Method for improving complexity of crack in pre-stage based on stress cyclic loading

Abstract

The invention discloses a method for improving the complexity of cracks in a pre-stage based on stress cyclic loading, and belongs to the technical field of improvement of the complexity of the cracks in the pre-stage of a tight oil reservoir. According to the method, the high-viscosity liquid and the low-viscosity liquid are injected alternately in the pre-stage, cyclic stress is generated, the complex degree of tight oil reservoir cracks is improved, and the yield of a single well is further increased; according to the method, the mode that high-viscosity liquid and low-viscosity liquid are alternately injected for multiple times in the prepad liquid stage to form cyclic stress loading is adopted, complex micro cracks are generated in rock, the complex degree of the cracks is increased, the net pressure in the cracks is increased through the high-viscosity liquid in the prepad stage, and therefore the transformation effect is improved, and the single well production yield is increased.

Description

Technical Field

[0001] The invention belongs to the technical field of improving the complexity of cracks in the pre-stage of a tight oil reservoir, and particularly relates to a method for improving the complexity of cracks in the pre-stage based on stress cyclic loading. Background Art

[0002] With the increase in exploration and development efforts, tight oil reservoirs with large resource reserves have become important replacement reserves. Practice has shown that the generation of complex fractures is an important method to achieve production increase in tight oil reservoirs, and fracturing technology has become the main means to generate complex fractures in tight reservoirs and increase the transformation volume. At present, the 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 changing the conditions of the fracturing stage, increase the reservoir transformation volume, and improve the production of a single well. For tight reservoirs, studies have shown that the formation of complex fractures in the initial stage is more conducive to the generation of complex fractures than in other stages. If the fractures are not sufficiently created in the early stage, it will be more difficult to add sand for fracturing in the later stage, and it will be difficult to achieve effective support.

[0003] The current methods for producing complex fractures include (1) staged fracturing, (2) variable viscosity fracturing, (3) temporary plugging fracturing, and (4) variable displacement fracturing. For tight reservoirs, increasing the complexity of fractures through staged sand fracturing has the risk of sand plugging, variable viscosity fracturing requires additional material costs, temporary plugging fracturing requires the introduction of special feeding equipment, and requires high professionalism of operators. Summary of the invention

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

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

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

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

[0008] S2: Calculate the reservoir and interlayer stresses based on the geostress profile, and calculate the net pressure under the conditions of the tight oil reservoir by using different fracturing displacements and pre-flushing fluids with different liquid viscosities, and obtain the viscosities and displacements of high-viscosity and low-viscosity liquids suitable for the tight oil reservoir to ensure that the interlayer is not compressed;

[0009] S3: In the pre-fluid stage, high-viscosity liquid and low-viscosity liquid are alternately injected into the wellbore to produce a cyclic stress effect, which causes complex microcracks to form at the wellbore fracturing position.

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

[0011] Furthermore, in S1, the calculated geostress profile is calculated based on 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 magnitudes of the reservoir and interlayer, and the reservoir-interlayer stress difference is calculated.

[0012] Furthermore, in S2, a fracturing test is performed on the tight oil reservoir in combination with the ground stress profile, and an active water injection reduction test is performed in sequence to obtain the friction resistance, and a base liquid injection reduction test is performed to obtain the liquid efficiency and formation parameters under the liquid system. Based on the results, the viscosity of the high-viscosity liquid and the low-viscosity liquid suitable for the tight oil reservoir is calculated.

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

[0014] Furthermore, the viscosity of the high-viscosity liquid is 24-30 mPa.s; 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, and then the low-viscosity liquid is injected, and the injection is performed 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 volume of the wellbore.

