Real time triaxial resistivity and pore pressure penetration measurements for monitoring saturation and electrical property alterations under stress

Abstract

The use of Archie's equation relating water saturation in clean sandstone formations to the electrical resistivity of the formations and the fluid properties has been key in electrical log interpretation. Archie's equation has been widely used for most sedimentary rocks for fluid saturation evaluation since its first application in sandstones. Recent studies have shown that Archie's equation is not an accurate representation of the electrical property representation in all formations (Herrick et al. 2001; Kennedy et al. 2012; Suman et al. 1997; Moran et al. 1979; Mahmood et al. 1991; Kennedy 2006). Due to the recent shale boom in the United States and throughout the world, characterization of these unconventional hydrocarbon bearing reservoirs including shale gas, tight oil, carbonates, and unconsolidated sand formations has become an area of interest. Application of Archie's equation in these formations is not considered to be accurate and may result in significant error when determining fluid saturation utilizing the electrical resistivity logs (Worthington 1982), and other properties correlated to resistivity response. Conductive mineralogy, low permeability complex pore structures, in situ stress state, and formation anisotropy are the leading causes of erroneous interpretation of the resistivity data collected in the field through resistivity logs in shale reservoirs. The results of an experimental investigation on the electrical impedance spectroscopy for sandstones and organic-rich shale (Archie and non-Archie) formations have been presented in this research study. The main objective of the study is to examine the effects of stress state, pore geometry changes, tracking of the fluid migration, and rock-fluid interactions on the electrical properties of the formations investigated. Berea Sandstone, Eagle Ford Shale, and Pierre Shale have been studied in detail and the results of the findings of the study are presented here. A resistivity measurement system has been designed and implemented in a triaxial cell. The core samples and the pore fluid injection system were electrically isolated for accurate measurements of the electrical properties. The resistivity measurements have been coupled with geomechanical deformation, compressional and shear wave velocities, absolute permeability, and XRD scans of specific core samples simultaneously studied. These parameters were monitored as a function of stress state, elevated pore pressure, and fluid composition to create a detailed understanding on the interdependence and correlations between various monitored parameters. Experimental results from Eagle Ford and Pierre Shale samples show that increasing stress on the rock increases the resistivity of the sample. This is mainly due to the closure of the natural fractures present in the sample, reduction of nano-pore space and elimination of part of the connectivity throughout the complex rock structure as a result of the closure of the pore space. In conjunction with the reduction in resistivity, permeability decrease is observed with increased stress. These observations represent the described pore geometry changes due to the increase in stress. Additionally, the resistivity measurements were used to track the imbibition of a brine solution through a Berea Sandstone sample. A correlation between the measured resistivity and brine salt concentration has been developed and compared to the predicted concentration from a numerical model. The computer model closely matched a portion of the measured resistivity data; however, some errors are apparent.