Numerical Simulation of Underground Electrical. Signals Caused by Hydrofracturing Operations

Abstract

When fluids are injected at high pressure into geothermal wells to create and/or enlarge fractures in the reservoir and stimulate production, the resulting fluid leakage away from the fracture surface into the relatively impermeable country rock can cause detectable transient electrical signals (voltages) to propagate outward into the reservoir through the mechanism of electrokinetic coupling (or “self-potential”). If nearby observation wells are equipped with appropriate sensors, these signals may be characterized and reveal information concerning fracture geometry and other fracture properties that is otherwise unobtainable or ambiguous based on microseismic monitoring alone.

A new numerical simulator (“SPFRAC”) has been developed to forward-calculate the electrical signals that will emanate from a fracture (or network of fractures) when pressurized in this manner. A representation of the fracture network as a collection of interconnected triangular planar elements is superimposed upon a conventional regular three-dimensional Cartesian computational grid describing the low-permeability country rock. The theoretical approach is presented, and the capabilities and limitations of the SPFRAC simulator are discussed. SPFRAC itself has been configured for operation on either a Windows PC or a Unix/Linux workstation, and is available through the United States Department of Energy.