Deep coal bed gas fracturing design method

Abstract

The invention discloses a deep coal bed gas fracturing design method which comprises the following steps that S100, a low-displacement pump is adopted to inject mud acid to reduce the fracturing pressure, and the displacement of the pump injected mud acid is 1-2 m < 3 > / min; s200, in the prepad fluid stage, low-concentration surfactant fracturing fluid is pumped in a variable displacement mode from low to high; in the prepad fluid stage, the displacement is changed from low to high, and the displacement is 2-4-6-8-10 m < 3 > / min; the viscosity of the low-concentration surfactant fracturing fluid under the ground condition is 5-10 mPa.s, continuous shearing is carried out for 2 h at 170 s <-1 > under the formation temperature condition, and the apparent viscosity is kept to be 5 mPa.s or above; s300, medium-concentration surfactant type fracturing fluid is pumped in a large-displacement mode at the sand-carrying fluid stage, a continuous sand adding mode is adopted, ultra-low-density proppant is carried according to the stepped sand ratio, a main crack is filled, and then a consolidated film covering proppant is carried according to the high sand ratio, and a crack opening is filled; s400, active water is adopted for large-displacement replacement, and it is guaranteed that a shaft is clean; the coal dust can be prevented from returning to the shaft, the production effect of the deep coal-bed gas well is improved, and technical support is provided for coal-bed gas field development.

Description

Technical Field

[0001] This invention relates to the field of coalbed methane exploration and development technology, specifically to a deep coalbed methane fracturing design method. Background Technology

[0002] Coalbed methane (CBM) is an unconventional natural gas generated and stored spontaneously during coal formation in coal-bearing strata, primarily composed of humic organic matter. Commonly known as methane, its main component is CH4, accounting for over 90%. CBM is generated within coal seams and stored in adsorbed and free states within the coal seam and adjacent strata. Extracting CBM requires establishing an effective communication channel between the coal seam and the wellbore, and the most effective way to create this channel is through hydraulic fracturing. CBM well fracturing differs significantly from conventional sandstone fracturing in terms of technology: coal seams have well-developed cleavage systems, resulting in severe filtration loss and making it difficult to form long fractures; coal seams are prone to adsorption and damage, making reservoir protection challenging, and post-fracturing coal dust is easily returned to the wellbore along with the CBM. Coal seams exceeding 1500m in depth are typically considered deep, and the technical difficulty of fracturing operations is far greater than in conventional CBM wells, generally resulting in less than ideal development outcomes.

[0003] In existing technologies, there are various patented techniques for coalbed methane fracturing, such as those with publication numbers CN102562022A, CN108533241A, CN107288610A, and CN106869889A. These patented technologies mainly employ high-volume fracturing with guar gum fracturing fluid, active water, or slickwater, using pulsed or continuous proppant injection methods, and carrying proppant of different particle sizes for fracturing design. Overall, their design methods are not significantly different from conventional sandstone fracturing. However, in practice, the problem of generally high fracturing pressure in deep coalbed methane formations remains unresolved. Furthermore, directly using high-volume fracturing can easily lead to excessive fracture height extension in the initial stage of fracturing, hindering fracture length extension. Using conventional proppant results in short propped fracture lengths. Additionally, the issue of coal powder backflow after coalbed methane fracturing has not been specifically addressed in the design. The patent with publication number CN112267867A mainly adopts the acidification and sand addition method. The entire process uses acid to reduce construction pressure, which is costly. The sand addition process adopts a continuous sand addition method. Due to the strong heterogeneity of the coal seam and the complex crack propagation, continuous sand addition is difficult. At the same time, there is no targeted design for the return of coal powder and proppant in the later stage. Summary of the Invention

[0004] The purpose of this invention is to provide a deep coalbed methane fracturing design method to solve the problems mentioned in the background art. Through pumping procedures and proppant design, it aims to reduce formation fracturing pressure, increase propped fracture length, and avoid coal dust return after fracturing.

