Method for reducing shale reservoir rock fracture pressure

Abstract

The invention relates to the technical field of shale reservoir yield increase and transformation, in particular to a method for reducing shale reservoir rock fracture pressure, which comprises the following steps: firstly, adding an oxidant solution into an acid solution according to the content of target block shale components organic matters, pyrite and carbonate minerals; and the oxidant solution and the existing acid liquor generate a synergistic breaking effect. According to the method, the oxidizing agent solution is added into the pre-acid liquid, so that shale component carbonate rock minerals are corroded, pyrite and organic matter can also be corroded, the shale reservoir rock fracture pressure is reduced through acidification-oxidation corrosion synergism, and the acidification fracture reduction effect is enhanced.

Description

Technical Field

[0001] The invention relates to the technical field of shale reservoir production enhancement and transformation, and in particular to a method for reducing rock fracture pressure of a shale reservoir. Background Art

[0002] Hydraulic fracturing uses the mechanical energy released by fracturing fluid to "break up" the shale matrix, forming a complex fracture network, expanding the gas leakage area, improving the gas transmission capacity, and realizing the commercial development of shale gas layers. The reservoir rock fracture pressure is mainly controlled by the rock mineral composition, content and distribution. According to the rock fracture pressure calculation formula for vertical fractures (or horizontal fractures) under open hole completion conditions, for a specific reservoir, the rock fracture pressure is closely related to the contact point porosity, Poisson's ratio and unidirectional tensile strength. On the macroscopic fracture mechanics scale, the chemical dissolution of rock component minerals will weaken the rock cementation strength, thereby reducing the cohesion of the rock and the internal friction angle; on the scale of microscopic damage mechanics, the generation of macroscopic fractures usually stems from the gradual expansion, convergence and penetration of a large number of microcracks inside the rock under stress, and after accumulating to a certain extent, macroscopic fractures are formed. The rock mechanical damage caused by chemical dissolution will aggravate the macroscopic fractures under stress. In order to enhance the effect of hydraulic fracturing, on-site fracturing construction usually pumps an appropriate amount of acid into the shale reservoir in advance to reduce the fracture pressure of the rock by dissolving carbonate minerals or other rock cements. The field practice of shale gas wells in the Bakken and Engle Ford regions of North America and shale gas wells in the Sichuan Basin of China has confirmed the effectiveness of this method. For example, the fracturing construction of shale gas wells in the Sichuan Basin usually uses dilute hydrochloric acid (15% HCl) or rare earth acid (15% HCl solution + 1.5% HF) as the pretreatment fluid, and the field construction results show that the "fracture reduction" effect of rare earth acid is good.

[0003] At present, the rock cracking of shale reservoirs mainly targets the acidification and dissolution of carbonate minerals in shale components, and has not yet fully utilized the characteristics of shale deposited in anoxic reducing environments and rich in pyrite and organic matter, which are easily oxidized and dissolved. Black shale is formed in anoxic reducing environments and is rich in organic matter and sulfide minerals such as pyrite. Pyrite is one of the characteristic products of strong reducing environment deposition and is prone to oxidation, hydrolysis and other chemical reactions in the oxygen-rich environment on the surface. Organic matter comes into contact with pore water rich in dissolved oxygen and is oxidized and decomposed under its own fermentation, accelerating the weathering process of shale. The acidic environmental water produced by the oxidation of black shale, in coordination with the free action of organic acids, accelerates the dissolution and transformation of rock mineral particles and generates clay minerals, dissolving the pore structure inside the rock. While weathering and oxidation increase the porosity of black shale, it deteriorates the rock strength, reduces the fracture toughness of the rock, and causes rock fracture. Although some scholars have proposed adding oxidants to the fracturing fluid of the slickwater system to achieve the interaction between the retained fracturing fluid and shale, and to generate dissolution pores and fractures through oxidation chemical dissolution, and to release heat through chemical reactions to promote the initiation and expansion of secondary fractures, the continuous micro-transformation after fracturing is mainly achieved through post-fracturing well sealing construction. However, the positive role of oxidants in promoting the formation of artificial fractures through hydraulic fracturing construction has not yet received attention and concern. Summary of the invention

[0004] The purpose of the present invention is to provide a method for reducing the rock fracture pressure of shale reservoirs, so as to solve the technical problem that in the prior art, due to the deep burial depth of deep shale, large pipeline friction and high construction pressure, in order to safely complete the fracturing construction task, in addition to requiring a low-friction fracturing fluid system, other measures are needed to reduce the construction pressure, increase the pressure window, and increase the on-site adjustment room as much as possible.

