Characterization of Fractured Reservoirs Using a Combination of Downhole Pressure and Self-Potential Transient Data

Abstract

In order to appraise the utility of self-potential (SP) measurements to characterize fractured reservoirs, we carried out continuous SP monitoring using multi Ag-AgCl electrodes installed within two open holes at the Kamaishi Mine, Japan. The observed ratio of SP change to pressure change associated with fluid flow showed different behaviors between intact host rock and fractured rock regions. Characteristic behavior peculiar to fractured reservoirs, which is predicted from numerical simulations of electrokinetic phenomena in MINC (multiple interacting continua) double-porosity media, was observed near the fractures. Semilog plots of the ratio of SP change to pressure change observed in one of the two wells show obvious transition from intermediate time increasing to late time stable trends, which indicate that the time required for pressure equilibration between the fracture and matrix regions is about 800 seconds. Fracture spacing was estimated to be a few meters assuming several micro-darcies (10-18 m2) of the matrix region permeability, which is consistent with geological and hydrological observations.

Application of Self-potential Measurements to Reservoir Engineering

“…To detect microscopic ΔSP/ΔP, an electrode array installed outside the insulated casing is desirable. Such observations were recently reported from an oil field by Chen et al 13) To detect macroscopic ΔSP/ΔP such as predicted by Ishido and Pritchett 7) , the conductive casing itself (with electrical continuity extending over a distance longer than the typical fracture spacing) can be used as an electrode. In open holes, the appearance of double porosity behavior depends on whether or not a skin zone is present.…”

“…Ishido and Pritchett 7) extended the EKP-postprocessor so as to calculate the drag current density in "MINC" media 8) . In the MINC representation, the contribution of mass exchange through the matrix region is neglected in mass exchange between adjacent macroscopic computational grid blocks.…”

“…Ishido and Pritchett 7) performed a pressure-transient simulation for a two-dimensional axi-symmetric horizontal reservoir model. The formation is represented by a "MINC" doubleporosity medium with the following properties:…”

“…In the calculations described in the previous section, Ishido and Pritchett 7) did not consider the "near-field" effects around a borehole. Ishido et al 9) constructed a reservoir model to treat a borehole and individual fractures explicitly instead of using the MINC double-porosity representation (Figure 3).…”

“…Changes in pressure and SP at a point ~5 m away from the injection well (open hole) during injection test for fractured medium7) . The pressure change for the equivalent porous medium is shown by broken curve.…”