Subsurface-to-surface resistivity method for monitoring fluid progression in improved oil recovery projects

Improved oil recovery (IOR) methods involve enhanced progression of fluids through an oil reservoir because of changes in petrophysical properties (re lative permeabilities, wettability, saturations) or applied boundary condit ions (pressure, temperature). Remote monitoring of IOR processes is primari ly based on an ability to map the changes in reservoir physical properties (electrical, acoustic, gravimetric...) that may occur during recovery. The time period of IOR processes is of the order of months to years. A repeated geophysical survey using a method sensitive to the appropriate physical pr operty changes can, in principle, be used to map the fluid front or the ext ent of the swept zone that arises as the result of the IOR process. Methods based on changes in strata resistivity comprise one family of geophysical approaches that may be used to monitor progression of IOR processes. Electrical resistivity measurements are, in principle, sensitive to IOR-ind uced fluctuations in reservoir conductivity. Seismic, deformation, or gravi ty methods alone cannot be used to detect these fluctuations if they arise largely through pore-fluid ionic concentration changes. Electrical resistiv ity methods for reservoir description and monitoring thus appear particular ly promising in such cases. We present herein several two-dimensional (2-D) numerical model studies add ressing the application of the subsurface-to-surface resistivity method for monitoring IOR processes involving hot-water and steam injection. The theo retical basis for this method and concept of sensitivity has been derived f rom previous work(1). Based on generic reservoir model studies for hot-wate r and steam injection, we believe that the feasibility for the application of resistivity methods to monitoring of IOR processes is clearly demonstrat ed.