Gao Qichao, Dong Pingchuan, Liu Chang
College of Petroleum Engineering, China University of Petroleum (Beijing), Beijing 102249, China.
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Beijing 102249, China.
ACS Omega. 2021 Mar 8;6(11):7312-7333. doi: 10.1021/acsomega.0c05389. eCollection 2021 Mar 23.
A large number of natural fractures are distributed in shale gas reservoirs. In-depth studying of the attitude of fractures is of great significance for the efficient development of shale gas. In previous studies, the complex three-dimensional discrete fracture networks (DFNs) and transport mechanisms were often not fully considered. In this study, the fully coupled multimechanism transport model and the complex discrete fracture networks (DFNs) model are developed to incorporate these complexities. The comprehensive transport model can couple multiple mechanisms such as slippage, diffusion, adsorption, and dissolution of shale gas. Moreover, the mechanisms of two-phase flow, reservoir deformation, real gas effect, and fracture closure are also considered. The three-dimensional DFN model can flexibly characterize the fracture attitudes, which means that the construction of the discrete fracture network is easier and faster. Under these frameworks, a series of partial differential equations (PDEs) were derived to describe transport mechanisms of shale gas in the shale fracture-matrix system. These PDEs were numerically discretized and solved by the finite element method. The proposed models are verified against gas production data from the field and validated against others' solutions. This study numerically simulates the influence of different fracture attitudes on shale gas transport and analyzes the sensitivity of the model. The results and sensitivity analysis reveal that both fracture dip angle and strike direction will significantly affect the gas production, and the smaller the angle between the strike direction and the flow direction, the higher the shale gas production. The length, density, area, and shape of fractures also play important roles in shale gas transport. There is an ideal fracture density in the fracture network, and the suggested excessive fracturing is not economic. The shale fracture-matrix system modeling and simulation methods can improve the development of shale gas reservoirs and increase the gas production of wells.
大量天然裂缝分布于页岩气储层中。深入研究裂缝产状对页岩气高效开发具有重要意义。在以往研究中，复杂的三维离散裂缝网络（DFN）和渗流机制往往未得到充分考虑。本研究开发了全耦合多机制渗流模型和复杂离散裂缝网络（DFN）模型，以纳入这些复杂性。综合渗流模型能够耦合页岩气的滑脱、扩散、吸附和溶解等多种机制。此外，还考虑了两相流、储层变形、真实气体效应和裂缝闭合等机制。三维DFN模型能够灵活表征裂缝产状，这意味着离散裂缝网络的构建更加容易和快速。在这些框架下，推导了一系列偏微分方程（PDE）来描述页岩气在页岩裂缝 - 基质系统中的渗流机制。这些PDE通过有限元法进行数值离散和求解。所提出的模型通过现场产气数据进行了验证，并与其他研究的解进行了对比验证。本研究对不同裂缝产状对页岩气运移的影响进行了数值模拟，并分析了模型的敏感性。结果和敏感性分析表明，裂缝倾角和走向都会显著影响产气，走向与流动方向之间的夹角越小，页岩气产量越高。裂缝的长度、密度、面积和形状在页岩气运移中也起着重要作用。裂缝网络中存在一个理想的裂缝密度，建议过度压裂并不经济。页岩裂缝 - 基质系统建模与模拟方法能够改善页岩气储层的开发效果，提高气井产量。