[0005] This invention is implemented as follows: a deep coalbed methane fracturing design method, comprising the following steps:

[0006] S100: Low-displacement pumps are used to inject acid to reduce rupture pressure;

[0007] S200, in the pre-flush stage, low-concentration surfactant-based fracturing fluid is pumped in a variable displacement manner from low to high, and ultra-low density proppant slugs are added in a pulse sand addition manner;

[0008] S300, during the sand-carrying fluid stage, a large-volume pump injects medium-concentration surfactant-based fracturing fluid, using a continuous sand-addition method, with a stepped sand ratio carrying ultra-low density proppant to fill the main fracture, followed by a high sand ratio carrying consolidable film proppant to fill the fracture opening.

[0009] S400 uses high-volume activated water to replace wastewater, ensuring the wellbore remains clean.

[0010] Furthermore, in step S100, the pumping rate of the injected acid is 1-2 m³ / h. 3 / min;

[0011] Furthermore, in step S200, the pre-fluidization stage uses a variable discharge rate from low to high, with a discharge rate of 2-4-6-8-10m³. 3 / min;

[0012] Furthermore, the low-concentration surfactant-based fracturing fluid has a viscosity of 5-10 mPa·s under surface conditions and is applied for 170 seconds under formation temperature conditions. -1 After continuous shearing for 2 hours, the apparent viscosity remained above 5 mPa·s.

[0013] Furthermore, in step S300, the medium-concentration surfactant-based fracturing fluid has a viscosity >40 mPa·s under surface conditions and is applied for 170 seconds under formation temperature conditions. -1 After continuous shearing for 2 hours, the apparent viscosity remained above 20 mPa·s, demonstrating good debonding performance. The residue content was <10 mg / L, and all indicators met industry standards.

[0014] Furthermore, in step S300, the surface coating material of the consolidateable membrane proppant can be softened and adhered under formation temperature and pressure conditions, and large pores still exist between the consolidateable membrane proppants after consolidation.

[0015] Furthermore, in step S400, the activated water uses 2% potassium chloride activated water, and the discharge rate of the potassium chloride activated water may include a similar stepped discharge design.

[0016] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention utilizes the dissolving effect of soil acid on rock skeleton particles to reduce the fracturing pressure of deep coalbed methane wells, thereby reducing construction difficulty; it controls fracture height by initiating fracturing with low-viscosity fracturing fluid at varying flow rates; it uses pulsed proppant filling to fill micro-fractures, reducing filtration loss, improving fracture creation efficiency, and extending fracture length; it uses high-viscosity fracturing fluid to carry ultra-low density proppant, improving proppant carrying capacity and fully supporting the main fracture; and it uses a consolidable proppant coating to fill the fracture opening. Under formation temperature and pressure, the consolidable proppant coating solidifies, forming a shielding layer in the near-wellbore zone, thereby preventing coal dust from returning to the wellbore, thus improving the production effect of deep coalbed methane wells and providing technical support for coalbed methane field development. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the proppant placement pattern in deep coalbed methane fracturing fractures.

[0018] Figure 2 This is a curve diagram of the deep coalbed methane fracturing operation in Well Y1. Detailed Implementation

[0019] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0020] The following description, in conjunction with the accompanying drawings and specific embodiments, provides further details:

[0021] Well Y1 is a coalbed methane well in a certain area. The target formation is the Shanxi Formation, with a depth of 1907.2-1909.2m and an apparent thickness of 2.7m. The lithology is black coal. The wellbore was completed using Φ139.7mm casing cementing. The target formation volume is 17.2m³. 3 .

[0022] Based on the geological indications of this well, the fracturing pump injection program design of this invention is shown in Table 1:

[0023] Table 1. Y1 Well Shanxi Formation Coalbed Gas Fracturing Pumping Procedure

[0024]

[0025] The fracturing pump injection procedure is described in text as follows:

[0026] 1. Use 2m 3 / min displacement pump injection 17m3 The soil is pretreated with acid.

[0027] 2. Use low-concentration surfactant-based fracturing fluid according to the 2-4-6-8m formula. 3 The discharge rate was gradually increased in a / min manner, and ceramsite was added at sand-liquid ratios of 3%-5%-7%-9%. The 3%-5% sand-liquid ratio stage used 70 / 140 mesh ultra-low density ceramsite, and the 7%-9% sand-liquid ratio stage used 40 / 70 mesh ultra-low density ceramsite.