[0005] The invention discloses a method for reducing the rock fracture pressure of a shale reservoir. First, an oxidant solution is added into an acid solution according to the contents of organic matter, pyrite and carbonate minerals in the shale of a target block.

[0006] Further, the method specifically includes the following steps:

[0007] S1. Determine the content of organic matter, pyrite and carbonate minerals in the shale components of the target block;

[0008] S2. According to the result of step S1, preparing an oxidant solution;

[0009] S3. According to the result of step S1, preparing an acid solution;

[0010] S4. During the pre-acid injection stage of the fracturing construction, the oxidant solution and the acid solution are mixed and injected into the reservoir to react before the subsequent steps are carried out.

[0011] Furthermore, the mass concentration percentage of the oxidant solution in step S2 is numerically equal to the total content percentage of organic matter and pyrite in the shale components of the target reservoir.

[0012] Furthermore, the percentage value of the acid mass concentration in step S3 is equal to the percentage of the total content of carbonate minerals in the shale component of the target reservoir.

[0013] Furthermore, the organic matter content of the shale component in the target block is greater than 3%, the pyrite content is greater than 2%, and the carbonate mineral content is greater than 10%.

[0014] Furthermore, the oxidant solution is an oxidant solution that can oxidize organic matter and pyrite in dissolved shale.

[0015] Furthermore, the oxidant solution is a hydrogen peroxide solution.

[0016] Furthermore, the mass concentration percentage of the hydrogen peroxide solution is ≥5%.

[0017] Furthermore, the acid solution is a dilute hydrochloric acid solution.

[0018] Furthermore, the mass concentration percentage of the dilute hydrochloric acid solution is ≥10%.

[0019] Furthermore, the reaction time in step S4 is ≥ 24 h.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The oxidant solution and the existing acid solution produce a synergistic fracture reduction effect. This method uses the addition of an oxidant solution to the pre-acid solution to dissolve not only the carbonate minerals of the shale component, but also pyrite and organic matter, thereby achieving acidification-oxidation dissolution to synergistically reduce the fracture pressure of the shale reservoir rock and enhance the acidification fracture reduction effect;

[0022] 2. The oxidant solution and the existing acid solution produce a synergistic drag reduction effect. This method uses the addition of an oxidant solution to the pre-acid solution to further oxidize and decompose organic scale in the wellbore while using the acidification effect to clean the wellbore and reduce friction, thereby enhancing the pickling effect;

[0023] 3. It is a complementary method to existing hydraulic fracturing transformation. This method uses the addition of an oxidant solution to the pre-acid solution to reduce the fracture pressure of organic-rich rocks, thereby assisting hydraulic fracturing to increase the density of artificial fracture networks;

[0024] 4. Hydrogen peroxide can maintain stability in an acidic environment. After hydrogen peroxide itself is oxidized and decomposed, it produces water and oxygen without introducing pollutants. At the same time, in the presence of formation iron ions, it induces the Fenton chain reaction to promote the efficient dissolution of organic matter. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only represent some embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0026] Figure 1 This is a comparison diagram of shale stress-strain curves under 10 MPa confining pressure during the triaxial rock mechanics experiment process of Example 2 of the present invention.

[0027] Figure 2 This is a comparison diagram of shale fracture conditions under 10 MPa confining pressure during the triaxial rock mechanics experiment process of Example 2 of the present invention.

