A method for improving crack complexity in the early stage based on stress cyclic loading

Abstract

This invention discloses a method for improving fracture complexity in the pre-stage of tight oil reservoirs based on cyclic stress loading, belonging to the technical field of improving fracture complexity in the pre-stage of tight oil reservoirs. The method of this invention generates cyclic stress by alternately injecting high-viscosity and low-viscosity fluids in the pre-stage, thereby increasing the complexity of fractures in tight oil reservoirs and further improving single-well production. This method employs multiple alternating injections of high-viscosity and low-viscosity fluids in the pre-stage to form a cyclic stress loading pattern, causing complex micro-fractures in the rock, increasing the complexity of the fractures. Furthermore, the high-viscosity fluid in the pre-stage increases the net pressure within the fractures, thus enhancing the stimulation effect and achieving increased single-well production.

Description

Technical Field

[0001] This invention belongs to the technical field of improving the complexity of fractures in the early stage of tight oil reservoirs, and specifically relates to a method for improving the complexity of fractures in the early stage based on stress cyclic loading. Background Technology

[0002] With increased exploration and development efforts, tight oil reservoirs with large resource reserves have become important replacement reserves. Practice has shown that generating complex fractures is a crucial method for increasing production in tight oil reservoirs, and fracturing technology has become the primary means of generating complex fractures and increasing the stimulated volume in tight reservoirs. Currently, methods for generating complex fractures include staged fracturing, variable viscosity fracturing, temporary plugging fracturing, and variable displacement fracturing. These methods all achieve the generation of complex fractures by altering the conditions at each fracturing stage, thereby increasing the reservoir stimulated volume and improving single-well production. For tight reservoirs, research indicates that the initial stage is more conducive to the generation of complex fractures than other stages. If early fracture formation is insufficient, subsequent proppant fracturing will be more difficult, making it challenging to achieve effective support.

[0003] Currently, methods for generating complex fractures include (1) staged fracturing; (2) variable viscosity fracturing; (3) temporary plugging fracturing; and (4) variable displacement fracturing. For tight reservoirs, staged fracturing with added sand increases the complexity of fractures, but there is a risk of sand plugging. Variable viscosity fracturing requires additional material costs, and temporary plugging fracturing requires the introduction of specialized feeding equipment and has high requirements for the professionalism of operators. Summary of the Invention

[0004] In order to overcome the shortcomings of the prior art, the present invention aims to provide a method for improving the complexity of fractures in the pre-stage based on stress cyclic loading, which solves the problems of high fracturing cost, insignificant fracture generation effect, and high professional requirements for operators in existing complex fracture generation methods for tight oil reservoirs.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] This invention discloses a method for improving the complexity of cracks in the early stage based on stress cyclic loading, comprising the following steps:

[0007] S1: Collect reservoir parameters of the tight oil reservoir where the wellbore is located, establish the basis for layer selection, and calculate the geostress profile;

[0008] S2: Based on the geostress profile, calculate the stress magnitude of the reservoir and the interlayer, and calculate the net pressure under the tight oil reservoir conditions by using different fracturing discharge rates and different liquid viscosities of the pre-flush fluid. Obtain the viscosity and discharge rate of the high-viscosity and low-viscosity fluids applicable to the tight oil reservoir to ensure that they do not penetrate into the interlayer.

[0009] S3: In the pre-flush stage, high-viscosity and low-viscosity fluids are injected alternately into the wellbore to generate a cyclic stress effect, which causes complex micro-cracks to be generated at the wellbore fracturing location.

[0010] Furthermore, in S1, reservoir parameters include permeability, porosity, and oil saturation characteristics.

[0011] Furthermore, in S1, the calculated geostress profile is obtained by calculating the maximum and minimum horizontal principal stresses based on well logging curve data, and the stress difference between the maximum and minimum horizontal principal stresses is calculated to obtain the stress magnitude of the reservoir and the interlayer, and the stress difference between the reservoir and the interlayer is calculated.

