China Coal Research Institute
CCTEG Xi’an Research Institute (Group) Co., Ltd.
No.5 Oil Production Plant PetroChina Qinghai Oilfield Company

复杂含煤地层顶板水平井射孔压裂是增加煤层透气性、提升瓦斯抽采效率的关键，而地层结构和射孔位置影响裂缝扩展形态。考虑地层结构特征及射孔位置，建立压裂工程地质模型；基于有限元方法构建数值模型，研究射孔位置、地层条件、垂向应力与水平应力差对裂缝扩展的影响，并进行工程验证，提出施工建议。结果表明：射孔孔眼位置存在全部位于煤层中、全部位于顶板岩层中和部分位于顶板部分位于煤层3种情况。孔眼位于煤层中，裂缝受到界面的“阻隔”作用，对煤层改造有利；孔眼位于顶板，当顶板层理发育，垂向应力与最小水平主应力差大于2 MPa时裂缝能够穿越层理和界面进入煤层，而顶板完整时，应力差大于−2 MPa裂缝即可在孔眼诱导作用下进入煤层，顶板层理和界面对裂缝垂向扩展具有“阻挡”作用，结构完整地层有利于裂缝的垂向穿层扩展；孔眼部分进入煤层，对裂缝起裂、扩展产生明显诱导作用，形成沿界面的水平缝和进入煤层的垂直缝，无论顶板是否完整，都能形成有效改造裂缝。当射孔孔眼距煤层较远、孔眼与煤层间弱面发育、水平应力大于垂向应力或压裂施工规模不足时，建议采用深穿透射孔、分支孔等能够沟通煤层的工程措施，以保证压裂效果。研究结果在陕西韩城某煤矿的井下分段压裂施工中进行了应用，试验孔瓦斯抽采效果良好，可为类似地质、工程条件下的压裂施工提供借鉴。

Horizontal well perforation and fracturing in complex coal-bearing strata are key to increasing coal seam permeability and enhancing gas extraction efficiency, with the formation structure and perforation location determining fracture propagation. Considering the characteristics of formation structure and perforation locations, a fracturing geological model is established. Based on the finite element method, a numerical model is constructed to study the effects of perforation location, formation conditions, and the difference between vertical stress and horizontal stress on fracture perforation. Engineering verification is conducted, and construction suggestions are proposed. Key findings are as follows: There are three scenarios for the perforation locations: all within the coal seam, all within the roof strata, partially in the roof strata and the other partially in the coal seam. (1) When the perforation is within the coal seam, the fracture is “blocked” by the rock strata, which is beneficial to the fracturing of the coal seam. (2) When the perforation is within the roof and the bedding of the roof is developed, fractures can penetrate the coal seam through the bedding and interface if the vertical stress and the difference between the minimum horizontal principal stress exceed 2 MPa. However, when the roof is intact, fractures with a stress difference greater than −2 MPa can penetrate the coal seam under the influence of perforation induction. The presence of bedding and interface has a “blocking” effect on the vertical extension of the fractures, while complete formation facilitates the vertical extension of the fracture. (3) If partial perforation enters the coal seam, it will have a significant induction effect on the fracture formation and extension. Horizontal fractures extending along the interface and vertical fractures entering the coal seam can form regardless of whether the roof is intact or not. When the perforation is far from the coal seam, weak interface develop between the perforation and the coal seam, horizontal stress exceeds vertical stress, or the fracturing construction scale is insufficient, it is recommended to use engineering measures such as deep penetrating perforation, branch holes and other engineering measures to ensure the fracturing effectiveness. The research results have been applied in segmented fracturing construction in an underground coal mine in Hancheng of Shaanxi, and the gas extraction effect of the test perforation is good. Indicating its potential for proposing fracturing construction schemes under similar engineering and geological conditions.

天地科技股份有限公司科技创新创业资金专项重点项目(2022-2-TD-ZD007)；中煤科工西安研究院(集团)有限公司顶层设计项目(2020XAYDC04-04)；陕西省自然科学基础研究计划项目(2024JC-YBMS-236)

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PANG Tao，JIANG Zaibing，HUI Jiangtao，et al. Fracture propagation mechanism in directional perforation and hydraulic fracturing of coal seam horizontal wells[J]. Coal Geology & Exploration，2024，52(4)：1−8.