Hydrofracture Characterization Using Downhole Electrical Monitoring

Abstract

Most proposed HDR development techniques seek to increase subsurface flow capacity by hydrofracturing hot but impermeable rock - pumping high-pressure fluid into one or more deep wells and enhancing permeability by opening pre-existing sealed fractures and/or creating new ones. During subsequent field operation, it is important that the hydraulic connections between production and injection wells be neither too poor (resulting in little fluid flow) nor too good (resulting in "short-circuiting" and rapid cooling). Unless the permeable fractures created by hydrofracturing can be accurately mapped, the cost of subsequent trial-and-error drilling to try to establish a suitable fluid circulation system is likely to dominate project economics and render HDR impractical. Theoretical calculations are presented to examine the feasibility of remotely monitoring hydrofracture propagation using downhole self-potential (SP) measurements in nearby shut-in observation wells. Combining microearthquake monitoring with downhole SP monitoring could provide more information than either technique alone. The computations suggest that SP signals far in excess of detection thresholds can propagate hundreds of meters from pressurized fractures within a few weeks. Practical issues pertaining to field deployment are also discussed.