Advances in understanding the relationship between rock wettability and high-frequency dielectric response

Previous studies have shown that wettability has an effect on the complex permittivity of rock in a wide frequency interval. At low frequencies ( < 10 kHz), the dielectric behaviour is primarily controlled by the connectivity of the water phase. At higher frequencies (10 kHz-10 GHz), the governing parameter is the shape of the water inclusions, i.e. large-surface inclusions, typical of a water wet system, and small-surface inclusions, generally associated with oil wet situations, show markedly different electrical responses. Maxwell-Wagner-Sillars polarisation governs these high-frequency effects, which are relatively independent of the level of water saturation of the system, allowing direct and easier determination of wettability. Several experiments carried out on synthetic porous media and reservoir rocks made strongly oil wet using a sylanisation process have been published by the authors [Rev. Inst. Fr. Pet. 53 (1994) 771; Bona, N., Rossi, E., Capaccioli, S., 1999a. Wettability and dielectric constant. Proc. of the 1999 Int. Symp. of the Society of Core Analysts, Golden, Colorado, paper SCA 9925; Bona, N., Rossi, E., Capaccioli, S., 1999b. Electrical measurements in the 100 Hz-10 GHz frequency range for efficient rock wettability evaluation. Proc. of the 1999 SPE Int. Symp. on Oilfield Chemistry, Houston, paper SPE 50720, 207-217; J. Phys. D: Appl. Phys. 33 (2000) 1036]. This paper summarizes the results obtained on "fresh-state" rocks with preserved wettability. Two reservoirs were studied. One is a fractured oil reservoir, with a permeability so low that it cannot be investigated using the conventional wettability test techniques. The other is a carbonate reservoir, with Arnott test results available. While the second reservoir was found to be strongly water wet, the first showed the coexistence of water wet and oil wet zones, which were clearly recognised by two distinct relaxation peaks in the dielectric spectra. Air-mercury pore size distribution measurements and CryoSEM analyses were also carried out. All the collected data were used to build a consistent model of the behaviour of the rock. (C) 2002 Elsevier Science B.V. All rights reserved