Pre-slit fracture control high pressure fracturing technology under low stress difference of storage and isolation layer and application

Abstract

The application provides a pre-seam-creating seam-controlling high-pressure fracturing process under low reservoir-separator stress difference and application. The pre-seam-creating seam-controlling high-pressure fracturing process under low reservoir-separator stress difference is applied to X14 well, and the seam height in the reservoir of the X14 well is controlled to be 14-20 m, and seam height control effect is good; the well is opened and flowback for 8.5 hours after being pressurized, and ignition is successful, and the highest daily production after production reaches 2.61x10 4 m 3 / d, the converted average daily production is 1.91x10 4 m 3 / d, compared with adjacent wells, the daily production is increased by 154%, and good seam height control and production increasing effect are achieved.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas reservoir development technology, and in particular relates to a pre-fracture and controlled high-pressure fracturing process and its application under low reservoir stress difference. Background Technology

[0002] Hydraulic fracturing can create highly conductive fractures, significantly increasing production in tight oil and gas reservoirs. However, with the same volume of fracturing fluid and proppant injected, fracture height control has a major impact on the final production enhancement. Uncontrolled fracture height can lead to problems such as water layers connecting the upper and lower layers of the producing formation, reservoir migration between layers with different pressure systems, waste of fracturing fluid and proppant, increased construction costs, and unsatisfactory fracturing production enhancement. Therefore, effectively controlling fracture height is one of the key technologies for fracturing stimulation. To achieve fracture height control, existing technologies have proposed artificial interlayer fracture height control techniques.

[0003] Artificial separator fracturing technology involves introducing a separator into the fracture using a carrier fluid before injecting the mixed sand fluid during fracturing. This forms an artificial separator at the top or bottom of the fracture, restricting the upward or downward movement of the mixed sand fluid, altering the distribution of vertical pressure within the fracture, and limiting its vertical propagation. The formation of the artificial separator effectively increases the barrier stress of the natural separators above and below, thereby widening the stress difference between the reservoir and the separator.

[0004] This shows that the stress difference between the reservoir and the caprock is a crucial factor influencing fracture height; as the stress difference decreases, the fracture height increases. This indicates that when the stress difference between the producing and caprock is high, fracture height extension is limited, but fracture length extension is beneficial. Therefore, a higher stress difference is advantageous for hydraulic fracturing. Consequently, for reservoirs with low reservoir-caprock stress differences, the small pressure difference between the upper and lower reservoir layers makes it difficult to control fracture height during fracturing, leading to excessively high fracture heights and potentially causing serious problems such as uncontrolled fracture height and reservoir cross-contamination. Currently, the commonly used carrying fluid technology allows the carried medium to easily sink during the carrying process, forming an artificial spacer at the bottom of the fracture. Therefore, controlling downward fracture extension is relatively easy. However, upward fracture extension control is typically achieved only through artificial spacers formed by hollow microparticles, but this method is less effective, especially for reservoirs with low reservoir-caprock stress differences, where fracture control at the fracture top is particularly poor.

[0005] Fracturing height control agents have a certain effect on controlling the vertical extension of fractures. However, the extension of fracture height is also affected by construction factors such as fracturing fluid viscosity, flow rate, and the timing and proportion of fracture height control agent addition. Therefore, it is necessary to comprehensively consider the construction conditions and the impact of fracture height control agents on fracture height control. Summary of the Invention

[0006] To address the issue of poor fracture control performance in existing fracture height control technologies under low reservoir stress differences, this invention provides a pre-fracturing and controlled high-pressure fracturing process and its application under low reservoir stress differences. Based on the combined use of air-suspended proppant and conventional proppant, it further subdivides reservoirs with low reservoir stress differences (not exceeding 6 MPa) and provides different pre-fracturing and controlled high-pressure fracturing processes for each reservoir level. Considering factors such as fracturing fluid viscosity, flow rate, type of fracture height control agent, timing of addition, and ratio, it achieves precise fracture height control for different reservoir levels.

[0007] This invention provides a pre-fracture control high-pressure fracturing process under low reservoir stress difference, comprising the following steps:

[0008] Step 1. Obtain the reservoir-interstitial stress difference of the target reservoir;

[0009] Step 2. Determine the grade of the target reservoir based on the stress difference between the reservoir and the interlayer;

[0010] Step 3. Provide a fracture-inducing and fracture-controlled high-pressure fracturing fluid comprising a base fluid, an air-suspended proppant, and a conventional proppant; determine the construction conditions of the fracture-inducing and fracture-controlled high-pressure fracturing fluid according to the grade of the target reservoir, and implement the pre-fracture-inducing and fracture-controlled high-pressure fracturing process.

[0011] According to the present invention, in step 2, the criteria for determining the grade of the target reservoir include: when the stress difference between the reservoir and the interlayer of the target reservoir is >4MPa and ≤6MPa, the grade of the target reservoir is determined to be Grade A;

[0012] When the reservoir-interlayer stress difference of the target reservoir meets the requirements of ≥2MPa and ≤4MPa, the target reservoir is determined to be of grade B.

[0013] When the stress difference between the reservoir and the interlayer of the target reservoir is less than 2 MPa, the target reservoir is classified as Grade C.

[0014] According to the present invention, in step 3, the construction conditions of the fracture-forming and fracture-controlling high-pressure fracturing fluid include the viscosity and / or discharge rate of the base fluid.

