基于多重烃类运移机制的页岩气藏多级压裂水平井渗流机理与模型研究

Shale gas reservoirs have complex, multimodal pore-size distributions and unique storage properties, causing shale gas to be transported by multiple flow mechanisms through the pore structure which include desorption from the surface of kerogen, diffusion in kerogen, viscous flow and stress sensitive. Multiply-fractured horizontal wells (MFHW) have been proved to be the most effective way to produce from shale gas reservoirs. Due to the complex fracture network created or induced by hydraulic fracturing, including primary fractures, second fractures, radial fractures in the immediate neighborhood of the horizontal well as a result of stress release and reconnection of natural fractures, which makes it more difficult to model gas flow in shale gas reservoirs with MFHW. Current gas flowing theories become inadequate to describe the multiple flow mechanisms in this kind of reservoir-well configuration, which will inevitably lead to blindness in practice and failure in developing strategy. Based on the consideration of multiple migration mechanisms in shale gas reservoirs and analysis of the complicated fracture network, this work will try to establish flowing models for MFHW in shale gas reservoirs, including transient well testing model and rate prediction model. The models will then be solved analytically or numerically; the transient pressure and rate dynamics of MFHW in shale gas reservoirs will be analyzed. The ultimate goal of this work is to form a complete and practical method for analyzing production dynamics of MFHW in shale gas reservoirs, which will give a series of important parameters pertinent to reservoir and MFHW. These parameters can be very helpful in the design of hydraulic fracturing and the development of shale gas reservoirs.

页岩气藏孔隙结构的多尺度性及特殊的烃类赋存方式，决定了页岩储层中复杂的烃类运移机制，包括吸附-解吸、扩散、粘性流、应力敏感等。此外，压裂水平井已被证实是开采页岩气藏的最有效方法，水平井压裂后在地层中形成复杂的裂缝网络系统，包括压裂缝、诱导缝、天然缝的连通以及应力释放形成的径向缝，使得本就复杂的页岩气藏渗流规律变得更加复杂。常规渗流理论不足以刻画页岩气藏中压裂水平井的渗流特征，而这将不可避免地导致矿场实践的盲动性甚至开发策略的失败。本课题将在综合考虑页岩气多重运移机制和地层中复杂裂缝网络的基础上，建立页岩气藏多级压裂水平井不稳定渗流模型（包括不稳定试井和产能预测模型），采用数学物理方法对其进行求解及编程实现，并对压力及产量动态的主要影响参数进行敏感性分析，从而形成一套较完整的页岩气藏不稳定试井分析及不稳态产能预测方法，可用于气藏及气井动态参数的反求和压裂参数的优化，科学指导页岩气藏高效开发。

我国页岩气资源丰富，但由于复杂的地质条件，其开采难度很大。弄清页岩气藏的渗流机理是页岩气藏科学有效开发的基础。然而，页岩气藏的渗流机理不同于常规气藏，除了与常规气藏一样的压力梯度作用下的渗流，还存在其它多种特殊的作用机制，如吸附-解吸、扩散、应力敏感等。此外，水平井多级压裂已被证实是开采页岩气藏的最有效的方式，但这种复杂的井型及水力压裂所形成的的复杂裂缝网络使得本就复杂的页岩气藏渗流问题变得更加复杂。常规渗流理论已不能有效地刻画页岩气藏中压裂水平井的渗流特征和渗流规律，而对页岩气渗流特征和渗流规律认识不清则将不可避免地导致矿场实践的盲动性甚至开发策略的失败。针对这些情况，本课题从页岩气多重输运机制的研究出发，通过实验测试和分析调研，确定了适当的页岩气吸附-解吸模型、扩散模型、页岩储层应力敏感规律方程。基于多重运移机制的研究和复杂裂缝网络形态的描述，建立了综合考虑多重输运机制的页岩气藏无限导流多级压裂水平井(IC-MFHW)试井模型、页岩气藏有限导流多级压裂水平井FC-MFHW)试井模型和页岩气藏三线性流多级压裂水平井TL-MFHW)试井模型以及等效产能预测模型，恰当地采用Pedrosa变换、摄动方法、Laplace变换法、积分方程的边界元求解法等数学方法对各类模型成功地进行了求解，并编制了相应程序，绘制了井底压力典型曲线和产量递减曲线，分析了不稳定渗流过程的流动阶段，弄清了页岩气生产过程中影响不稳定压力动态和产能大小的主要因素，为页岩气藏多级压裂水平井不稳定试井及产能预测提供了理论支撑，其成果可用于页岩气藏及气井动态参数（如渗透率、压裂缝长、压裂缝导流能力、表皮污染系数，等等）的反求和页岩气井产能的预测，从而指导页岩气藏的高效的开发。