裂缝性油气储层水力裂缝模拟的增强有限元方法

Existing hydraulic fracturing models cannot handle the interactions between hydraulic fractures and natural fractures as well as the complex fracture coalescence, crossing and branching behavior during the hydraulic fracturing of naturally fractured oil/gas bearing formations, and thus commonly fail to faithfully predict the resulting complex fracture geometries. Recently, we have developed an efficient and robust finite element method, the augmented finite element method (AFEM), for modeling the initiation and propagation of multiple cracks in solids as well as their interactions. In this present research, we propose to develop a novel and efficient augmented finite element method for accurately modeling the initiation and propagation of hydraulic fractures in naturally fractured formations, by extending the existing AFEM to account for the fluid flow within the crack and its nonlinear coupling with the deformation of the rock mass as well as the fracture propagation. Laboratory experiments will be performed for validating the proposed new augmented finite element method. Based on the new AFEM, the initiation and propagation characteristics of hydraulic fractures in naturally fractured formations will be investigated systematically, and insights on the complex patterns of hydraulic fractures are to be obtained, which will be beneficial to the design and optimization of hydraulic fracturing in naturally fractured formations.

裂缝性油气储层水力压裂涉及天然裂缝与水力裂缝之间的相互影响、裂缝合并、贯穿与分岔等复杂因素，现有的水力裂缝数值模拟方法尚难以全面考虑，无法准确预测裂缝性储层中复杂的水力裂缝形态。在前期的研究中，我们发展了一种高效的断裂力学有限元方法—增强有限元方法（augmented finite element method，AFEM），用于分析固体材料中多裂缝起裂、扩展以及裂缝间的复杂相互作用。本项目拟将现有增强有限元方法加以扩展，进一步考虑裂缝内的流体流动及其与岩石受力与变形、裂缝扩展的非线性耦合，发展适用于裂缝性储层水力裂缝起裂与扩展模拟的新型、高效的流固耦合增强有限元方法，通过开展物理模拟实验验证所发展的新型增强有限元方法，并利用所发展的方法系统研究裂缝性储层中水力裂缝起裂与扩展规律，总结裂缝性储层中水力裂缝的复杂几何形态特征，为裂缝性储层水力压裂设计与优化提供一定的理论依据。

裂缝性油气储层水力压裂涉及天然裂缝与水力裂缝之间的相互影响、裂缝合并、贯穿与分岔等复杂因素，现有的水力裂缝数值模拟方法尚难以全面考虑，无法准确预测裂缝性储层中复杂的水力裂缝形态。在本项目研究过程中，考虑岩石受力变形、裂缝扩展、缝内流体流动耦合，推导了水力压裂流固耦合有限元列式，构建了一套用于裂缝性储层水力裂缝起裂与扩展模拟的流固耦合有限元方法和相应的计算程序，集成至ABAQUS有限元软件平台，并利用经典问题的解析解进行了验证。采用所发展的裂缝性储层水力裂缝模拟有限元方法，建立了包含水力裂缝以及天然裂缝的模型，分析了天然裂缝与水力裂缝的相互作用，揭示了天然裂缝力学性质与走向、压裂液粘度以及注入速率等因素对于水力裂缝的影响，总结了裂缝性储层水力裂缝起裂与扩展以及几何形态演化规律，研究结果对裂缝性油气储层压裂改造设计具有一定的指导意义。