Fracture Evaluation by VEP Electrical Resistivity Method

针对热干岩(HDR)地热能开发中存在的问题，建立了在注入井与生产井之间建立人工裂缝作为地热储的技术，并对其在现场的分布和流动能力进行了评价。日本电力工业中央研究所在Akinomiya HDR项目的大规模水力压裂施工中，采用了以套管为电流电极的垂直电剖面法(VEP)来监测流体流动行为。通过现场个人计算机控制的数字记录系统，在水力压裂井周围的地面上用多个非极化电极连续检测和监测注入的流体流动。由检测到的流动路径估计的裂缝分布与井下电视(BHTV)和声发射(AE)测量获得的裂缝几何形状具有良好的一致性。压裂作业还进行了井间(井间)测量，以更准确地定位主要流动路径。VEP(孔-地)测量提供了水力压裂作业产生的主要裂缝的横向分布，而井间(井对井)测量提供了现场注采井之间流动路径的深度信息，因此，通过井对地(VEP)和井对孔(地质层析)电阻率测量与现场计算机控制的数据采集系统相结合，可以获得具有时变和主要裂缝的三维流体流动路径。

For Hot Dry Rock (HDR) geothermal energy development, it is necessary to establish technologies for creating artificial fractures between injection and production wells as geothermal reservoirs and for evaluating their distribution and flow capacity on the site.In this study, Vertical Electric Profiling (VEP) method using a casing pipe as a current electrode has been conducted for monitoring fluid-flow behaviors during massive hydraulic fracturing operations'on HDR project at Akinomiya by Central Research Institute of Electric Power Industry in Japan. The injected fluid-flow could be continuously detected and monitored with multiple non-polarizing electrodes on the ground surface surrounding the hydraulic fracturing well by the digital recording system controlled by a Personal Computer on the site.The distribution of fractues estimated from the detected flow paths showed good agreements with the geometry of fractures obtained by Borehole Television (BHTV) and Acoustic Emission (AE) survey data.Hole-to-hole (crosshole) survey was also conducted during fracturing operations for locating the major flow paths more accurately with depth.The VEP (hole-to-surface) survey provides lateral distribution of major fractures created by hydraulic fracturing operations, while the crosshole (hole-to-hole) survey gives information about the depths of flow paths connected between injection and production wells on the site.Therefore, we could obtain the fluid-flow paths with time variations and major fractures in three-dimensions by the joint use of hole-to-surface (VEP) and hole-to-hole (geotomography) electrical resistivity surveys with computer cotrolled data acquisition system on the site.