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

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

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

[0020] The invention discloses a method for improving the complexity of fractures in a pre-stage based on stress cyclic loading. On the basis of not increasing the cost of fracturing, based on the research on the compressibility of tight oil reservoirs, a method of alternately injecting high-viscosity and low-viscosity liquids in the pre-stage is used to generate cyclic stress to improve the complexity of fractures in tight oil reservoirs, and further improve the production of a single well. When the high-viscosity and low-viscosity liquids are alternately injected, the high-viscosity liquid suddenly changes to the low-viscosity liquid, and the pressure of the liquid on the reservoir is suddenly released, causing the rock to transfer energy to the liquid. The sudden release of energy causes the rock to lose its supporting effect and break, and then multi-stage cyclic injection is performed. Multiple injections form a cyclic stress loading mode, and the high-viscosity liquid increases the net pressure in the pre-stage, so that the rock produces more complex micro-cracks. After further sand fracturing, more supporting fracture networks can be generated, thereby increasing the reservoir transformation volume and improving the production of a single well. This method adopts the method of alternately injecting high-viscosity liquid and low-viscosity liquid multiple times in the pre-liquid stage to form a cyclic stress loading mode, so that the rock produces complex micro-cracks, increasing the complexity of the cracks, and the high-viscosity liquid in the pre-stage increases the net pressure in the fractures, thereby improving the transformation effect and achieving the improvement of the production of a single well. This method can form more complex cracks without adding extra equipment and material costs, does not require special training for operators, is highly operational, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the mode of alternately injecting the pre-fluid combined with the sand carrying stage using the method of the present invention. DETAILED DESCRIPTION

[0022] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work 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 and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0024] The present invention discloses a method for improving the complexity of fractures in the pre-stage based on stress cyclic loading, which generates cyclic stress loading by alternately injecting high-viscosity liquid and low-viscosity liquid pre-fluid, forces rock to break and generate complex microscopic fractures, and improves the complexity of fractures in tight oil reservoirs. The technical solution comprises the following steps:

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

[0026] S2, collect reservoir parameters. Collect reservoir parameters of tight oil reservoirs, and collect and analyze data such as 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, and the stress magnitudes of the reservoir and the interlayer are obtained by calculation.

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

[0029] S5, the fracturing fluid system at different stages is preferably selected. The selected low-viscosity fluid has no wall-building performance in fluid loss, and the high-viscosity fluid has a low fluid loss coefficient.

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

[0031] S7, optimize fracturing construction parameters. Based on the above analysis, optimize and establish fracturing construction parameters that match the technical ideas, reservoir characteristics, fracturing fluid, etc.

[0032] Furthermore, the reservoir type is determined in step S1, and the tight oil reservoir has reservoir characteristics of strong compressibility, no bottom water, good shielding conditions and no high-angle fractures penetrating the mudstone.

[0033] Furthermore, the reservoir parameters collected in step S2 need to be associated with the permeability, porosity, oil saturation and other characteristics in the core analysis with the interpretation results corresponding to the well logging interpretation, so as to establish a basis for layer selection for the cyclic stress loading fracturing scheme.

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

[0035] Furthermore, the study of the net pressure of different displacements and liquid viscosities under reservoir conditions described in step S4 requires a small-scale fracturing test on the formation, followed by an active water injection reduction displacement test to obtain the friction resistance, and a base liquid injection reduction displacement test to obtain the liquid efficiency and various formation parameters under the liquid system, and then the viscosity of the high-viscosity liquid and low-viscosity liquid suitable for the well and the fracturing construction displacement are calculated based on the results.

[0036] Furthermore, the preferred fracturing fluid system at different stages in step S5 needs to optimize different types of high-viscosity and low-viscosity fracturing fluids in the pre-fluid stage based on the different degrees of fatigue damage of the rock. The low-viscosity liquid should not have wall-building properties, and the high-viscosity liquid has the characteristic of low filtration coefficient.

[0037] Furthermore, in step S6, the compatibility of the fracturing fluid system is optimized. Conventional volume fracturing uses only a single liquid injection. The present invention involves the alternating injection of fracturing fluids of different viscosities in the pre-fluid stage. Therefore, it is necessary to consider the compatibility of the liquid and the formation fluid under the condition of changing the viscosity to avoid changes in concentration and composition caused by the reaction, thereby affecting 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 to be alternately injected. The present invention alternately injects high-viscosity liquid and low-viscosity liquid. Therefore, the injection volume of each high-viscosity liquid and low-viscosity liquid needs to reach 1.2 times the volume of the wellbore to produce the effect of cyclic stress; (2) the cycle of alternating injection. Since the number of times the liquid is alternately injected can cause multiple fatigue damage to the rock, optimizing the cycle of alternating injection can represent fatigue damage of 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 occurrence of uncontrolled fracture height. Too large a displacement is likely to form a main fracture, which is not conducive to the formation of micro fractures; (4) the order of alternating injection. First, high-viscosity liquid needs to be injected to press open a certain fracture so that space can be created for the subsequent injection of low-viscosity liquid to release stress.

