基于细观离散元方法的低渗透储层水力压裂数值模拟

Numerical simulation of hydraulic fracturing for low permeability reservoirs based on particle flow code-discrete element method

针对连续介质理论研究水力压裂需预设裂缝界面单元的不足,引入离散元—颗粒流方法模拟低渗透储层的水力压裂行为。采用圆形颗粒模拟低渗透储层,通过Navier-Stokes方程描述压裂液的流体运动,建立了考虑流固耦合作用的低渗透储层水力压裂颗粒流数值模型。分析了颗粒间平行连接断裂的失效判据,讨论了基于N-S方程的低渗透储层水力压裂PFC-CFD流固耦合求解方法,揭示了不同参数对水力压裂裂缝起裂及延伸行为的影响。结果表明:起裂压力与最小水平主应力、颗粒法向接触刚度、颗粒法向及切向连接强度近似呈正比例关系,而裂缝最大长度及宽度与上述参数呈反比例关系;起裂压力、裂缝长度、宽度与最大水平主应力、切向接触刚度基本不相关。

Aimed at the shortcoming of macroscopic continuum theory in investigating hydraulic fracturing by presetting crack interface element, the particle flow method of micro-mechanical discrete element theory was introduced to simulate mechanical behavior of hydraulic fracturing for low permeability reservoir. The reservoir was simulated as round particle, while the fluid movement of the fracturing fluid was described by Navier-Stokes equation, and particle flow numerical model of hydraulic fracturing for low permeability reservoir was established considering fluid-solid coupling. Fracturing criteria for contact-bond model were analyzed, and PFC-CFD fluid-solid coupling solution for hydraulic fracturing in low permeability reservoir was discussed based on N-S equation, and the effects of different parameters on the fracturing behavior were discovered. The results showed that the fracture initiation pressure is proportional to the minimum horizontal stress, particle normal contact stiffness, particle normal and tangential connection strength, while the fracture length and width are inversely proportional to above parameters. In addition, the initiation pressure, fracture length and width are independent of maximal horizontal stress and tangential contact stiffness.