Stress Dependence of Sandstone Electrical Properties and Deviations From Archie’s Law
Authors: Carpenter Chris 
Published: August 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 181484, “Experimental and Numerical Investigations on Stress Dependence of Sandstone Electrical Properties and Deviations From Archie’s Law,” by M.F. Farid, J.-Y. Arns, W.V. Pinczewski, and C.H. Arns, University of New South Wales, prepared for the 2016 SPE Annual Technical Conference and Exhibition, Dubai, 26–28 September. The paper has not been peer reviewed.

The resistivity index (RI) of Fontainebleau and Bentheimer sandstones was investigated at ambient and reservoir pressures down to low water saturations. The RI measurements show that both sandstones display Archie behavior at elevated pressure. However, at ambient pressure, the RI for Fontainebleau sandstone deviates from Archie behavior at low water saturations. The pore-space images suggest that the deviation from Archie behavior is attributable to the presence of conductive percolating grain-contact regions.

Introduction

Deviations from Archie’s law are known to occur, particularly at low water saturations, even for clean sandstones. It is unclear whether the deviations from Archie behavior observed at low pressures are also displayed at elevated pressures. In this paper, the authors present laboratory measurements of RI for two strongly water-wet sandstones at ambient and elevated pressures. The measurements are supplemented with high-resolution microcomputed-tomography (CT) imaging in dry and wet states at ambient pressure to determine an accurate description of the open resolved pore space and to attribute a finite porosity to fluid-saturated grain contacts at elevated pressure. Assuming that the main elements responding to effective stress are the grain contacts, grain-contact conductivities are estimated at elevated confining pressures with actual formation-factor measurements for saturated samples at the same confining pressures. These are compared with computations on the micro-CT images. For Bentheimer, which contains image-resolvable clay regions, the clay regions are considered as additional conductive pathways with different stress dependence.

Methodology

Experimental Procedure. Rock. The Fontainebleau sandstone samples used exhibit relatively low porosity in the range of 4 to 5%, with sample permeabilities of approximately 0.03 to 2.2 md at confining pressures of 500 to 6,000 psi. The Bentheimer sandstone samples used have a porosity of approximately 23.7 to 25.9%, with permeability of the intact samples at approximately 2,000 md over a range of 500 to 6,000 psi in confining pressure.

Fluid. RI and drainage capillary pressure experiments were performed with brine (2% by weight sodium chloride) and air as the wetting and the non-wetting phases, respectively. The density and viscosity of brine were measured to be 1.0166 g/cm3 and 1.0 cp, while 0.00129  g/cm3 and 0.0185 cp were  used for the  density and viscosity of air, respectively.

Core preparation, as well as image analysis and acquisition, is described in detail in the complete paper. In Fig. 1, the authors illustrate the final phase assignments for Fontainebleau sandstone, including grain/grain boundary labels for a few different saturations and boundary assignments.