[0028] Figure 3 The microstructure diagram of Example 3 of the present invention is shown in Figure 1. SEM observation of shale original sample (a), hydration treatment sample (b), acidification treatment sample (c) and oxidation treatment sample (d) DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0030] Example 1

[0031] This embodiment discloses a method for reducing the rock fracture pressure of a shale reservoir, wherein an oxidant solution is added to a pre-acid solution, and oxidation-acid dissolution is used to reduce the rock fracture pressure of a shale reservoir, and acid-oxidation dissolution is used to synergistically reduce the rock fracture pressure of a shale reservoir, thereby enhancing the effect of hydration fracturing transformation, and also being conducive to reducing the construction pressure under the same fracturing scale. Specifically, the following process is included:

[0032] (1) Preferably, shale reservoirs with an organic matter content greater than 3%, a pyrite content greater than 2%, and a carbonate mineral content greater than 10% are selected, and the contents of organic matter, pyrite, and carbonate minerals in the shale of the target block are measured;

[0033] (2) Based on the measurement results of step (1), prepare a hydrogen peroxide solution with a mass concentration percentage of ≥ 5%, wherein the mass concentration percentage of hydrogen peroxide is numerically equal to the total content percentage of organic matter and pyrite in the target reservoir shale;

[0034] (3) Based on the measurement results of step (1), a dilute hydrochloric acid solution having a mass concentration percentage of ≥10% is prepared, wherein the mass concentration percentage of the dilute hydrochloric acid is equal to the total content percentage of carbonate minerals in the target reservoir shale;

[0035] (4) During the pre-acid injection stage of the fracturing construction, the hydrogen peroxide solution of step (2) and the dilute hydrochloric acid solution of step (3) are mixed and injected into the reservoir, and the reaction time is ≥ 24 hours.

[0036] Example 2

[0037] In order to prove that oxidative dissolution reduces the fracture pressure of shale rocks, outcrop shale of the Longmaxi Formation (organic matter content 4.1%, pyrite content 2.6%, carbonate mineral content 10.7%) was selected, and several core columns were drilled parallel to the bedding at similar positions in the same rock block. In order to reduce the comparison error, long core columns with a length of about 10 cm were drilled along the parallel bedding, and then cut in half to obtain core columns A and B with similar physical properties, both of which were about 5 cm long. The prepared core columns were soaked in distilled water and oxidant solution (10% hydrogen peroxide and 5% sodium hypochlorite) for 240 hours. After the experiment, the rock samples were dried at low temperature (60°C) for 48 hours, and then triaxial compressive rock mechanics tests were carried out. The experimental results show that oxidative dissolution reduces the differential stress corresponding to the fracture of the rock sample ( Figure 1 ), which proves that the fracture strength of shale rock is reduced after being treated with oxidant solution; at the same time, the fracture effect of rock sample under axial stress is better ( Figure 2 ), which proves that the fracture creation effect of rock samples under stress is improved after being treated with oxidant solution.

[0038] Example 3

[0039] In order to distinguish the differences in the pore structure response of shale under hydration, acidification and oxidation, four centimeter-level SEM rock samples were drilled along the parallel bedding direction from the outcrop shale of the Longmaxi Formation (organic matter content 4.1%, pyrite content 2.6%, carbonate mineral content 10.7%), of which three rock samples were soaked in distilled water, 5% acid solution and 10% hydrogen peroxide solution for 240 hours (solid-liquid ratio, 1g:20mL). After the experiment, the rock samples were dried at 60℃ in an oven for 48 hours, and then the surface morphology of all rock samples was observed by SEM.

[0040] The surface structure of the original shale is dense, and the clay minerals are distributed along the horizontal bedding direction and the directional arrangement is very obvious. No discernible microcracks are found. Figure 3 a). After being soaked in distilled water, no changes were observed in the structural density and mineral distribution characteristics of the shale. Although clay minerals objectively expand due to water absorption, their hydration-induced cracking effect is weak, and no hydration-induced microcracks were observed under the SEM electron microscope ( Figure 3 b). After being soaked in hydrochloric acid, some dissolution pores can be observed on the shale surface ( Figure 3c); after being soaked in the oxidizing solution, very obvious microcracks appeared on the surface of the rock sample, with a length of hundreds of micrometers and a spacing of less than 100 μm. They all extended along the horizontal bedding direction and were accompanied by a small amount of dissolved pores ( Figure 3 d). Comparative analysis shows that under the experimental conditions, the phenomenon of hydration-induced fracture in Longmaxi Formation shale is not obvious, acid dissolution is mainly reflected in new pores, while oxidation dissolution not only produces dissolution pores but also induces microcracks.