[0015] According to the present invention, when the target reservoir is classified as Grade A, the viscosity of the base fluid is 9-12 mPa·s; and / or the flow rate is 8-10 m³ / s. 3 / min; or

[0016] When the target reservoir is classified as Grade B, the viscosity of the base fluid is 9-12 mPa·s; and / or the flow rate is 6-8 m³ / s. 3 / min; or

[0017] When the target reservoir is classified as Grade C, the viscosity of the base fluid is 6-9 mPa·s; and / or the flow rate is 4-6 m³ / s. 3 / min.

[0018] According to the present invention, in step 3, the construction conditions of the fracture-controlled high-pressure fracturing fluid further include the method of injecting the fracture-controlled high-pressure fracturing fluid into the target reservoir, including:

[0019] The injected portion of the base fluid serves as the joint plug I;

[0020] The injected portion of the base liquid carries the gas-suspended proppant and is accompanied by liquid nitrogen injection, serving as an upward-controlling high-segment plug;

[0021] The injected portion of the base liquid serves as the joint plug II;

[0022] The injected portion of the base fluid carries the conventional proppant as a downward-controlled high-segment plug;

[0023] The injected portion of the base fluid serves as the joint plug III.

[0024] According to the present invention, the construction conditions of the fracture-creating and control-fracture high-pressure fracturing fluid further include the dosage of the base fluid in the fracture-creating slug I, the upward fracture-controlling high slug, the fracture-creating slug II, the downward fracture-controlling high slug, and the fracture-creating slug III; and / or

[0025] The amount of gas-suspended proppant used in the upward-controlled high-segment plug; and / or the amount of liquid nitrogen injected; and / or

[0026] The amount of conventional proppant used in the downward controlled-slit high-segment plug.

[0027] According to the present invention, when the target reservoir is classified as Grade A, the total volume of the base fluid in the fracture-controlling high-pressure fracturing fluid is taken as 100%, the volume percentage of the base fluid in fracture-starting slug I is 40-50%, the volume percentage of the base fluid in the upward-controlling high-slug is 15-20%, the volume percentage of the base fluid in fracture-starting slug II is 10%, the volume percentage of the base fluid in the downward-controlling high-slug is 15-20%, and the volume percentage of the base fluid in fracture-starting slug III is 10%; or

[0028] When the target reservoir is classified as Grade B or Grade C, the total volume of the base fluid in the fracture-controlling high-pressure fracturing fluid is taken as 100%. The volume percentage of the base fluid in the fracture-starting slug I is 30-40%, the volume percentage of the base fluid in the upward-controlling high-slug is 20-25%, the volume percentage of the base fluid in the fracture-starting slug II is 10%, the volume percentage of the base fluid in the downward-controlling high-slug is 20-25%, and the volume percentage of the base fluid in the fracture-starting slug III is 10%.

[0029] According to the present invention, when the target reservoir is classified as Grade A, Grade B, or Grade C, the volume of the base fluid in the upward-controlled fracture high-segment plug is taken as 100%, and the volume percentages of the gas-suspended proppant and the accompanying liquid nitrogen are independently 10-15%; and / or

[0030] The volume of the base fluid in the downward controlled-slit high-segment plug is 100%, and the volume percentage of the conventional proppant is 10-15%.

[0031] According to the present invention, the fracture-forming and fracture-controlling high-pressure fracturing fluid further includes a drag-reducing agent and / or an anti-swelling agent;

[0032] Preferably, the total mass of the base fluid in the fracture-forming and fracture-controlling high-pressure fracturing fluid is 100%, the mass percentage of the drag-reducing agent is not higher than 0.2 wt%, and / or the mass percentage of the anti-swelling agent is not higher than 0.4 wt%.

[0033] According to the present invention, the fracture-creating and fracture-controlling high-pressure fracturing fluid further includes a drainage aid and / or a foaming agent;

[0034] Preferably, in the fracture-forming and fracture-controlling high-pressure fracturing fluid, the mass concentration of the drainage aid and the foaming agent is independently not higher than 200 ppm.

[0035] According to the present invention, the low reservoir stress difference refers to a reservoir stress difference not exceeding 6 MPa.

[0036] The process described in this invention is used as a pre-fracture control stage in the entire fracturing process of the target reservoir.

[0037] Preferably, the construction conditions also include the injection volume of the high-pressure fracturing fluid used for fracturing and controlling the fracture throughout the entire fracturing process.

[0038] According to the present invention, the total volume of fracturing fluid injected in the entire fracturing process is taken as 100%. When the target reservoir is classified as Grade A, the injection volume of the fracturing fluid for fracture creation and control is 25-35%; or

[0039] When the target reservoir is classified as Grade B, the injection volume of the fracture-creating and fracture-controlling high-pressure fracturing fluid accounts for 20-30%; or

[0040] When the target reservoir is classified as Grade C, the injection volume ratio of the fracture-creating and controlled-fracture high-pressure fracturing fluid is 10-20%. The beneficial effects of this invention are:

[0041] To address the poor performance of existing fracture control technologies in reservoirs with low reservoir-interstitial stress differences, particularly in controlling fracture extension to the top, this invention provides a pre-fracturing and controlled fracture high-pressure fracturing process and its application under low reservoir-interstitial stress differences. This pre-fracturing and controlled fracture high-pressure fracturing process utilizes a combination of air-suspended proppant and conventional proppant. The air-suspended proppant and conventional proppant are injected into the target reservoir in a slug-like manner, forming artificial interlayers at the top and bottom of the fracture, respectively, to prevent vertical fracture propagation during fracturing. This effectively compensates for the shortcomings of existing fracture control technologies in preventing fracture extension to the top. Furthermore, since reservoirs with low reservoir-interstitial stress differences are more prone to losing fracture height control during fracturing, the inventors noted a "one-size-fits-all" flaw in current fracture control technologies for these reservoirs. They fail to further subdivide the numerous reservoirs with reservoir-interstitial stress differences (not exceeding 6 MPa) based on these differences, lacking specificity. This is one reason for the poor performance of existing fracture control technologies. Based on this, this invention, building upon the existing range of low reservoir stress difference, further subdivides the grades of reservoirs with low reservoir stress difference. For different grades of reservoirs, considering factors such as the viscosity and flow rate of the base fluid carrying gas-suspended proppant and conventional proppant, the formulation of the fracture-creating and controlling high-pressure fracturing fluid, and the injection timing of gas-suspended proppant and conventional proppant, it enables more precise and effective pre-fracture-creating and controlling high-pressure fracturing of the target reservoir. This significantly improves the fracture control effect of the target reservoir and ultimately enhances the fracturing production of the target reservoir. Specifically, as demonstrated by the actual application of the pre-fracture-creating and controlling high-pressure fracturing technology under low reservoir stress difference provided by this invention in well X14, the fracture height within the X14 reservoir was controlled at 14-20m after fracturing, demonstrating good fracture height control. The well successfully ignited 8.5 hours after fracturing and flowback, and the highest daily production after commissioning reached 2.61 × 10⁻⁶. 4 m 3 / d, equivalent to an average daily output of 1.91×10 4 m 3 / d, compared to the adjacent well, the daily production increased by 154%, achieving a good effect of controlled fracture and high production. Attached Figure Description

[0042] Figure 1 This invention demonstrates the high-pressure fracturing principle of pre-fracturing and fracturing control under low reservoir pressure differential provided by the present invention. Detailed Implementation

[0043] The present invention will be further described below with reference to the embodiments. However, the embodiments of the present invention are merely illustrative examples and should not be construed as limiting the present invention under any circumstances.

[0044] In existing technologies, reservoirs with low reservoir-interceptor stress differences mainly refer to reservoirs where the stress difference between the reservoir and interceptor is no higher than 6 MPa. Reservoir-interceptor stress difference is a crucial factor affecting fracture height. As the stress difference decreases, the fracture height increases. When the stress difference between the producing and caprocks is high, fracture height extension is limited, which is conducive to fracture length growth. However, when the pressure difference between the upper and lower reservoirs is small, it becomes difficult to limit fracture height during fracturing, easily leading to severe fracture height runaway and reservoir migration problems. Therefore, controlling fracture height during fracturing is more challenging in reservoirs with low reservoir-interceptor stress differences. However, current fracture height control technologies for low reservoir stress difference reservoirs only target the broad category of reservoirs with low reservoir stress difference, without further subdividing them. Combined with the inherent difficulties in controlling fracture height in low reservoir stress difference reservoirs, existing fracture height control technologies for low reservoir stress difference reservoirs are not very effective in controlling fracture height in more subdivided low reservoir stress difference reservoirs with a reservoir stress difference not exceeding 6 MPa, especially in controlling the upward extension of fractures.

[0045] To address the aforementioned problems, embodiments of the present invention classify reservoirs with a stress difference not exceeding 6 MPa based on the reservoir-interstitial stress difference, and adjust construction conditions accordingly. Specifically, embodiments of the present invention provide a pre-fracture and controlled high-pressure fracturing process under low reservoir-interstitial stress difference, comprising the following steps:

[0046] Step 1. Obtain the reservoir-interstitial stress difference of the target reservoir;

[0047] Step 2. Determine the grade of the target reservoir based on the stress difference between the reservoir and the interlayer;

[0048] Step 3. Provide a fracture-inducing and fracture-controlled high-pressure fracturing fluid comprising a base fluid, an air-suspended proppant, and a conventional proppant; determine the construction conditions of the fracture-inducing and fracture-controlled high-pressure fracturing fluid according to the grade of the target reservoir, and implement the pre-fracture-inducing and fracture-controlled high-pressure fracturing process.

[0049] In this embodiment of the invention, the reservoir-interlayer stress difference of the target reservoir is obtained by analyzing the logging and well logging data of the target reservoir. The acquisition of the logging and well logging data of the target reservoir is prior art and will not be described in detail here.

[0050] In this embodiment of the invention, the air-suspended proppant can be a commercially available air-suspended proppant (e.g., quartz sand), or it can be an air-suspended proppant provided in Chinese Patent CN116064026B (Direct-addition suspending agent for real-time modification of proppant during fracturing and its application), Chinese Patent CN112708413B (An air-filled proppant for airbag shell and its preparation method), Chinese Patent CN114032085B (A high-efficiency air-suspended proppant for fracturing and its preparation method), Chinese Patent CN110724515B (An air-suspended proppant for fracturing and its construction method), Chinese Patent CN113901664B (A method for optimizing proppant suspension parameters based on bubble bridge effect and a suspension method), or an air-suspended proppant prepared by the device provided in Chinese Patent CN215823390U (An online continuous spraying device for gas-suspended proppant for fracturing site).

[0051] In this embodiment of the invention, the conventional proppant may be a commercially available ceramsite proppant, such as ceramsite proppant QXS-Z and / or ceramsite proppant QXA-ZJ.