[0039] Example 1

[0040] For the implementation well, the method of this embodiment is adopted, and the design mode of alternately injecting high-viscosity and low-viscosity liquids is adopted in the pre-stage to carry out the fracturing process. The pumping program table is shown in Table 1. Figure 1 This is a schematic diagram of the liquid injection mode corresponding to the pumping program.

[0041] Table 1 Typical alternate injection pre-filling mode pumping program

[0042]

[0043] From Table 1 and Figure 1 It can be seen that the design of alternately injecting high-viscosity and low-viscosity pre-liquids forms a cyclic stress loading pattern, and the high-viscosity liquid increases the net pressure in the pre-stage, which can cause complex micro-cracks in the rock, reduce the large amount of filtration loss of low-viscosity liquid, and effectively increase the three-dimensional morphology of length, width and height of the cracks, thereby further increasing the reservoir transformation volume.

[0044] Table 2 is a comparison result of single well fracture simulation between the design mode of alternately injecting 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 the method of the present invention can produce complex microscopic cracks in the rock, increase the complexity of the cracks, 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 liquids and the conventional design mode

[0046]

[0047]

[0048] Example 2

[0049] This embodiment adopts a fracturing method provided by the present invention to improve the complexity of tight oil reservoir fractures by alternately injecting pre-fluids, and is operated according to the following construction steps:

[0050] S1, propose technical ideas.

[0051] Through preliminary analysis, the design mode of alternate injection of pre-fluid is targeted at tight oil reservoirs. During the pre-fluid injection stage, high-viscosity liquid and low-viscosity liquid are alternately injected to form cyclic stress and generate more micro-cracks. After multiple alternating injections of high-viscosity liquid and low-viscosity liquid pre-fluid, the micro-cracks will continue to become more complex, eventually causing fatigue damage to the rock, thereby achieving the purpose of increasing the complexity of the cracks and increasing the volume of reservoir transformation;

[0052] The main steps of the above process are: first, by analyzing the parameters such as the permeability of the tight reservoir, the thickness of the oil layer, the lithology, and the clay minerals, the net pressure of the formation is calculated in combination with the reservoir and barrier layer ground stress and a small-scale fracturing test, and the effective permeability of the reservoir and the fracturing fluid loss coefficient are analyzed to determine the displacement of the high-viscosity liquid injected in the first stage of the pre-fluid and the liquid viscosity of 24 to 30 mPa.s. The injection volume of the high-viscosity liquid needs to reach 1.1-1.3 times the wellbore volume in order to create space for the subsequent injection of low-viscosity liquid to release stress.

[0053] Secondly, through the high stress loading in the high viscosity liquid injection stage, the displacement size and viscosity of the low viscosity liquid in the second stage of low stress loading of the pre-fluid are calculated to be 6-9mPa.s to ensure that the interlayer is not compressed.

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

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

[0056] The alternating injection of multiple high-viscosity liquids and low-viscosity liquids needs to consider the compatibility of the liquids and the degree of fatigue damage caused by the injected liquid concentration to the rock. High-viscosity liquids and low-viscosity liquids will produce a pressure difference. If the concentration difference between the two is too large, the stress difference will be too high, which is not conducive to the formation of cyclic stress and has a poor effect on the fatigue damage of the rock; low-viscosity liquids will produce a certain amount of filtration, while high-viscosity liquids will lose relatively slowly. Therefore, when selecting low-viscosity liquids, slippery water or base liquid can be selected, and cross-linked guar gum can be selected for high-viscosity liquids; high-viscosity liquids should have low filtration loss and a viscosity of 24 to 30 mPa.s. Low-viscosity liquids have no wall-building properties and a viscosity of 6 to 9 mPa.s. The viscosity difference is prone to asymmetric stress loading to form microcracks, but excessive viscosity is prone to rock penetration.

[0057] S3, optimize fracturing construction parameters.