[0041] Example 4

[0042] According to the basic parameters of JY1HF shale gas well (Table 1), based on the changes in pores during shale oxidation and dissolution as shown in Table 2, the rock fracture pressure calculation formula 1-1 under open hole completion conditions was used to calculate the changes in rock fracture pressure corresponding to the initiation of shale vertical fractures under the condition of oxidation and dissolution reducing the reservoir.

[0043] (Table 2) shows the degradation effect of hydrogen peroxide solution oxidative dissolution on shale reservoir rocks.

[0044]

[0045]

[0046]

[0047] Where σ f ——Uniaxial tensile stress strength of rock (MPa);

[0048] σ h , σ H ——Minimum and maximum horizontal principal stresses (MPa);

[0049] P o ——pore fluid pressure (MPa);

[0050] P bv ——Vertical crack initiation pressure (MPa);

[0051] P bh ——Horizontal crack initiation pressure (MPa);

[0052] P ob ——overburden pressure (MPa);

[0053] φ c ——Porosity of rock contact point (%);

[0054] φ——rock porosity (%);

[0055] ν——Poisson’s ratio;

[0056] Table 1 Basic parameters of JY1HF shale gas well

[0057]

[0058] Table 2 Calculation of rock fracture pressure during shale oxidation and dissolution process

[0059]

[0060] The above are the implementation methods listed in this embodiment, but this embodiment is not limited to the above optional implementation methods. Those skilled in the art can arbitrarily combine the above methods to obtain other various implementation methods. Anyone can derive other various forms of implementation methods under the inspiration of this embodiment. The above specific implementation methods should not be understood as limiting the scope of protection of this embodiment. The scope of protection of this embodiment shall be based on the definition in the claims, and the description can be used to interpret the claims.

Claims

1. A method for reducing rock fracture pressure in a shale reservoir, characterized in that: First, an oxidant solution is added to the acid solution according to the content of organic matter, pyrite and carbonate minerals in the shale of the target block.

2. A method for reducing rock fracture pressure in shale reservoirs according to claim 1, characterized in that: The specific steps include: S1. Determine the content of organic matter, pyrite and carbonate minerals in the shale components of the target block; S2. According to the result of step S1, preparing an oxidant solution; S3. According to the result of step S1, preparing an acid solution; S4. During the pre-acid injection stage of the fracturing construction, the oxidant solution and the acid solution are mixed and injected into the reservoir to react before the subsequent steps are carried out.

3. A method for reducing rock fracture pressure in shale reservoirs according to claim 2, characterized in that: The mass concentration percentage of the oxidant solution in step S2 is numerically equal to the total content percentage of organic matter and pyrite in the shale components of the target reservoir.

4. A method for reducing rock fracture pressure in shale reservoirs according to claim 3, characterized in that: The percentage value of the acid mass concentration in step S3 is equal to the percentage of the total content of carbonate minerals in the shale component of the target reservoir.

5. A method for reducing rock fracture pressure in shale reservoirs according to claim 1 or 2, characterized in that: The shale components in the target area have an organic matter content greater than 3%, a pyrite content greater than 2%, and a carbonate mineral content greater than 10%.

6. A method for reducing rock fracture pressure in shale reservoirs according to claim 1 or 2, characterized in that: The oxidant solution is an oxidant solution capable of oxidizing organic matter and pyrite in dissolved shale.

7. A method for reducing rock fracture pressure in shale reservoirs according to claim 6, characterized in that: The oxidant solution is a hydrogen peroxide solution.

8. A method for reducing rock fracture pressure in shale reservoirs according to claim 7, characterized in that: The mass concentration percentage of the hydrogen peroxide solution is ≥5%.

9. A method for reducing rock fracture pressure in shale reservoirs according to claim 1 or 2, characterized in that: The acid solution is a dilute hydrochloric acid solution.

10. A method for reducing rock fracture pressure in shale reservoirs according to claim 9, characterized in that: The mass concentration percentage of the dilute hydrochloric acid solution is ≥10%.

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