[0052] In this embodiment of the invention, the particle size of the air-suspended proppant and the conventional proppant is determined by the vertical depth of the target reservoir:

[0053] When the vertical depth of the target reservoir is ≥1500m, the particle size of the air-suspended proppant and the conventional proppant are independently 70 / 140 mesh;

[0054] When the vertical depth of the target reservoir is less than 1500m, the particle size of the air-suspended proppant and the conventional proppant are independently 40 / 70 mesh proppant.

[0055] In this embodiment of the invention, in step 2, the criteria for determining the grade of the target reservoir include: when the stress difference between the reservoir and the interlayer of the target reservoir is >4MPa and ≤6MPa, the grade of the target reservoir is determined to be Grade A;

[0056] When the reservoir-interlayer stress difference of the target reservoir meets the requirements of ≥2MPa and ≤4MPa, the target reservoir is determined to be of grade B.

[0057] When the stress difference between the reservoir and the interlayer of the target reservoir is less than 2 MPa, the target reservoir is classified as Grade C.

[0058] This invention classifies reservoirs with a stress difference of no more than 6 MPa into grades of 2 MPa each. In actual production, reservoirs with low stress differences can be further classified into more detailed grades according to the need for fracture height control. Construction conditions can be adjusted for different grades of reservoirs, and the pre-fracture and controlled high-pressure fracturing process under low stress differences can be implemented to achieve more precise fracture height control.

[0059] In this embodiment of the invention, in step 3, the construction conditions of the fracture-creating and fracture-controlling high-pressure fracturing fluid include the viscosity and / or discharge rate of the base fluid.

[0060] The viscosity of the base fluid affects its proppant-carrying capacity, especially its ability to carry conventional proppant, which in turn affects the fracture control effect during the fracture-making process. The discharge rate of the base fluid affects the fracture-making speed and impact of the high-pressure fracturing fluid on the reservoir, and also affects the fracture control effect during the fracture-making process. Since the fracture height extension of the reservoir is more sensitive under low reservoir-interstitial pressure differential, it is necessary to limit the viscosity and / or discharge rate of the base fluid.

[0061] In this embodiment of the invention, when the target reservoir is classified as Grade A, the viscosity of the base fluid is 9-12 mPa·s; and / or the flow rate is 8-10 m³ / s. 3 / min; or

[0062] When the target reservoir is classified as Grade B, the viscosity of the base fluid is 9-12 mPa·s; and / or the flow rate is 6-8 m³ / s. 3 / min; or

[0063] When the target reservoir is classified as Grade C, the viscosity of the base fluid is 6-9 mPa·s; and / or the flow rate is 4-6 m³ / s. 3 / min.

[0064] In this embodiment of the invention, the base liquid is drag-reducing water that meets the above viscosity conditions.

[0065] Considering that the vertical propagation of fractures in reservoirs with low reservoir stress difference is more sensitive, compared with injecting the fracture-creating and fracture-controlling high-pressure fracturing fluid all at once, designing a scientific and reasonable combination of slugs to inject the fracture-creating and fracture-controlling high-pressure fracturing fluid into the target reservoir in stages is beneficial to improving the accuracy of fracture height control. While ensuring sufficient fracture depth, the fracture height is limited to an ideal range.

[0066] In this embodiment of the invention, step 3, the construction conditions for the fracture-controlling high-pressure fracturing fluid further include the method of injecting the fracture-controlling high-pressure fracturing fluid into the target reservoir, including:

[0067] The injected portion of the base fluid serves as the joint plug I;

[0068] The injected portion of the base liquid carries the gas-suspended proppant and is accompanied by liquid nitrogen injection, serving as an upward-controlling high-segment plug;

[0069] The injected portion of the base liquid serves as the joint plug II;

[0070] The injected portion of the base fluid carries the conventional proppant as a downward-controlled high-segment plug;

[0071] The injected portion of the base fluid serves as the joint plug III.

[0072] In addition to injecting the fracture-controlling high-pressure fracturing fluid into the target reservoir in a slug manner, it is also necessary to define the construction conditions during the injection process of each slug for different reservoir grades in order to ensure effective and accurate fracture height control for each reservoir grade.

[0073] In this embodiment of the invention, the construction conditions of the fracture-creating and control-fracture high-pressure fracturing fluid further include the dosage of the base fluid in the fracture-creating slug I, the upward fracture-controlling high slug, the fracture-creating slug II, the downward fracture-controlling high slug, and the fracture-creating slug III; and / or

[0074] The amount of gas-suspended proppant used in the upward-controlled high-segment plug; and / or the amount of liquid nitrogen injected; and / or

[0075] The amount of conventional proppant used in the downward controlled-slit high-segment plug.

[0076] In this embodiment of the invention, when the target reservoir is classified as Grade A, the total volume of the base fluid in the fracture-controlling high-pressure fracturing fluid is taken as 100%, the volume percentage of the base fluid in fracture slug I is 40-50%, the volume percentage of the base fluid in the upward-controlling high-slug is 15-20%, the volume percentage of the base fluid in fracture slug II is 10%, the volume percentage of the base fluid in the downward-controlling high-slug is 15-20%, and the volume percentage of the base fluid in fracture slug III is 10%; or

[0077] When the target reservoir is classified as Grade B or Grade C, the total volume of the base fluid in the fracture-controlling high-pressure fracturing fluid is taken as 100%. The volume percentage of the base fluid in the fracture-starting slug I is 30-40%, the volume percentage of the base fluid in the upward-controlling high-slug is 20-25%, the volume percentage of the base fluid in the fracture-starting slug II is 10%, the volume percentage of the base fluid in the downward-controlling high-slug is 20-25%, and the volume percentage of the base fluid in the fracture-starting slug III is 10%.