[0028] 3. Use medium-concentration surfactant-based fracturing fluid at 8m... 3 The sand is added continuously at a sand-to-liquid ratio of 12%-15%-17%; the 12%-15% sand-to-liquid ratio stage uses 40 / 70 mesh ultra-low density ceramsite, and the 17% sand-to-liquid ratio stage uses 30 / 50 mesh ultra-low density ceramsite.

[0029] 4. Use 8m 3 / min displacement, pumping medium concentration surfactant-based fracturing fluid, carrying 19-20% sand-to-fluid ratio to solidify 30 / 50 mesh ceramsite.

[0030] 5. Use 8m 3 / min pumping volume 17.2m 3 Use 2% potassium chloride activated water, stop the pump, and end the fracturing operation.

[0031] A schematic diagram of the proppant placement pattern in deep coalbed methane fracturing fractures is shown below. Figure 1 As shown, the specific construction curves are attached. Figure 2 .

[0032] This invention utilizes the dissolving effect of soil acid on rock skeleton particles to reduce the fracturing pressure of deep coalbed methane wells, thereby reducing construction difficulty; it controls fracture height by initiating fracturing with low-viscosity fracturing fluid at varying flow rates; it uses pulsed proppant filling of microfractures to reduce filtration loss, improve fracture creation efficiency, and extend fracture length; it uses high-viscosity fracturing fluid to carry ultra-low density proppant, improving proppant carrying capacity and fully supporting the main fracture; and it uses a consolidable film proppant to fill the fracture opening. Under formation temperature and pressure, the consolidable film proppant consolidates, forming a shielding layer in the near-wellbore zone, thereby preventing coal dust from returning to the wellbore, thus improving the production efficiency of deep coalbed methane wells and providing technical support for coalbed methane field development.

[0033] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A deep coalbed methane fracturing design method, characterized in that: Includes the following steps: S100: Low-displacement pumps are used to inject acid to reduce rupture pressure; S200, in the pre-flush stage, low-concentration surfactant-based fracturing fluid is pumped in a variable displacement manner from low to high, and ultra-low density proppant slugs are added in a pulse sand addition manner; S300, during the sand-carrying fluid stage, a large-volume pump injects medium-concentration surfactant-based fracturing fluid, using a continuous sand-addition method, with a stepped sand ratio carrying ultra-low density proppant to fill the main fracture, followed by a high sand ratio carrying consolidable film proppant to fill the fracture opening. S400 uses high-volume activated water to replace wastewater, ensuring the wellbore remains clean.

2. The deep coalbed methane fracturing design method according to claim 1, characterized in that, In step S100, the pumping rate of the acid is 1-2 m³. 3 / min.

3. The deep coalbed methane fracturing design method according to claim 1, characterized in that, In step S200, the pre-fluidization stage uses a variable discharge rate from low to high, with a discharge rate of 2-4-6-8-10m. 3 / min.

4. The deep coalbed methane fracturing design method according to claim 1, characterized in that, The low-concentration surfactant-based fracturing fluid has a viscosity of 5-10 mPa·s under surface conditions and is applied for 170 seconds under formation temperature conditions. -1 After continuous shearing for 2 hours, the apparent viscosity remained above 5 mPa·s.

5. The deep coalbed methane fracturing design method according to claim 1, characterized in that, In step S300, the medium-concentration surfactant-based fracturing fluid has a viscosity >40 mPa·s under surface conditions and is applied for 170 seconds under formation temperature conditions. -1 After continuous shearing for 2 hours, the apparent viscosity remained above 20 mPa·s, and the residue content was <10 mg / L.

6. The deep coalbed methane fracturing design method according to claim 1, characterized in that, In step S300, the consolidable proppant can be softened and adhered to the surface coating material under formation temperature and pressure conditions.

7. The deep coalbed methane fracturing design method according to claim 1, characterized in that, In step S400, the activated water is 2% potassium chloride activated water, and the discharge rate of the potassium chloride activated water may include a similar stepped discharge design.

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