[0012] Furthermore, in S2, combined with the geostress profile, fracturing tests are conducted on the tight oil reservoir. Then, active water injection and discharge reduction tests are performed sequentially to determine the frictional resistance, and base fluid injection and discharge reduction tests are performed to determine the fluid efficiency and formation parameters under the fluid system. Based on the results, the viscosities of the high-viscosity and low-viscosity fluids suitable for the tight oil reservoir are calculated.

[0013] Furthermore, the filtration loss coefficient of the high-viscosity liquid is 5.56*10. -4 m / min 1 / 2 The low-viscosity liquid does not have wall-forming properties.

[0014] Furthermore, the viscosity of the high-viscosity liquid is 24–30 mPa·s; and the viscosity of the low-viscosity liquid is 6–9 mPa·s.

[0015] Furthermore, in S3, when the high-viscosity liquid and the low-viscosity liquid are injected alternately, the high-viscosity liquid is injected first, followed by the low-viscosity liquid, and this process is repeated alternately.

[0016] Furthermore, in S3, the total volume of the high-viscosity liquid and the low-viscosity liquid injected is 1.1 to 1.3 times the wellbore volume.

[0017] Furthermore, in S3, the time difference between the alternating injection of the high-viscosity liquid and the low-viscosity liquid is within 10 minutes.

[0018] Furthermore, in S3, the alternating injection cycle of the high-viscosity liquid and the low-viscosity liquid is 2-3 times.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] This invention discloses a method for increasing fracture complexity in the pre-fracturing stage based on cyclic stress loading. Without increasing fracturing costs, and based on research into the compressibility of tight oil reservoirs, this method uses alternating injection of high-viscosity and low-viscosity fluids in the pre-fracturing stage to generate cyclic stress, thereby increasing fracture complexity and ultimately improving single-well production. During the alternating injection of high-viscosity and low-viscosity fluids, the high-viscosity fluid abruptly changes to low-viscosity fluid, causing a sudden release of pressure on the reservoir. This results in the rock transferring energy to the fluid, and the sudden release of energy causes the rock to lose its support and fracture. This leads to multi-stage cyclic injection, creating a cyclic stress loading pattern. Furthermore, the high-viscosity fluid increases the net pressure in the pre-fracturing stage, resulting in more complex micro-fractures in the rock. Subsequent proppant fracturing can generate more support fracture networks, thereby increasing the reservoir stimulation volume and improving single-well production. This method employs multiple alternating injections of high-viscosity and low-viscosity fluids in the pre-fracturing stage to create a cyclic stress loading pattern, resulting in complex micro-fractures in the rock and increasing fracture complexity. The high-viscosity fluid in the pre-fracturing stage also increases the net pressure within the fractures, thus enhancing the stimulation effect and improving single-well production. This method can create more complex cracks without increasing the cost of additional equipment and materials, requires no special training for operators, is highly operable, and has broad application prospects. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the interactive injection of pre-fluid combined with the sand-carrying stage using the method of the present invention. Detailed Implementation

[0022] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0023] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0024] This invention discloses a method for increasing the complexity of fractures in the pre-fracturing stage based on cyclic stress loading. This method generates cyclic stress loading by alternately injecting high-viscosity and low-viscosity pre-fracturing fluids, forcing the rock to fracture and generate complex micro-fractures, thereby increasing the fracture complexity of tight oil reservoirs. The technical solution includes the following steps:

[0025] S1. Identify reservoir types. Classify various reservoir types based on electrical and physical property data.

[0026] S2, Collect reservoir parameters. Collect and analyze data on tight oil reservoir parameters, including reservoir permeability, oil layer thickness, lithology, and clay minerals.

[0027] S3, calculate the geostress profile. For the selected reservoir, the maximum and minimum horizontal principal stresses of the reservoir, as well as the stress magnitudes of the reservoir and interlayers, are calculated.

[0028] S4, to study the net pressure under reservoir conditions with different displacement and liquid viscosity.

[0029] S5 is a preferred fracturing fluid system for different stages. The selected low-viscosity fluid has no wall-building properties and the high-viscosity fluid has a low filtration coefficient.

[0030] S6, Optimize the compatibility of the fracturing fluid system. The fracturing fluid should have good compatibility so that the stress it generates does not affect the effect of cyclic loading.