[0078] In this embodiment of the invention, when the target reservoir is classified as Grade A, Grade B, or Grade C, the volume of the base fluid in the upward-controlled fracture high-segment plug is taken as 100%, and the volume percentages of the gas-suspended proppant and the accompanying liquid nitrogen are independently 10-15%; and / or

[0079] The volume of the base fluid in the downward controlled-slit high-segment plug is 100%, and the volume percentage of the conventional proppant is 10-15%.

[0080] In this embodiment of the invention, the fracture-forming and control-fracturing high-pressure fracturing fluid further includes drag-reducing agents and / or anti-swelling agents.

[0081] Preferably, the total mass of the base fluid in the fracture-forming and fracture-controlling high-pressure fracturing fluid is 100%, the mass percentage of the drag-reducing agent is not higher than 0.2 wt%, and / or the mass percentage of the anti-swelling agent is not higher than 0.4 wt%.

[0082] In this embodiment of the invention, the fracture-creating and fracture-controlling high-pressure fracturing fluid further includes a drainage aid and / or a foaming agent.

[0083] Preferably, in the fracture-forming and fracture-controlling high-pressure fracturing fluid, the mass concentration of the drainage aid and the foaming agent is independently not higher than 200 ppm.

[0084] In this embodiment of the invention, the specific types of drag-reducing agents, anti-swelling agents, drainage aids, and foaming agents are not limited. They can be selected from commercially available drag-reducing agents, anti-swelling agents, drainage aids, and foaming agents based on the base liquid. The selection of suitable drag-reducing agents, anti-swelling agents, drainage aids, and foaming agents based on the base liquid is prior art and will not be elaborated here. The drag-reducing agents, anti-swelling agents, drainage aids, and foaming agents can be added to the base liquid of the fracture-creating and fracture-controlling high-pressure fracturing fluid.

[0085] In this embodiment of the invention, the low reservoir stress difference refers to a reservoir stress difference not exceeding 6 MPa.

[0086] The process described in the embodiments of the present invention is used as a pre-fracture and control phase in the entire fracturing process of reservoirs with low reservoir stress difference.

[0087] Preferably, the construction conditions also include the injection volume of the fracture-creating and fracture-controlling high-pressure fracturing fluid in the entire fracturing process. For example, after applying the pre-fracture-creating and fracture-controlling high-pressure fracturing process provided in the embodiments of the present invention to the target reservoir, a sand-mixing fluid (in which air-suspended proppant can be optionally added) and a displacement fluid are then injected sequentially into the target reservoir to complete the entire fracturing process for the target reservoir.

[0088] For reservoirs of different grades, the injection volume of the high-pressure fracturing fluid injected into the target reservoir during the pre-fracturing and fracturing high-stage can vary slightly and can be adjusted according to the actual situation of the target reservoir.

[0089] In this embodiment of the invention, the total volume of fracturing fluid injected in the entire fracturing process is considered as 100%. When the target reservoir is classified as Grade A, the injection volume of the fracturing fluid for fracture creation and control is 25-35%; or

[0090] When the target reservoir is classified as Grade B, the injection volume of the fracture-creating and fracture-controlling high-pressure fracturing fluid accounts for 20-30%; or

[0091] When the target reservoir is classified as grade C, the injection volume ratio of the fracture-creating and fracture-controlling high-pressure fracturing fluid is 10-20%.

[0092] Combination Figure 1 The present invention provides a detailed description of the conventional fracturing process in existing technologies and the specific process of injecting high-pressure fracturing fluid into the target reservoir using a slug injection method as described in this invention: Figure 1 The left side of the diagram shows a schematic diagram of conventional fracturing technology in reservoirs. As can be seen from the diagram, with the injection of fracturing fluid, the fracture state formed in the reservoir changes from ①' to ②' and then to ③'. As the fracture extends deeper into the reservoir, its vertical height also gradually increases. The fracture expands significantly at both the top and bottom. Due to the lack of effective artificial barriers at the top and bottom of the fracture, control of the fracture height fails. Figure 1 As shown on the right, the present invention first injects a portion of the base fluid as a fracture-forming plug I into the target reservoir, thereby generating preliminary fractures (i.e., Figure 1 The fracture in area ① lays the foundation for subsequent fracture creation and height control. Then, a portion of the base fluid carrying the gas-suspended proppant is injected, along with liquid nitrogen as an accompanying gas, as an upward-controlling-fracture-height plug into the target reservoir. The injected liquid nitrogen generates bubbles that the gas-suspended proppant can absorb, accumulating at the top of the fracture to form a top artificial barrier. The formation of this top artificial barrier prevents the fracture height from extending upwards during fracturing, thus achieving upward fracture height control. Then, as... Figure 1 As shown in Figure ②, a portion of the base fluid is injected again into the target reservoir as a fracture-building slug II, allowing the fracture to extend deeper into the reservoir. At this point, due to the formation of the top artificial barrier, the fracture mainly increases in depth and does not extend further upwards. Next, a portion of the base fluid carrying the conventional proppant is injected into the target reservoir as a downward fracture height control slug. The conventional proppant deposits at the bottom of the fracture, forming a bottom artificial barrier, preventing the fracture height from extending downwards during fracturing, thus achieving downward fracture height control. Finally, as... Figure 1 As shown in ③, the remaining base fluid is injected into the target reservoir as a fracture-making plug III, allowing the fractures to extend further into the reservoir. While successfully controlling the fracture height, the ideal fracture depth is achieved, thus completing the pre-fracture-making and controlled high-pressure fracturing process for the target reservoir.

