考虑压裂缝网动态滤失的停泵压降模型

Pump-stopping pressure drop model considering transient leak-off of fracture network

通过在传统主裂缝物质平衡方程中引入主裂缝-基质、次裂缝-基质、主裂缝-次裂缝耦合流动表达式,建立了“主裂缝-次裂缝-基质”滤失耦合流动模型,同时将主、次裂缝缝宽的应力敏感方程和井筒注入量方程进行压力-流量连续性耦合求解,实现了滑溜水体积压裂改造停泵后井底压降和缝网闭合的模拟计算。研究表明:停泵井底压降导数双对数曲线呈现5个特征斜率段,反映了停泵后“缝间窜流控制”、“缝网滤失控制”、“缝网闭合控制”和“残余滤失控制”4个主控阶段。水平井体积压裂改造停泵初始时刻,主、次裂缝间的窜流现象明显,随后滤失占据主导,主、次裂缝的滤失呈现出非均匀的递减趋势:主裂缝滤失速度慢,次裂缝滤失速度快,缝网整体呈现先快、后慢,直至接近于零的滤失规律。缝网导流能力对停泵压降曲线形态的影响相对弱于缝网规模;缝网导流能力与滤失量、裂缝闭合程度正相关;次裂缝规模与滤失量、次裂缝闭合程度正相关,但与主裂缝闭合程度负相关。经现场施工数据检验,该模型能够有效地反映缝网的闭合特征,解释结果可靠且能够反映实际水平井各压裂段的非均匀改造效果。

By introducing the coupling flow expressions of main fracture-matrix,secondary fracture-matrix and main fracture-secondary fracture into the traditional main fracture material balance equation,the“main fracture-secondary fracture-matrix”leak-off coupling flow model is established.The pressure-dependent fracture width equation and the wellbore injection volume equation are coupled to solve the pressure-rate continuity problem.The simulation and calculation of the bottomhole pressure drop and fracture network closure after the pump stopping in slickwater volumetric fracturing treatment are realized.The research results show that the log-log curve of pump-stopping bottomhole pressure drop derivative presents five characteristic slope segments,reflecting four dominant stages,i.e.inter-fracture crossflow,fracture network leak-off,fracture network closure and residual leak-off,after pump shutdown.At the initial time of pump shutdown for volumetric fracturing treatment of horizontal well,the crossflow between main and secondary fractures is obvious,and then the leak-off becomes dominant.The leak-off of main and secondary fractures shows a non-uniform decreasing trend.Specifically,the leak-off of main fractures is slow,while that of secondary fractures is fast;the fracture network as a whole presents the leak-off law of fast first,then slow,until close to zero.The influence of fracture network conductivity on the shape of pressure decline curve is relatively weaker than that of fracture network size.The fracture network conductivity is positively correlated with leak-off volume and fracture closure.The secondary fracture size is positively correlated with leakoff volume and closure of the secondary fracture,but negatively correlated with closure of the main fracture.Field data validation proves that the proposed model and simulation results can effectively reflect the closure characteristics of the fracture network,and the interpretation results are reliable and can reflect the non-uniform stimulation performance of each fracturing stage of an actual horizontal well.