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

[0059] (2) Fatigue levels of alternating injection. Since the levels of alternating injection of liquid can cause multiple fatigue damage to the rock, combined with the stress-strain characteristics, for reservoirs with small stress difference, setting 2-3 alternating cycles of high-viscosity and low-viscosity liquids in the pre-stage can achieve fatigue damage of the rock. In the sand-carrying stage, the conventional step-by-step sand-adding mode can be used to achieve an effective pre-stage alternating fracturing method. The cycle setting can be specifically referred to Table 1.

[0060] (3) The order of liquid injection: Through small-scale test closure 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) Alternate injection displacement: Combined with reservoir characteristics, the displacement of injected liquid is controlled to prevent the occurrence of uncontrolled fracture height. Since high-viscosity and low-viscosity liquids will produce a pressure difference, especially if the displacement of high-viscosity liquid is too large, it is easy to cause excessive stress, which is not conducive to the development of micro-fractures. Therefore, it is advisable to keep the displacement of high-viscosity liquid and low-viscosity liquid balanced. For specific displacement settings, please refer to Table 1.

[0062] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for increasing the complexity of cracks in the pre-stage based on stress cyclic loading, characterized in that: The following steps are involved: S1: Collect the reservoir parameters of the tight oil reservoir where the wellbore is located, establish the basis for layer selection, and calculate the ground stress profile; S2: Calculate the reservoir and interlayer stresses based on the geostress profile, and calculate the net pressure under the conditions of the tight oil reservoir by using different fracturing displacements and pre-flushing fluids with different liquid viscosities, and obtain the viscosities and displacements of high-viscosity and low-viscosity liquids suitable for the tight oil reservoir to ensure that the interlayer is not compressed; S3: In the pre-fluid stage, high-viscosity liquid and low-viscosity liquid are alternately injected into the wellbore to produce a cyclic stress effect, which causes complex microcracks to form at the wellbore fracturing position.

2. The method for increasing the complexity of cracks in the pre-stage based on stress cyclic loading according to claim 1, characterized in that: In S1, reservoir parameters include permeability, porosity and oil saturation characteristics.

3. The method for increasing the complexity of cracks in the pre-stage based on stress cyclic loading according to claim 1 is characterized in that: In S1, the calculation of the geostress profile is performed based on the well 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 magnitudes of the reservoir and interlayer, and the reservoir-interlayer stress difference is calculated.

4. The method for increasing the complexity of cracks in the pre-stage based on stress cyclic loading according to claim 1 is characterized in that: In S2, a fracturing test is performed on the tight oil reservoir in combination with the ground stress profile, and an active water injection reduction test is performed in sequence to obtain the friction resistance, and a base liquid injection reduction test is performed to obtain the liquid efficiency and formation parameters under the liquid system. Based on the results, the viscosity of the high-viscosity liquid and low-viscosity liquid suitable for the tight oil reservoir is calculated.

5. The method for increasing the complexity of cracks in the pre-stage based on stress cyclic loading according to claim 4 is characterized in that: The filtration coefficient of the high viscosity liquid is 5.56*10 -4 m / min 1 / 2 ; The low-viscosity liquid does not have wall-building properties.

6. The method for increasing the complexity of cracks in the pre-stage based on stress cyclic loading according to claim 5, characterized in that: The viscosity of the high-viscosity liquid is 24-30 mPa.s; the viscosity of the low-viscosity liquid is 6-9 mPa.s.

7. The method for increasing the complexity of cracks in the pre-stage based on stress cyclic loading according to claim 1, characterized in that: In S3, when the high-viscosity liquid and the low-viscosity liquid are injected alternately, the high-viscosity liquid is injected first, and then the low-viscosity liquid is injected, and the process is performed alternately.

8. The method for increasing the complexity of cracks in the pre-stage based on stress cyclic loading according to claim 1, characterized in that: In S3, the total volume of the high-viscosity liquid and the low-viscosity liquid injected is 1.1 to 1.3 times the volume of the wellbore.

9. The method for increasing the complexity of cracks in the pre-stage based on stress cyclic loading according to claim 1, characterized in that: In S3, the time difference between the alternate injection of the high-viscosity liquid and the low-viscosity liquid is within 10 minutes.

10. The method for increasing the complexity of cracks in the pre-stage based on stress cyclic loading according to claim 1, characterized in that: In S3, the high-viscosity liquid and the low-viscosity liquid are alternately injected for 2-3 cycles.

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