[0093] The following are application examples of the pre-fracture and controlled-fracture high-pressure fracturing process under low reservoir stress difference provided by this invention. The anti-swelling agent and drag-reducing agent used below were all purchased from Dongying Shipuri Petroleum Engineering Technology Co., Ltd.

[0094] Example 1

[0095] The pre-fracture and controlled high-pressure fracturing process under low reservoir stress difference was implemented in well X14 of a gas reservoir in Sichuan Province. The specific steps are as follows.

[0096] Step S1. Obtain the reservoir-interstitial stress difference of the target reservoir.

[0097] Based on the analysis of the logging and well logging data obtained from the X14 gas reservoir, the basic parameters of the X14 gas reservoir shown in Table 1 are obtained.

[0098] Table 1. Basic parameters of well X14

[0099] Parameter name numerical values Porosity / % 9.8 Permeability / mD 0.36 Reservoir thickness / m 16 Layer thickness / m 15 Maximum horizontal principal stress / MPa 24.77 Minimum horizontal principal stress / MPa 19.7 reservoir stress difference / MPa 1.2 Perforation position / m 1131-1131.5

[0100] Step S2. Determine the grade of the target reservoir based on the stress difference of the reservoir layer.

[0101] As can be seen from the basic parameters of well X14 in Table 1, the reservoir stress difference of well X14 is 1.2 MPa, which is less than 2 MPa, and the reservoir grade is C.

[0102] Step S3. Provide a fracture-inducing and fracture-controlled high-pressure fracturing fluid comprising a base fluid, an air-suspended proppant, and a conventional proppant; determine the construction conditions of the fracture-inducing and fracture-controlled high-pressure fracturing fluid according to the grade of the target reservoir, and implement the pre-fracture-inducing and fracture-controlled high-pressure fracturing process.

[0103] This embodiment uses a fracture-creating and fracture-controlling high-pressure fracturing fluid comprising a base fluid, an air-suspended proppant, and a ceramsite proppant. The base fluid is drag-reducing water, the air-suspended proppant is 40 / 70 mesh 35MPa quartz sand, and the ceramsite proppant is 40 / 70 mesh 52MPa ceramsite. The base fluid contains drag-reducing agents and anti-swelling agents. Taking the total mass of the base fluid in the fracture-creating and fracture-controlling high-pressure fracturing fluid as 100%, the drag-reducing agent accounts for 0.2% of the mass, and the anti-swelling agent accounts for 0.1% of the mass.

[0104] Based on the C-level reservoir determined in step S2, the construction conditions for fracture-controlled high-pressure fracturing fluid in this embodiment are determined as follows:

[0105] Viscosity of the base liquid: 9 mPa·s;

[0106] Base liquid discharge rate: 5-6m 3 / min.

[0107] Considering that the vertical propagation of fractures in reservoirs is more sensitive under low reservoir stress difference, compared with injecting high-pressure fracturing fluid for fracture creation and control all at once, a scientifically designed and reasonable combination of slugs is used to inject high-pressure fracturing fluid for fracture creation and control in stages into well X14. This is beneficial to improve the accuracy of fracture height control, ensuring sufficient fracture depth while limiting the fracture height within an ideal range.

[0108] Based on the C-level reservoir determined in step S2, the construction conditions for fracture-controlled high-pressure fracturing fluid in this embodiment also include the method of injecting fracture-controlled high-pressure fracturing fluid into well X14, including:

[0109] Inject a portion of the base fluid as the joint plug I;

[0110] A portion of the base liquid carrying the gas-suspended proppant is injected, along with liquid nitrogen, as a high-segment plug for upward control of the seam;

[0111] Inject a portion of the base fluid as the joint plug II;

[0112] A portion of the base fluid carrying conventional proppant is injected as a high-segment plug for downward controlled seam.

[0113] A portion of the base fluid was injected as the joint plug III.

[0114] Based on the determination to inject high-pressure fracturing fluid for fracture control into well X14 using a slug injection method, the construction conditions during the injection process of each slug are limited for the reservoir of well X14 to ensure effective and precise fracture height control for reservoirs of all grades.

[0115] The construction conditions for the high-pressure fracturing fluid used for joint creation and control in this embodiment also include:

[0116] Volume percentage of base fluid in Fracturing Segment Plug I: The total volume of base fluid in the fracture control high-pressure fracturing fluid is taken as 100%, and the volume percentage of base fluid in Fracturing Segment Plug I is 30%.

[0117] In the upward-controlled fracture high-segment plug: the total volume of the base fluid in the fracture-controlling high-pressure fracturing fluid is taken as 100%, and the volume ratio of the base fluid in the upward-controlled fracture high-segment plug is 25%.

[0118] The volume of the base fluid in the upper section of the upward-controlled joint plug is 100%, and the volume of the air-suspended proppant is 15%; the volume of the base fluid in the upper section of the upward-controlled joint plug is 100%, and the volume of the accompanying liquid nitrogen is 15%.

[0119] Volume percentage of base fluid in Fracturing Segment Plug II: The total volume of base fluid in the fracture control high-pressure fracturing fluid is taken as 100%, and the volume percentage of base fluid in Fracturing Segment Plug II is 10%.

[0120] In the downward controlled fracture high-segment plug: the total volume of the base fluid in the fracture control high-pressure fracturing fluid is taken as 100%, and the volume ratio of the base fluid in the downward controlled fracture high-segment plug is 25%.

