supply assembly includes a pump body, a liquid supply tank and a collection tank communicating with the pump body, and a fluid guide for containing the first rock slab and the second rock slab.

[0026] Furthermore, the fluid guide has a first opening and a second opening that connect to the splitting crack, wherein the first opening is connected to the pump body and the collection tank, and the second opening is connected to the pump body and the supply tank.

[0027] Furthermore, the guide fluid is also provided with a first pressure-transmitting hole that connects to the first opening and a second pressure-transmitting hole that connects to the second opening.

[0028] Furthermore, the liquid supply assembly includes a first valve disposed between the pump body and the second opening, a second valve disposed between the pump body and the first opening, a third valve disposed between the liquid supply tank and the second opening, and a fourth valve disposed between the first opening and the collection tank.

[0029] Furthermore, by opening the second valve and the third valve and closing the first valve and the fourth valve, the liquid supply assembly can provide a flow path along the first direction from the first opening to the second opening.

[0030] Furthermore, by opening the first valve and the fourth valve and closing the second valve and the third valve, the liquid supply assembly can provide a flow path along the second direction from the second opening to the first opening.

[0031] The beneficial effects of this invention are as follows: The method for proppant backflow testing provided by this invention, on the one hand, forms a fractured surface in the rock sample through a splitting process, thereby more realistically simulating the roughness of the fractured surface formed by the fracturing of underground rock. On the other hand, placing the splitting fracture in a vertical state and adopting a horizontal loading method more accurately simulates the real state of fractures in the formation. During the test, the flow process of fluids during the pumping and subsequent flowback phases of hydraulic fracturing operations in actual production is simulated, and some test parameters are changed to facilitate the plotting of the critical backflow velocity map of the proppant. Therefore, the influence of factors such as the magnitude of stress acting on the proppant, the amount of proppant used, and the type of proppant on proppant backflow can be obtained, making the test data more reliable and providing comprehensive and effective guidance for pumping scheme design and actual flowback work in the field. Attached Figure Description

[0032] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0033] Figure 1 The diagram shown is a top view of a device used for proppant reflux testing.

[0034] Figure 2 The diagram shown is a schematic representation of the rock mass.

[0035] Figure 3 The diagram shows the structure of the rock block after it has split.

[0036] In the figure, the following labels are used: 10, hydraulic press; 11, first pressure plate; 12, second pressure plate;

[0037] 20. Liquid supply assembly; 21. Pump body; 22. Liquid supply tank; 23. Collection tank; 24. Fluid guide; 241. First opening; 242. Second opening; 243. First pressure transmission hole; 244. Second pressure transmission hole; 25. Sealing ring; 26. First valve; 27. Second valve; 28. Third valve; 29. ​​Fourth valve.

[0038] 30. Rock block; 31. First rock slab; 32. Second rock slab; 33. Crack. Detailed Implementation

[0039] To make the technical problem to be solved, the technical solution, and the beneficial effects of this invention clearer, the invention will now be described in detail with reference to the accompanying drawings. This drawing is a simplified schematic diagram, illustrating only the basic structure of the invention, and therefore only shows the components relevant to the invention. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0040] refer to Figure 1 As shown, the present invention provides an apparatus for proppant reflux testing, comprising a hydraulic press 10 and a fluid supply assembly 20. A first rock plate 31 and a second rock plate 32 required for the test can be mounted on the hydraulic press 10. The hydraulic press 10 is used to provide the stress required for the test to the first rock plate 31 and the second rock plate 32. The fluid supply assembly 20 is used to supply or collect the fracturing fluid, proppant, and flowback fluid required for the test.

[0041] refer to Figure 2 and Figure 3As shown, in some embodiments, the first rock plate 31 and the second rock plate 32 are made by splitting a single rock block 30 along its length. Irregularly cleaved surfaces corresponding to and fitting together are formed on the first rock plate 31 and the second rock plate 32, thereby approximating the fracturing surface formed after hydraulic fracturing of the underground rock strata through splitting. The first rock plate 31 and the second rock plate 32 are installed at the output end of the hydraulic press 10 with their cleaved surfaces facing each other. The gap between the first rock plate 31 and the second rock plate 32 is the splitting fracture 33. In a specific implementation, the hydraulic press 10 simultaneously compresses the first rock plate 31 and the second rock plate 32 relative to each other. The fluid supply component 20 first injects fracturing fluid containing proppant into the splitting fracture 33, and then injects flowback fluid in the reverse direction into the splitting fracture 33 to discharge the fracturing fluid. The flowback of the proppant is observed, thus realistically simulating the influence of the underground rock fracture state on the proppant flowback.

