Estimation of accurate electrical property parameters, such as the "cementation exponent" m and "saturation exponent" n is critical for characterizing hydrocarbon saturations with resistivity based models for completions decisions and original oil-in-place estimation. These exponents are known to vary with formation brine salinity, clay-content, porosity, grain-size distribution, pore geometry, stress and fluid wettability. Consequently, within large complex reservoirs electrical parameters will vary stratigraphically, lithologically and geographically. Characterizing these variations in a field petrophysical model is critical for successful reservoir characterization and field development.
Traditional methods for estimating m and n are problematic. Core-based measurements of m and n are time consuming and expensive. It is also difficult to reconstruct in-situ restored state conditions for accurate estimation of m and n with core-based methods. Because of the high paraffin content in our oil, the cleaning processes involved in preparing core samples for electrical property measurements can alter the original clay structure and wettability resulting in inaccurate estimations of m and n. Standard log-based methods for estimating m rely on analysis in a "wet" sand, often not present in unconventional fields and extremely rare at Monument Butte. NMR logging can be used to measure saturations in the flushed zone, but can be prohibitively expensive for routine application in a complex reservoir.
A method for estimating m and n using dielectric data in combination with standard triple combo logs has been developed (Ramakrishna et al, 2012). We expand upon the dielectric model and apply this methodology to over 40 wells within the Green River Formation of the Greater Monument Butte Unit (GMBU) in northeastern Utah, USA. The GMBU covers approximately 100,000 acres with numerous productive stratigraphic units and variable clastic and carbonate depositional facies. This methodology has the distinct advantage of measuring the properties in-situ and allows us to economically extend the analysis to non-cored intervals and additional regions within the field. In this study we illustrate the variations in m and n with variations in rock quality, facies, wettability and stratigraphy within the complex Green River reservoirs.