[0121] The volume of the base fluid in the high-segment plug of the downward controlled seam is taken as 100%, and the volume of the conventional proppant is 15%.

[0122] The volume percentage of the base fluid in the fracture slug III: Taking the total volume of the base fluid in the fracture control high-pressure fracturing fluid as 100%, the volume percentage of the base fluid in the fracture slug III is 10%.

[0123] To better demonstrate the production enhancement effect of the pre-fracture and controlled fracture high-pressure fracturing process under low reservoir stress difference provided by this invention on the target reservoir (here, well X14) after fracture and controlled fracture, it was determined that after fracture and controlled fracture in well X14, a mixed sand fluid (100% by total mass of mixed sand fluid, including 0.1-0.2wt% drag reducer, 0.1wt% anti-swelling agent, 10-35wt% 40 / 70 mesh 35MPa quartz sand and 64.7-89.8wt% formation water) and a displacement fluid (specifically formation water) were injected to implement the entire fracturing process.

[0124] In this embodiment, the construction conditions for fracture-creating and controlled high-pressure fracturing fluid, determined according to the reservoir grade of well X14, also include:

[0125] The total volume of fracturing fluid injected in the entire fracturing process of well X14 is considered as 100%, while the total volume of high-pressure fracturing fluid injected for fracturing and fracturing control accounts for 15%.

[0126] Further, based on the determined proportion of the injection volume of the fracturing fluid for fracturing and controlling fracture, it was determined that the injection volume of the sand-mixing fluid and the injection volume of the displacement fluid accounted for 84% and 1% of the total injection volume of fracturing fluid in the entire fracturing process of well X14, respectively.

[0127] According to the aforementioned construction conditions, firstly, 30% of the total volume of the base fluid in the high-pressure fracturing fluid for fracture control was injected into well X14 as the fracture-building slug I, generating preliminary fractures in the reservoir and laying the foundation for the subsequent fracture-building and fracture-control process. Next, 25% of the total volume of the base fluid in the high-pressure fracturing fluid for fracture control, carrying gas-suspended proppant (taking the volume of base fluid in the upward fracture-control slug as 100%, and the volume of gas-suspended proppant as 15%), was injected into well X14 as the upward fracture-control slug. Simultaneously, liquid nitrogen was injected as accompanying gas (taking the volume of base fluid in the upward fracture-control slug as 100%, and the volume of accompanying liquid nitrogen as 15%), and injected as the upward fracture-control slug. The accompanying liquid nitrogen generated bubbles for the gas-suspended proppant to absorb, which accumulated at the top of the fracture along with the proppant, forming a top artificial barrier. The formation of this top artificial barrier prevented the fracture height from extending upwards during fracturing, achieving upward fracture height control. Then, fracture control was used again... 10% of the total volume of the base fluid in the fracture-control high-pressure fracturing fluid was injected into well X14 as fracture-building slug II, allowing the fractures to extend further into the reservoir. At this point, due to the formation of the top artificial barrier, the fractures mainly increased in depth and did not extend further upwards. Next, 25% of the total volume of the base fluid in the fracture-control high-pressure fracturing fluid, carrying ceramic proppant (taking the volume of the base fluid in the upward fracture-control high-pressure slug as 100%, and the volume of the ceramic proppant as 15%), was injected into well X14 as a downward fracture-control high-pressure slug. The ceramic proppant deposited at the bottom of the fracture to form a bottom artificial barrier, preventing the fracture height from extending downwards during fracturing, thus achieving downward fracture height control. Finally, 10% of the total volume of the base fluid in the fracture-control high-pressure fracturing fluid was injected into well X14 as fracture-building slug III, allowing the fractures to extend further into the reservoir to reach the required fracture depth. This completed the pre-fracture-control high-pressure fracturing process for well X14. Then, sand-mixing fluid, accounting for 84% of the total volume of fracturing fluid injected in the entire fracturing process, and formation water, accounting for 1% of the total volume of fracturing fluid injected in the entire fracturing process, are injected into well X14 as displacement fluids. The aforementioned high-pressure fracturing fluid for fracturing and control and sand-mixing fluid are then injected into the reservoir of well X14 to complete the entire fracturing process of well X14.

[0128] Measurements showed that after fracturing, the fracture height in the X14 well reservoir was controlled at 14-20m, proving that the pre-fracturing and controlled fracturing high-pressure fracturing process with low reservoir stress difference provided by this invention has good fracture height control effect. The well successfully ignited 8.5 hours after fracturing and well opening / flowback, and its highest daily production after commissioning reached 2.61 × 10⁻⁶ m. 4 m 3 / d, equivalent to an average daily output of 1.91×10 4 m 3 / d, compared to the adjacent well, the daily production increased by 154%, achieving a good effect of controlled fracture and high production.

[0129] While the present invention has been described with reference to specific embodiments, those skilled in the art will understand that various changes can be made without departing from the true spirit and scope of the invention. Furthermore, numerous modifications can be made to the subject, spirit, and scope of the invention to suit specific situations, materials, material compositions, and methods. All such modifications are included within the scope of the claims of the present invention.

Claims

1. A pre-fracture controlled high-pressure fracturing process under low reservoir stress difference, comprising the following steps: Step 1. Obtain the reservoir-interstitial stress difference of the target reservoir; Step 2. Determine the grade of the target reservoir based on the stress difference between the reservoir and the interlayer; Step 3. Provide a fracture-inducing and fracture-controlled high-pressure fracturing fluid comprising a base fluid, an air-suspended proppant, and a conventional proppant; determine the construction conditions of the fracture-inducing and fracture-controlled high-pressure fracturing fluid according to the grade of the target reservoir, and implement the pre-fracture-inducing and fracture-controlled high-pressure fracturing process.