[0042] Recombined Figure 1 As shown, in some embodiments, the output end of the hydraulic press 10 is equipped with a first pressure plate 11 and a second pressure plate 12. The surface of the first pressure plate 11 near the second pressure plate 12 is used to connect with the surface of the first rock slab 31 away from the splitting surface. The surface of the second pressure plate 12 near the first pressure plate 11 is used to connect with the surface of the second rock slab 32 away from the splitting surface. In this embodiment, both the first pressure plate 11 and the second pressure plate 12 are arranged perpendicular to the ground, thereby facilitating the application of horizontal forces to the first rock slab 31 and the second rock slab 32.

[0043] In some embodiments, the liquid supply assembly 20 includes a pump body 21, a liquid supply tank 22 connected to the pump body 21 via a pipe, a collection tank 23 connected to the pump body 21 via a pipe, and a fluid guide 24 for receiving the first rock slab 31 and the second rock slab 32. One end of the fluid guide 24 is provided with a first opening 241 that can communicate with the splitting fissure 33, and the other end of the fluid guide 24 is provided with a second opening 242 that can communicate with the splitting fissure 33. The first opening 241 is connected to the pump body 21 and the collection tank 23 via a pipe, and the second opening 242 is connected to the pump body 21 and the liquid supply tank 22 via a pipe.

[0044] In some embodiments, the fluid guide 24 is further provided with a first pressure transmitting hole 243 and a second pressure transmitting hole 244 for mounting pressure sensors. The first pressure transmitting hole 243 is located next to and communicates with the first opening 241, and the second pressure transmitting hole 244 is located next to and communicates with the second opening 242. The pressure sensors installed at the first pressure transmitting hole 243 and the second pressure transmitting hole 244 monitor the fluid pressure at the first opening 241 and the second opening 242, allowing technicians to adjust the operating state of the pump body 21 based on the monitored fluid pressure, thereby correcting the fluid pressure at the first opening 241 and the second opening 242 to more realistically simulate the state of fluid in underground rock fissures.

[0045] In some embodiments, the fluid guide 24 is also sealed to the first pressure plate 11 and the second pressure plate 12 of the hydraulic press 10 to prevent liquid leakage. In one specific embodiment, the fluid guide 24 is sealed to the first pressure plate 11 and the second pressure plate 12 by a sealing ring 25.

[0046] The liquid supply assembly 20 also includes valves disposed on each pipeline, including a first valve 26 disposed between the pump body 21 and the second opening 242, a second valve 27 disposed between the pump body 21 and the first opening 241, a third valve 28 disposed between the liquid supply tank 22 and the second opening 242, and a fourth valve 29 disposed between the first opening 241 and the collection tank 23.

[0047] Open the second valve 27 and the third valve 28, and close the first valve 26 and the fourth valve 29. Under the pumping action of the pump body 21, the liquid in the supply tank 22 can sequentially pass through the pump body 21 and the second valve 27, enter the fracture 33 between the first rock slab 31 and the second rock slab 32 through the first opening 241, and then return to the supply tank 22 through the second opening 242 and the third valve 28. This is the first direction of liquid flow. The above process simulates the process of proppant entering the underground rock fracture with the fracturing fluid. It should be noted that the supply tank 22 provides fracturing fluid containing proppant during this process.

[0048] Open the first valve 26 and the fourth valve 29, and close the second valve 27 and the third valve 28. Under the pumping action of the pump body 21, the liquid in the supply tank 22 can sequentially pass through the pump body 21 and the first valve 26, enter the fracture 33 between the first rock plate 31 and the second rock plate 32 through the second opening 242, and then enter the collection tank 23 through the first opening 241 and the fourth valve 29. This is the second direction of liquid flow. The above process simulates the process of flowback fluid displacing fracturing fluid. It should be noted that the supply tank 22 provides flowback fluid in this process. Moreover, in the fracture 33, the flow direction of the flowback fluid is opposite to the flow direction of the fracturing fluid. That is, the second direction is opposite to the first direction.

[0049] Based on the above-mentioned testing apparatus, the present invention also provides a method for proppant reflux testing, comprising the following steps.

[0050] In step S1, a rock sample is collected and then split into two rock slabs with interlocking split surfaces. Finally, the two rock slabs are mounted on the testing device with their split surfaces interlocked and in a vertical position, and initial stress is applied to the two rock slabs in the horizontal direction through the testing device.

[0051] The initial stress is selected from 1.0 MPa to 5.0 MPa, preferably 2.0 MPa.

[0052] In practice, the rock samples can be selected from rocks drilled underground or from rocks collected from the surface of the construction area. In a preferred embodiment, the rock samples are selected from rocks collected from the surface of the construction area. Rock samples collected from the surface of the construction area are plentiful and inexpensive, sufficient to meet the needs of multiple experiments, and can effectively avoid errors from a single experiment.

