2.5维频率域可控源电磁法水力压裂监测正演模拟

2.5D controlled-source audio-frequency magnetotelluric modeling for hydraulic fracturing monitoring

水力压裂技术在非常规油气资源开采过程中发挥着重要作用,水力压裂监测对指导生产具有重要意义。目前广泛应用的微地震监测不能准确获得水力压裂的有效压裂体积参数,而电磁法对低阻体反应灵敏,可以用来监测裂缝的空间特征和扩展范围。采用频率域可控源电磁法(CSAMT)对水力压裂模型进行了正演数值模拟,详细推导了2.5维CSAMT有限元计算方程并实现了正演数值算法。通过数值解与解析解的对比,验证了算法的可靠性。分别对发射源靠近井口和发射源远离井口两个水力压裂模型进行了数值模拟。为了考察压裂前、后的相对变化,分别求取了各压裂段与仅含套管时的视电阻率相对差异和相邻两个压裂段的视电阻率相对差异。研究结果表明,随着压裂进程的持续,视电阻率的相对差异曲线的极值会朝着裂缝新生成的方向发生偏移,其移动的范围与裂缝的位置具有良好的对应关系,故可以根据视电阻率的相对差异曲线极值的位置对裂缝进行大致定位。根据对视电阻率残差拟断面图的分析可知,在裂缝所对应的位置处,视电阻率产生了足够大的残差数值,虽然由于场源的影响,异常区域的中心与裂缝的中心位置并不一一对应,而是稍微偏向裂缝远离场源一侧的方向,但是可以根据相邻两个压裂段的视电阻率残差结果对裂缝的位置进行初步定位。研究结果表明,频率域可控源电磁法对于水力压裂的裂缝定位是可行的,可以为水力压裂电磁法动态监测提供理论方法和技术支持。

Hydraulic fracturing is an important technique in unconventional hydrocarbon recovery,and thus how to monitor its performance is crucial to production.Microseismic monitoring,which is a common practice,fails to obtain some useful parameters of the stimulated reservoir.In contrast,magnetotelluric responses are sensitive to low resistivities and could then be used to monitor spatial distribution of fractures.We formulate the 2.5D finite-element equations of controlled-source audio-frequency magnetotellurics(CSAMT)and numeric algorithm for CSAMT forward modeling.The algorithm is validated using numerical solutions and analytic solutions for two hydraulic fracturing models:one with the emission source close to the wellhead and the other with the emission source far away from the wellhead.To examine pre-frac and post-frac variations,we calculate relative apparent resistivity differences between each fractured stage and casing pipe and between two neighboring fractured stages.The results show that the extremum on the relative difference curve will move toward new fractures generated as hydraulic fracturing goes on,and the movement correlates with fracture location.This means that hydraulic fractures may be roughly located in accordance with the position of the extremum on the relative difference curve.Pseudo-section map shows sufficiently large apparent resistivity residuals at the positions of fractures,although the center of anomalies is slightly biased toward the side away from the field source instead of being exactly aligned with the center of fractures owing to the influence of the field source;this means that hydraulic fractures may be tentatively located in accordance with apparent resistivity residuals at two neighboring fractured stages.As per the model tests,CSAMT is feasible for hydraulic fracture positioning;our study offers a theoretical approach and technical support to dynamic electromagnetic monitoring of hydraulic fracturing.