Petrophysical Properties of Sandstones Under True Triaxial Stresses I: Directional Transport Characteristics and Pore Volume Change

Abstract

Transport properties are important parameters in controlling reservoir performance, and an understanding of how fluids flow through a reservoir can aid successful management of hydrocarbon recovery. Traditional laboratory measurements of permeability and resistivity are carried out in one direction which does not take into account any anisotropy in the rock properties. Furthermore, laboratory measurements of these properties are often carried out under conditions of equivalent hydrostatic stress, or sometimes anisotropic triaxial loading in which two of the principal stresses are equal. However, permeability and resistivity may vary in the three principal directions, and pore volume change may differ under different stress paths. A novel method for measuring directional permeability and resistivity on a cubic sample under realistic true triaxial stresses has been developed and used in this study. This paper presents laboratory measurements of directional permeability, directional resistivity and pore volume change under conditions of hydrostatic and true triaxial stresses. Measurements of permeability and resistivity were carried out in the three principal directions using cubic rock samples of outcrop and reservoir sandstones. The main objectives of the work was to investigate directional and stress dependence of transport properties under realistic true triaxial stresses. Results have shown greater pore volume change to occur under comparable hydrostatic compared to true triaxial stresses. The samples with laminations showed high degrees 1Currently with Petroleum Development Oman (PDO)of anisotropy in permeability and resistivity under conditions of hydrostatic and true triaxial stresses. The degree of anisotropy was found to be constant between low and high stress. The permeability values were found to be higher under true triaxial stresses than under their corresponding equivalent hydrostatic stresses, whilst resistivity showed little differences. Thus the results have shown the importance of measuring directional transport properties and pore volume changes under realistic stress conditions.