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    井下页岩层压裂裂缝的数值模拟与瞬变电磁响应特征分析

    Numerical simulation and transient electromagnetic response characteristics of fracturing-induced fractures in underground shale formations

    • 摘要: 水力压裂通过人工制造裂缝改造页岩层,是当前油田增产的重要技术手段之一。了解压裂裂缝的发育过程、深度、长度特征及空间分布特征,对提升页岩层油气开采效率至关重要。针对在工程实践中地面电磁法受探测深度和探测精度制约的问题,提出井下井网与地面同步观测的联合监测方案。研究了垂直井下不同页岩层参数对电磁测量结果的影响规律,根据瞬变电磁法原理,使用接地电偶源发射装置。对套管内壁一处或多处进行水力压裂,通过地面压裂设备向套管内壁挤压页岩周围的砂体形成裂缝后,注入含盐电解液,使得井壁周围呈现一定规模的低阻裂缝,研究不同电导率的压裂液注入垂直方向裂缝后所形成的地质模型。在地面同心圆阵列式测线上布置了72个测点,这些测点均匀分布于3条环线上。通过对这些测点所接收的地电信息进行信息采集,并且基于COMSOL Multiphysics有限元数值仿真软件,对页岩层中不同形态的压裂裂缝进行响应特征分析。研究结果表明,该方法可有效确定井下裂缝的方位、埋深和裂缝长度,为研究井下低阻体的压裂裂缝形态提供了理论依据和技术思路。

       

      Abstract: Hydraulic fracturing is currently one of the most important techniques for enhancing oilfield production, with the primary objective of transforming shale formations to generate artificial fractures. Understanding the development process, depth, length characteristics, and distribution of these fracturing-induced fractures is crucial for the efficient exploitation of shale formations and the improvement of production capacity. In practical engineering applications, surface-based electromagnetic methods are significantly limited by detection depth and accuracy. To enhance the efficiency and accuracy of inter-well detection, it is feasible to conduct simultaneous observations from both downhole well patterns and surface arrays. The influence of various shale formation parameters in vertical wells on measurement results was investigated. By utilizing the principles of transient electromagnetic (TEM) methods, a grounded electric dipole source was employed to transmit signals. Hydraulic fracturing was carried out at one or multiple locations along the inner wall of the well casing, with surface fracturing equipment used to compress the sandstone around the shale, thus inducing fractures. A saline electrolyte was then injected, creating low-resistivity fractures around the wellbore. This resulted in a vertically distributed geological model with varying conductivity due to the injected fracturing fluid. Geoelectric data was acquired through a surface concentric circular array composed of three loops, with a total of 72 measurement points arranged along the loops, enabling the monitoring of the development of underground fracturing-induced fractures. Based on finite element numerical simulations conducted using COMSOL Multiphysics software, the response characteristics of fractures with different geometries in shale formations were analyzed. The results demonstrate that this method effectively determines the orientation, burial depth, and length of subsurface fractures, thereby providing a theoretical foundation and technical approach for the characterization of fracturing-induced low-resistivity fractures in subsurface formations.

       

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