2. The process according to claim 1, characterized in that, In step 2, the criteria for determining the grade of the target reservoir include: when the stress difference between the reservoir and the interlayer of the target reservoir is >4MPa and ≤6MPa, the grade of the target reservoir is determined to be Grade A; When the reservoir-interlayer stress difference of the target reservoir meets the requirements of ≥2MPa and ≤4MPa, the target reservoir is determined to be of grade B. When the stress difference between the reservoir and the interlayer of the target reservoir is less than 2 MPa, the target reservoir is classified as Grade C.

3. The process according to claim 2, characterized in that, In step 3, the construction conditions of the fracture control high-pressure fracturing fluid include the viscosity and / or flow rate of the base fluid.

4. The process according to claim 3, characterized in that, When the target reservoir is classified as Grade A, the viscosity of the base fluid is 9-12 mPa·s; and / or the flow rate is 8-10 m³ / s. 3 / min; or When the target reservoir is classified as Grade B, the viscosity of the base fluid is 9-12 mPa·s; and / or the flow rate is 6-8 m³ / s. 3 / min; or When the target reservoir is classified as Grade C, the viscosity of the base fluid is 6-9 mPa·s; and / or the flow rate is 4-6 m³ / s. 3 / min.

5. The process according to claim 3 or 4, characterized in that, In step 3, the construction conditions for the fracture-controlled high-pressure fracturing fluid also include the method of injecting the fracture-controlled high-pressure fracturing fluid into the target reservoir, including: The injected portion of the base fluid serves as the joint plug I; The injected portion of the base liquid carries the gas-suspended proppant and is accompanied by liquid nitrogen injection, serving as an upward-controlling high-segment plug; The injected portion of the base liquid serves as the joint plug II; The injected portion of the base fluid carries the conventional proppant as a downward-controlled high-segment plug; The injected portion of the base fluid serves as the joint plug III.

6. The process according to claim 5, characterized in that, The construction conditions for the fracture-creating and control-fracturing high-pressure fracturing fluid also include the dosage of the base fluid in the fracture-creating slug I, the upward fracture-controlling high slug, the fracture-creating slug II, the downward fracture-controlling high slug, and the fracture-creating slug III; and / or The amount of gas-suspended proppant used in the upward-controlled high-segment plug; and / or the amount of liquid nitrogen injected; and / or The amount of conventional proppant used in the downward controlled-slit high-segment plug.

7. The process according to claim 6, characterized in that, When the target reservoir is classified as Grade A, the total volume of the base fluid in the fracture-controlled high-pressure fracturing fluid is taken as 100%. The volume percentage of the base fluid in fracture slug I is 40-50%, the volume percentage of the base fluid in the upward fracture-controlled high-slug is 15-20%, the volume percentage of the base fluid in fracture slug II is 10%, the volume percentage of the base fluid in the downward fracture-controlled high-slug is 15-20%, and the volume percentage of the base fluid in fracture slug III is 10%; or When the target reservoir is classified as Grade B or Grade C, the total volume of the base fluid in the fracture-controlling high-pressure fracturing fluid is taken as 100%. The volume percentage of the base fluid in the fracture-starting slug I is 30-40%, the volume percentage of the base fluid in the upward-controlling high-slug is 20-25%, the volume percentage of the base fluid in the fracture-starting slug II is 10%, the volume percentage of the base fluid in the downward-controlling high-slug is 20-25%, and the volume percentage of the base fluid in the fracture-starting slug III is 10%.

8. The process according to claim 6 or 7, characterized in that, When the target reservoir is classified as Grade A, B, or C, the volume of the base fluid in the upward-controlled fracture high-segment plug is taken as 100%, and the volume percentages of the gas-suspended proppant and the accompanying liquid nitrogen are independently 10-15%; and / or The volume of the base fluid in the downward controlled-slit high-segment plug is 100%, and the volume percentage of the conventional proppant is 10-15%.

9. The process according to any one of claims 1 to 8, characterized in that, The fracture-creating and control-fracture high-pressure fracturing fluid also includes drag-reducing agents and / or anti-swelling agents; Preferably, the total mass of the base fluid in the fracture-forming and fracture-controlling high-pressure fracturing fluid is 100%, the mass percentage of the drag-reducing agent is not higher than 0.2 wt%, and / or the mass percentage of the anti-swelling agent is not higher than 0.4 wt%.

10. The process according to any one of claims 1 to 9, characterized in that, The term "low reservoir stress difference" refers to a reservoir stress difference that is not higher than 6 MPa.

11. The application of the process according to any one of claims 1 to 10 as a pre-fracturing and control stage in the entire fracturing process of the target reservoir; Preferably, the construction conditions for the fracture-creating and control-fracture high-pressure fracturing fluid also include the injection volume of the fracture-creating and control-fracture high-pressure fracturing fluid throughout the entire fracturing process.

12. The application according to claim 11, characterized in that, Taking the total volume of fracturing fluid injected in the entire fracturing process as 100%, when the target reservoir is classified as Class A, the injection volume of the fracturing fluid for fracture creation and control is 25-35%; or When the target reservoir is classified as Grade B, the injection volume of the fracture-creating and fracture-controlling high-pressure fracturing fluid accounts for 20-30%; or When the target reservoir is classified as Grade C, the injection volume of the fracture-creating and fracture-controlling high-pressure fracturing fluid accounts for 10-20%.

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