基于高精度边界元法的多尺度多机制复杂缝网页岩气藏压裂水平井动态模拟

Multi-scaled pores in shales lead to multiple storage and flowing mechanisms of shale gas, such as viscous flow, diffusive flow, desorption, stress-sensitivity, et al. Shale gas flowing problems become even more complicated because of the complex fracture network caused by multi-stage hydraulic fracturing in shale gas reservoirs. The fracture network, which is composed of natural fracture, primary hydraulic fracture and induced fractures, usually has irregular shape. Furthermore, the conductivities of fractures of different types in the fracture network can also be quite different, causing difficulties in simulating gas flow of fractured horizontal wells in shale gas reservoirs. Many theoretical models and experimental discussions on multiple flowing mechanisms of shale as well as multi-scaled pore structures in shales can be found in recent literatures; however, most of the models in literatures can not accurately represent the complex fracture network (irregular shape and different conductivities of primary hydraulic fracture, induced fracture and natural fracture) around fractured horizontal wells in shale gas reservoirs; thus corresponding analysis on transient pressure and production dynamics always has a certain deviation from actual field production performance. Multi-stage fractured horizontal wells (MFHWs) have been proven to be the most-efficient technology in exploitation of shale gas. Accurate simulation of transient pressure and production dynamics is of great importance in determination of important dynamic parameters of shale gas reservoirs, performance prediction of shale gas wells, production planning of shale gas and optimization of hydraulic fracturing of horizontal wells in shale gas reservoirs. Considering the advantages of the boundary element method (BEM) in dimensionality reduction and the treatment of complex boundary shape and its high precision, the project aims to propose a series of theoretical models to accurately simulate transient pressure as well as production dynamics of MFHWs in shale gas reservoirs based on experimental and theoretical research on multiple flowing mechanisms of shale gas in multi-scaled pores. Prospective research results of the project have important theoretical significance and application value in scientific and efficient development of shale gas reservoirs.

页岩储层多尺度孔隙特征决定了烃类赋存和运移方式的多样性：黏性流、扩散、解吸、应力敏感等。更为复杂的是，页岩气藏水平井多级压裂时，常形成复杂的缝网结构——天然裂缝与主裂缝及各级次级裂缝相互交错的复杂缝网，其形态常不规则，且各种裂缝导流能力也存在明显差异，故其流动机理也变得异常复杂。近年来的页岩气渗流理论研究虽对页岩储层多尺度多机制特征进行了不少讨论，但对复杂缝网仍考虑不足，这导致建立的页岩气井不稳定压力和产量动态模型过于简单，与实际情况差异太大。作为开采页岩气藏最有效的多级压裂水平井，正确模拟其压力和产量动态对做好页岩气藏参数识别、气井生产动态预测、天然气生产规划以及压裂参数优化均具有非常重要的意义。为此，本课题将在深入研究多尺度多机制复杂缝网页岩气藏渗流机理的基础上，利用边界元法的降维性、高精度性和处理复杂形状边界的灵活性，建立更符合实际的页岩气井动态模型，为页岩气开发提供理论和技术支撑。

本课题从页岩储层孔隙特征研究出发，深入分析和研究了多尺度多机制复杂缝网页岩气的渗流机理。在此基础上，利用边界元方法的降维性、高精度性以及在处理复杂形状内边界渗流问题时的灵活性，建立了更加符合实际情况的页岩气藏多级压裂水平井不稳定渗流模型，模拟并分析了压力动态特征，划分出了典型流动阶段，并对不稳定产量动态行了模拟和分析，研究成果可为页岩气试井分析、页岩气井产量动态模拟以及页岩气高效开发提供重要的理论和技术支撑。.页岩微观孔隙结构特征分析表明，页岩储层孔隙结构复杂，多尺度孔隙并存，这决定了页岩气赋存方式与流动机制的多样化：在赋存方式上既有游离态，也有吸附态，还有少量溶解态；在流动机制上，既有压力差作用下的粘性渗流，又有浓度差作用下的扩散，在流动过程中还存在较显著的渗透率应力敏感效应。页岩气藏多级压裂水平井体积压裂所形成的人工裂缝除了有限条主裂缝外，还有大量纵横交错的次级裂缝，此外气藏中还有天然微裂缝。主裂缝可视为离散裂缝，而次级裂缝和天然微裂缝则由于数量众多，可作为存在物性差异的连续介质来进行处理。.本项目综合考虑多尺度多流动机制，基于高精度边界元方法建立了页岩气藏压裂直井和压裂水平井不稳定渗流模型，并成功地进行了求解和编程计算。结果表明，裂缝导流系数、裂缝半长、裂缝条数、扩散系数、吸附解吸系数、内外区流度比、外边界形态等对井底压力动态具有显著影响；压裂改造区渗透率、裂缝条数、裂缝半长、体积压裂改造范围、裂缝导流系数等均是影响页岩气井产量大小的重要因素。利用本项目所建模型，对我国某页岩气区块实际气井测试资料进行了试井解释，实现了较好拟合，获得了页岩储层渗透率、压裂缝半长等压后参数。