[0031] S7, Optimize fracturing operation parameters. Based on the above analysis, optimize and establish fracturing stimulation operation parameters that match the technical approach, reservoir characteristics, fracturing fluid, etc.

[0032] Furthermore, the reservoir type described in step S1 is a tight oil reservoir characterized by strong compressibility, no bottom water, good shielding conditions, and no high-angle fractures penetrating the mudstone.

[0033] Furthermore, the collection of reservoir parameters in step S2 requires coring the permeability, porosity, oil saturation, and other characteristics from the core analysis with the interpretation results corresponding to the well logging, in order to establish a basis for selecting reservoir layers for the cyclic stress loading fracturing scheme.

[0034] Furthermore, the calculation of the geostress profile in step S3 requires calculation based on well logging curve data to obtain the maximum and minimum horizontal principal stresses and calculate the stress difference between the two; to obtain the magnitude of reservoir and interlayer stresses and calculate the reservoir-interlayer stress difference.

[0035] Furthermore, the study of net pressure under reservoir conditions with different discharge rates and liquid viscosities described in step S4 requires small-scale fracturing tests on the formation, followed by active water injection to reduce discharge rate and determine frictional resistance, and base fluid injection to reduce discharge rate and determine the liquid efficiency and various formation parameters under the liquid system. Then, based on the results, the viscosity of the high-viscosity and low-viscosity liquids applicable to the well and the fracturing operation discharge rate are calculated.

[0036] Furthermore, the selection of fracturing fluid systems for different stages in step S5 needs to be based on the different degrees of fatigue damage to the rock. Different types of high-viscosity and low-viscosity fracturing fluids in the pre-fracturing stage should be optimized. Low-viscosity fluids should not have wall-building properties, while high-viscosity fluids should have a low filtration coefficient.

[0037] Furthermore, in step S6, the compatibility of the fracturing fluid system is optimized. Conventional volumetric fracturing only uses a single liquid injection. This invention relates to the alternating injection of fracturing fluids of different viscosities in the pre-fracturing stage. Therefore, it is necessary to consider the compatibility between the liquid and the formation fluid under different viscosity conditions to avoid changes in concentration and composition caused by the reaction, which in turn affects the fatigue damage effect caused by cyclic stress.

[0038] Furthermore, the optimization of fracturing construction parameters in step S7 includes the following points: (1) The amount of liquid injected alternately. In this invention, high-viscosity liquid and low-viscosity liquid are injected alternately. Therefore, the injection volume of high-viscosity liquid and low-viscosity liquid needs to reach 1.2 times the wellbore volume to produce the effect of cyclic stress; (2) The cycle of alternating injection. Since the number of times of alternating injection can cause multiple fatigue damages to the rock, optimizing the cycle of alternating injection can represent fatigue damage in different cycles; (3) The displacement of alternating injection. The displacement of the injected liquid is controlled in combination with the reservoir characteristics to prevent the fracture height from running out of control. If the displacement is too large, it is easy to form the main fracture, which is not conducive to the formation of micro fractures; (4) The sequence of alternating injection. First, high-viscosity liquid needs to be injected to open up a certain amount of fractures in order to create space for subsequent low-viscosity liquid injection to release stress.

[0039] Example 1

[0040] For the well to be implemented, the method of this embodiment adopts a design mode of alternating injection of high viscosity and low viscosity fluids in the pre-fracturing stage for the fracturing process, and the pumping procedure table is shown in Table 1. Figure 1 This is a schematic diagram of the liquid injection mode corresponding to this pumping procedure.

[0041] Table 1 Typical Interactive Injection Pre-Fluid Pumping Procedure Table

[0042]

[0043] From Table 1 and Figure 1 As can be seen, the design of alternating injection of high-viscosity and low-viscosity fluids before the injection forms a cyclic stress loading mode. The high-viscosity fluid increases the net pressure in the pre-injection stage, which can cause complex micro-cracks in the rock, reduce the large amount of filtration loss of low-viscosity fluid, and effectively improve the three-dimensional morphology of the crack length, width and height, thereby further increasing the reservoir stimulation volume.