[0053] In one specific embodiment, the collected rock sample can be prepared as follows: Figure 2 and Figure 3 The rock block, as shown in the diagram, is then split along the direction of the splitting line.

[0054] In step S2, fracturing fluid containing proppant is pumped between the two rock slabs along a first direction using the testing device, wherein the amount of proppant used is the product of the projected area of ​​the cleavage surface of the rock slab and the average proppant concentration.

[0055] In some implementations, proppant may be added between the two rock plates in advance, and then fracturing fluid may be pumped between the two rock plates along the first direction.

[0056] In some embodiments, the duration for which the testing device pumps fracturing fluid containing proppant into the fracture can be estimated based on the actual pumping time required on site. In one specific embodiment, the duration for which the testing device pumps fracturing fluid and proppant between the two rock slabs is between 15 min and 45 min, preferably 30 min.

[0057] The average proppant concentration is obtained by the following formula:

[0058]

[0059] In the above formula, n is the average proppant concentration (kg / m²); Q is the total amount of proppant used in a well (kg); c is the number of open fracture clusters; l f To support the crack length, m; h f To support the crack height, m.

[0060] In step S3, the initial stress is gradually increased to the target stress using the testing device.

[0061] The target stress is obtained by the following formula:

[0062] σ=σ c -ρgh0-p0

[0063] In the above formula, σ is the stress acting on the proppant, in MPa; c ρ is the closure stress, MPa; ρ is the actual density of the fluid inside the wellbore, g / cm³. 3 g is the acceleration due to gravity; h0 is the vertical depth at the center of the fracture, in meters; p0 is the wellhead pressure at the initial stage of fluid drainage after fracturing, in MPa.

[0064] It should be noted that the basic data required for testing can be obtained during actual on-site production and construction, such as the in-situ stress, wellbore trajectory, wellbore structure, total amount and type of proppant used in the fracturing well, as well as fracture monitoring parameters (number of open fracture clusters, propped fracture height, and propped fracture length), and the wellhead pressure at the initial stage of fluid drainage after fracturing. The closure stress can be calculated based on the obtained in-situ stress and wellbore trajectory. The calculation method for the closure stress can be obtained from the data during construction and production using theoretical formulas or computer calculations, all of which are well known to those skilled in the art and will not be described in detail here.

[0065] In step S4, the backflow fluid is pumped between the two rock slabs through the testing device in a second direction opposite to the first direction until the proppant return reaches a stable level.

[0066] In step S5, the flow rate of the backflow liquid is changed, and step S4 is repeated to obtain the cumulative proppant return curve under different flow rates, thereby obtaining the critical flow rate of proppant backflow under the current target stress.

[0067] In step S6, the value of the target stress is changed, and steps S3 to S4 are repeated to obtain the critical flow rate of proppant recirculation under different target stresses.

[0068] In step S7, the amount of proppant is changed, and steps S2 to S4 are repeated to obtain the critical flow rate of proppant recirculation under different proppant concentrations.

[0069] In step S8, the proppant type is changed, and steps S2 to S4 are repeated to obtain the critical flow rate of proppant reflux under different proppant types.

[0070] In step S9, a critical reflux velocity chart of the proppant is plotted based on the critical flow rates obtained in steps S6 to S8.

[0071] Based on the critical backflow velocity chart of the proppant obtained from the above test methods, it is easy to obtain the influence of factors such as the magnitude of stress acting on the proppant, the amount of proppant used, and the type of proppant on the backflow of the proppant, so as to provide comprehensive and effective guidance for the backflow work on site.

[0072] The beneficial effects of the apparatus and method for proppant reflow testing provided by this invention are as follows: Firstly, the apparatus and method create a fractured surface in the rock sample through a splitting mechanism, thereby more realistically simulating the roughness of the fractured surface formed by underground rock fracturing. Secondly, by placing the splitting fracture in a vertical state and applying a horizontal loading method, the actual state of fractures in the formation can be more accurately simulated. During the test, the flow process of fluids during the pumping and subsequent flowback phases of hydraulic fracturing operations in actual production is simulated, and some test parameters are changed to facilitate the plotting of the critical flowback velocity map of the proppant. Thus, the influence of factors such as the magnitude of stress acting on the proppant, the amount of proppant used, and the type of proppant on proppant reflow can be obtained, making the test data more reliable and providing comprehensive and effective guidance for actual flowback work in the field.

[0073] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0074] It should be understood that the terms "length", "width", "up", "down", "front and back", "left and right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the present invention.

[0075] The above description, based on the preferred embodiments of the present invention, provides guidance. Those skilled in the art can make various changes and modifications without departing from the scope of the invention. The technical scope of this invention is not limited to the contents of the specification but must be determined according to the claims.

