基于FDEM-Flow研究地应力对水力压裂的影响

Effect of in-situ stress on hydraulic fracturing based on FDEM-Flow

用提出的FDEM-Flow(考虑流固耦合的离散元-有限元耦合方法)方法作为工具,研究了地应力对水力压裂的影响。通过一个注水圆孔的算例,研究不同地应力状态对压裂裂隙的走向和形态的影响。研究结果表明,起裂压力的大小和压裂裂隙的方位均与地应力有密切关系。在竖向地应力v?保持不变的情况下,且侧压力系数?>1时,随着?的增大,起裂压力和失稳压力均减小;?>1时且取值较小时,主要产生水平向的裂隙,并有斜向裂隙产生;?>1时且取值较大时,裂隙严格按照最大主压应力的方向扩展,不再出现斜向裂隙;?<1时,主要产生竖向的裂隙;?=1时,水平地应力和竖向地应力相等,裂隙的扩展不存在优势方向。不同侧压力系数条件下,裂隙的扩展方向与最大主应力的方向一致,水力压裂裂隙的起裂和扩展主要由最大主拉应力控制,裂隙在拉应力集中的区域起裂。这些结果与已有的试验及理论认识是相符的,进一步验证了FDEM-Flow方法用于模拟水力压裂问题的有效性。

With the proposed FDEM-Flow（combined finite-discrete element method considering fluid-solid coupling） method as a tool, we study the effects of in-situ stress on hydraulic fracturing. Through a numerical examples of an injection hole under the state of different in-situ stresses, the influence of in-situ stress on the direction and morphology of fracturing crack is studied. The results show that both the initiation pressure and direction of fracturing fractures are closely related to in-situ stress. When the vertical in-situ stress σv keeps constant and the lateral pressure coefficient λ〉1, the initiation pressure and breakdown pressure are decreased. When λ〉1 and its value is comparatively small, horizontal cracks are mainly produced with some oblique cracks generating. However, when λ〉1 and the value is larger, cracks are strictly extended along the direction of the maximum principal stress and oblique cracks no longer occur. When λ〈1, fracture pattern is dominated by vertical cracks. But when λ〉=1, the horizontal in-situ stress and vertical in-situ stress are equal, there is no advantage direction for crack extension. Under the conditions of different lateral pressure coefficients, the direction of crack propagation always coincides with that of the maximum principal stress. The fracture initiation and propagation are mainly controlled by the maximum principal tensile stress, and fractures are initiated in the concentration zones of tensile stress. These results agree well with the available experimental results and theoretical analysis, which demonstrate the effectiveness of FDEM-Flow method to simulate hydraulic fracturing.