[0044] Table 2 shows the comparison results of single-well fracture simulation between the design mode of alternating injection of high-viscosity and low-viscosity liquids in the pre-stage provided by the present invention and the conventional design mode. It can be seen that by using the method of the present invention, complex micro-fractures are generated in the rock, increasing the complexity of the fractures, thereby improving the transformation effect and achieving an increase in single-well production output.

[0045] Table 2 Comparison of fracturing simulation results between the alternating injection of high-viscosity and low-viscosity fluids and the conventional design mode.

[0046]

[0047]

[0048] Example 2

[0049] This embodiment employs a fracturing method provided by the present invention, which improves the complexity of fractures in tight oil reservoirs through alternating injection of pre-flush fluid. The operation is carried out according to the following steps:

[0050] S1, proposing technical ideas.

[0051] Based on preliminary analysis, the interactive injection pre-fluid design mode is designed for tight oil reservoirs. By alternately injecting high-viscosity and low-viscosity liquids during the pre-fluid injection stage, cyclic stress is formed, resulting in more micro-fractures. After multiple alternating injections of high-viscosity and low-viscosity liquid pre-fluids, the micro-fractures will continue to become more complex, eventually causing fatigue damage to the rock, thereby achieving the purpose of increasing the complexity of the fractures and increasing the reservoir stimulation volume.

[0052] The main steps of the above process are as follows: First, by analyzing parameters such as tight reservoir permeability, oil layer thickness, lithology, and clay minerals, and combining the reservoir stress and small-scale fracturing tests to calculate the formation net pressure, the effective permeability of the reservoir and the fracturing fluid loss coefficient are obtained. This determines the discharge rate and viscosity of the high-viscosity fluid injected in the first stage of the pre-fracturing fluid injection, which should be 24-30 mPa·s. The injection volume of the high-viscosity fluid needs to reach 1.1-1.3 times the wellbore volume to create space for subsequent low-viscosity fluid injection to release stress.

[0053] Secondly, by applying high stress during the high-viscosity liquid injection stage, the displacement and viscosity of the low-viscosity liquid in the second stage of the pre-flush liquid under low stress were calculated to be 6-9 mPa·s, ensuring that the interlayer is not compressed.

[0054] Then, by optimizing the fracturing process using the above parameters, the cycle of alternating injection is determined. The number of times the liquid is injected alternately can cause multiple fatigue damages to the rock. Therefore, optimizing the cycle of alternating injection can represent the fatigue damage of different cycles.

[0055] S2 optimizes the fracturing fluid system at different stages.

[0056] The alternating injection of high-viscosity and low-viscosity liquids requires consideration of the compatibility between the liquids and the degree of fatigue damage to the rock caused by the concentration of the injected liquid. A pressure difference between high-viscosity and low-viscosity liquids can lead to a high stress difference if the concentration difference is too large, which is not conducive to the formation of cyclic stress and results in poor fatigue damage to the rock. Low-viscosity liquids will have some filtration loss, while high-viscosity liquids will have relatively slower filtration loss. Therefore, when selecting low-viscosity liquids, slickwater or base liquids can be chosen, while cross-linked guar gum can be selected for high-viscosity liquids. High-viscosity liquids should have low filtration loss, with a viscosity of 24–30 mPa·s. Low-viscosity liquids have no wall-building properties, with a viscosity of 6–9 mPa·s. The viscosity difference can easily generate asymmetrical stress loading, forming microcracks, but excessive viscosity can easily lead to rock perforation.

[0057] S3 optimizes fracturing operation parameters.

[0058] (1) The time of alternating injection: Since low viscosity liquid will produce a certain amount of filtration loss, while high viscosity liquid filtration loss is relatively slow, the time difference between alternating injection of high viscosity liquid and low viscosity liquid should not be too large, and should be kept within 10 minutes. Otherwise, it is easy to cause excessive single stress, which is more likely to generate the main crack and is not conducive to the formation of complex cracks. The specific time setting can be referred to Table 1.