Claims

1. A method for proppant reflux testing, comprising the following steps, Step S1: Collect a rock sample, then split the rock sample into two rock slabs with split surfaces that can fit together. Install the two rock slabs on the testing device with their split surfaces touching and in a vertical position, and apply initial stress to the two rock slabs in the horizontal direction through the testing device. Step S2: Using the testing device, fracturing fluid containing proppant is pumped between the two rock plates along the first direction, wherein... The amount of proppant used is the product of the projected area of ​​the split surface of the rock slab and the average proppant concentration. Step S3: Gradually increase the initial stress to the target stress using the testing device; Step S4: Pump backflow fluid between the two rock slabs through the testing device in a second direction opposite to the first direction until the proppant return reaches a stable level; Step S5: Change the flow rate of the backflow liquid and repeat step S4 to obtain the cumulative proppant return curve under different flow rates, and then obtain the critical flow rate of proppant backflow under the current target stress. Step S6: Change the value of the target stress and repeat steps S3 to S4 to obtain the critical flow rate of proppant recirculation under different target stresses; Step S7: Change the amount of proppant and repeat steps S2 to S4 to obtain the critical flow rate of proppant recirculation under different proppant concentrations. Step S8: Change the proppant type and repeat steps S2 to S4 to obtain the critical flow rate of proppant reflux under different proppant types; Step S9: Based on the critical flow rates obtained in steps S6 to S8, obtain the critical reflux velocity chart of the proppant.

2. The method for proppant reflux testing according to claim 1, characterized in that, The target stress is obtained by the following formula: s = s c -ρgh0-p0 Where σ is the target stress acting on the proppant, in MPa; σ c ρ is the closure stress, MPa; ρ is the actual density of the fluid inside the wellbore, g / cm³. 3 g is the acceleration due to gravity; h0 is the vertical depth at the center of the fracture, in meters; p0 is the wellhead pressure at the initial stage of fluid drainage after fracturing, in MPa.

3. The method for proppant reflux testing according to claim 1, characterized in that, The average proppant concentration is obtained by the following formula: Where n is the average proppant concentration, kg / m³ 2 Q represents the total amount of proppant used in a single well, in kg; c represents the number of fracture clusters opened; l f To support the crack length, m; h f To support the crack height, m.

4. The method for proppant reflux testing according to any one of claims 1-3, characterized in that, In step S1, the magnitude of the initial stress is in the range of 1.0 MPa to 5.0 MPa.

5. The method for proppant reflux testing according to any one of claims 1-3, characterized in that, In step 2, the duration of pumping fracturing fluid and proppant between the two rock plates by the testing device is between 15 min and 45 min.

6. The method for proppant reflux testing according to any one of claims 1-3, characterized in that, The testing apparatus includes: A hydraulic press (10) is used to install the first rock plate (31) and the second rock plate (32) required for testing; and The fluid supply assembly (20) is used to inject fracturing fluid or flowback fluid containing proppant into the splitting fracture (33) formed between the first rock plate (31) and the second rock plate (32).

7. The method for proppant reflux testing according to claim 6, characterized in that, The liquid supply assembly (20) includes a pump body (21), a liquid supply tank and a collection tank (23) connected to the pump body (21), and a fluid guide (24) for containing the first rock plate (31) and the second rock plate (32).

8. The method for proppant reflux testing according to claim 7, characterized in that, The fluid guide (24) has a first opening (241) and a second opening (242) that connect to the splitting crack (33). The first opening (241) is connected to the pump body (21) and the collection tank (23), and the second opening (242) is connected to the pump body (21) and the supply tank (22).

9. The method for proppant reflux testing according to claim 8, characterized in that, The guide fluid (24) is also provided with a first pressure transmission hole (243) that connects to the first opening (241) and a second pressure transmission hole (244) that connects to the second opening (242).

10. The method for proppant reflux testing according to claim 8, characterized in that, The liquid supply assembly (20) includes a first valve (26) disposed between the pump body (21) and the second opening (242), a second valve (27) disposed between the pump body (21) and the first opening (241), a third valve (28) disposed between the liquid supply tank (22) and the second opening (242), and a fourth valve (29) disposed between the first opening (241) and the collection tank (23).

11. The method for proppant reflux testing according to claim 10, characterized in that, By opening the second valve (27) and the third valve (28) and closing the first valve (26) and the fourth valve (29), the liquid supply assembly (20) is able to provide a flow path along the first direction from the first opening (241) to the second opening (242).

12. The method for proppant reflux testing according to claim 10, characterized in that, By opening the first valve (26) and the fourth valve (29) and closing the second valve (27) and the third valve (28), the liquid supply assembly (20) is able to provide a flow path along the second direction from the second opening (242) to the first opening (241).

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