A classification for the pressure‐sensitivity properties of a sandstone rock frame

Abstract

A three‐parameter formula is proposed to describe the sensitivity of the bulk modulus (κ) or shear rigidity (μ) for a sandstone rock frame under applied isotropic loading. The theoretical basis for this relation is the use of excess compliance as a pseudo function to describe all internal weaknesses in the rock, regardless of their origin. The law is fitted to 179 sets of laboratory measurements on unsaturated reservoir core and outcrop sandstones that have low to moderate porosity and a range of clay fractions and cementation. A stress index, defined to quantitatively rank the reservoir sands, indicates that laboratory‐derived pressure sensitivity is highest (10% per MPa maximum) for the clean, moderate‐porosity Paleocene sands from the Forties Formation and West of Shetlands, whereas the lower‐porosity, cemented, and more‐consolidated Permian sands rank as least pressure sensitive (1% per MPa maximum). Higher‐porosity sands are stress sensitive because of the overall rock‐frame compressibility. All of the sands appear to contain microcracks induced by core unloading, with some harder rocks being more susceptible to this damage. Theory predicts, and laboratory data confirm, that VP/VS generally increases with pressure, although a small proportion of the samples do show the opposite trend. The sign and magnitude of the pressure sensitivity for η [(VS/VP)2] may relate to the intensity of the microcracking in the samples.