[0059] (2) The number of fatigue stages of alternating injection: Since the number of alternating injection stages can cause multiple fatigue damages to the rock, combined with the stress-strain characteristics, for reservoirs with small stress differences, the fatigue damage of the rock can be achieved by setting 2-3 alternating cycles of high and low viscosity liquids in the pre-stage. The effective pre-stage alternating fracturing method can be achieved by using the conventional step-by-step sand addition mode in the sand-carrying stage. For specific cycle settings, please refer to Table 1.

[0060] (3) Liquid injection sequence: Through small-scale test closed stress calculation and net pressure fitting, when high viscosity liquid is loaded first, the stress of the liquid on the rock accumulates. Once it suddenly changes to low viscosity liquid, the accumulated stress will be released instantly. Therefore, high viscosity liquid is injected first during construction.

[0061] (4) The injection rate should be adjusted according to the reservoir characteristics to prevent runaway fracture height. Since high-viscosity and low-viscosity liquids will generate a pressure difference, especially if the injection rate of high-viscosity liquid is too high, it will easily lead to excessive stress, which is not conducive to the development of micro fractures. Therefore, it is advisable to keep the injection rates of high-viscosity and low-viscosity liquids in balance. The specific injection rate settings can be found in Table 1.

[0062] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A method for improving fracture complexity in a pre-shearing stage based on stress cycling loading, characterized by, The method comprises the following steps: S1: collecting reservoir parameters of the tight oil reservoir where the wellbore is located, establishing the basis for selecting layers, and calculating the ground stress profile; S2: calculating the stress of the reservoir and the barrier layer according to the ground stress profile, and calculating the net pressure under the condition of the tight oil reservoir through different fracture discharge amounts and different viscosities of the preflush, so as to obtain the viscosity and discharge amount of the high-viscosity liquid and the low-viscosity liquid suitable for the tight oil reservoir, and ensure that the barrier layer is not channeling; S3: in the preflush stage, the high-viscosity liquid and the low-viscosity liquid are alternately injected into the wellbore to produce a cyclic stress effect to generate complex microfractures at the fracturing position of the wellbore; In S2, the tight oil reservoir is fractured and tested in combination with the ground stress profile, active water injection and discharge reduction testing are sequentially performed to obtain the friction, base fluid injection and discharge reduction testing are sequentially performed to obtain the liquid efficiency and formation parameters under the liquid system, and the viscosity of the high-viscosity liquid and the low-viscosity liquid suitable for the tight oil reservoir is calculated according to the results. In S3, the alternating period of the high-viscosity liquid and the low-viscosity liquid is 2-3.

2. The method of claim 1, wherein the stress cycle loading is applied to the pre-stage fracture to increase the complexity of the pre-stage fracture. In S1, the reservoir parameters include permeability, porosity and oil saturation characteristics.

3. The method of claim 1, wherein the stress cycle loading is applied to the pre-stage fracture to increase the complexity of the pre-stage fracture. In S1, the ground stress profile is calculated according to the logging curve data to obtain the maximum and minimum horizontal principal stresses, and the stress difference between the maximum and minimum horizontal principal stresses is calculated to obtain the stress of the reservoir and the barrier layer, and the stress difference between the reservoir and the barrier layer is calculated.

4. The method of claim 1, wherein the method is characterized by, The high viscous liquid has a filtration coefficient of 5.56*10 -4 m / min 1 / 2 ; the low viscous liquid has no wall-building property.

5. The method of claim 4, wherein the stress cycle loading is applied to the pre-stage fracture to increase the complexity of the fracture. The viscosity of the high-viscosity liquid is 24-30 mPa.s, and the viscosity of the low-viscosity liquid is 6-9 mPa.s.

6. The method of claim 1, wherein the method is characterized by, In S3, the high-viscosity liquid is injected first, and then the low-viscosity liquid is injected, and the injection is alternated.

7. The method of claim 1, wherein the method is characterized by, In S3, the total volume of the high-viscosity liquid and the low-viscosity liquid injected respectively is 1.1-1.3 times the volume of the wellbore.

8. The method of claim 1, wherein the method is characterized by, In S3, the time difference between the injection of the high-viscosity liquid and the low-viscosity liquid is within 10 minutes.

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