水平井压裂多裂缝扩展诱发光纤应变演化机理

Evolution mechanism of optical fiber strain induced by multi-fracture growth during fracturing in horizontal wells

在平面三维多裂缝扩展模型基础上,构建了压裂监测井光纤应变与应变率的计算模型,提出了水平井压裂多裂缝扩展诱发光纤应变正演计算方法。基于该方法,开展了水平井压裂多裂缝扩展诱发光纤应变与应变率数值模拟分析,结果表明,裂缝扩展诱发光纤应变演化可分为应变增强、收缩汇聚和直线状汇聚3个阶段;应变率演化可分为应变率增强、收缩汇聚、直线状汇聚、停泵应变率反转4个阶段。光纤应变停泵后不变,但应变率会出现反转,光纤应变率反转可反映注入动态;根据分布式光纤应变与应变率的直线状汇聚带,可判断裂缝扩展至光纤的时刻与井间压窜,同时可依据各簇裂缝扩展到光纤监测井的时刻识别多裂缝非均匀扩展现象,评价裂缝非均匀扩展程度;光纤监测井水平段位于裂缝高度范围内时,可识别裂缝延伸至光纤的时刻,而光纤监测井水平段位于裂缝高度以外时,汇聚条带不明显;多段压裂施工中,受上一压裂段应力干扰的影响,后续压裂段的光纤应变可能不再出现压裂初期的张应变区域,但光纤应变率动态可有效呈现每一段压裂裂缝的扩展动态,单段压裂的光纤应变率演化规律适用于多段压裂。

A forward model for optical fiber strain was established based on a planar 3D multi-fracture model.Then the forward method calculating distributed fiber strain induced by multi-fracture growth was proposed.Based on this method,fiber strain evolution during fracturing of horizontal well was numerically simulated.Fiber strain evolution induced by fracture growth can be divided into three stages:strain increasing,shrinkage convergence,and straight line convergence;whereas the evolution of fiber strain rate has four stages:strain rate increasing,shrinkage convergence,straight line convergence,and strain rate reversal after pumping stop.Fiber strain does not flip after pumping stop,while the strain rate flips after pumping stop,so strain rate can reflect injection dynamics.The time when the fracture extends to the fiber and inter-well pressure channeling can be identified by the straight line convergence band of distributed fiber strain or strain rate,and the non-uniform growth of multiple fractures can be evaluated by using the instants of fractures reaching the fiber monitoring well.When the horizontal section of the fiber monitoring well is within the height range of a hydraulic fracture,the instant of the fracture reaching the fiber can be identified,otherwise,the converging band is not obvious.In multi-stage fracturing,under the influence of stress shadow from previous fracturing stages,the tensile region of fiber strain may not appear,but the fiber strain rate can effectively show the fracture growth behavior in each stage.The evolution law of fiber strain rate in single-stage fracturing can be applied to multi-stage fracturing.