Laboratory‐Determined Transport Properties of Core From the Salton Sea Scientific Drilling Project

Abstract

Two cores from the Salton Sea Scientific Drilling Project have been studied in the laboratory to determine electrical resistivity, ultrasonic velocity, and brine permeability at pressures and temperatures close to estimated borehole conditions. Both samples were siltstones; the first sample was from 1158‐m depth, and the other was from 919‐m depth. A synthetic brine with 13.6 weight percent NaCl, 7.5 weight percent CaCl2, and 3.2 weight percent KCl was used as a pore fluid. The dry bulk density of the first sample was 2.44 Mg m−3 with an effective porosity of 8.7%. The second sample had a dry bulk density of 2.06 Mg m−3 with an effective porosity of 22.2%. At the midplane of the first sample, electrical impedance tomography was used to map the spatial variation of resistivity during the experiment. Also, at the midplane of both samples, ultrasonic tomography was used to map the spatial variation of P wave velocity. In addition, resistivity was measured with six pairs of electrodes along the sample axis. Both samples showed a strong anisotropy in resistivity and ultrasonic velocity measured perpendicular and parallel to the sample axis. The brine permeability of the first sample decreased from 5 μD to about 1.6 μD during the experiment. The second sample permeability had the same trend, but the permeability values were about 3 orders of magnitude larger. The second sample was subjected to temperatures and pressures exceeding those experienced in situ. Permeability, resistivity, and ultrasonic velocity of this sample showed a discontinuous change just beyond these in situ conditions. This discontinuity implies a structural change in the rock under conditions which would be found below its origin depth in the borehole. A model is proposed to explain the observed velocity anisotropy and variations in velocity, electrical resistivity anisotropy, and permeability with effective depth. When in situ stress is released, microcracking may occur along grain boundaries preferentially oriented parallel to bedding. This microcracking controls velocity and resistivity anisotropy at room conditions. When pressure and temperature are restored, competing effects of compaction and thermal softening of the minerals cause a reversal in the anisotropy. At temperatures and pressures above those at in situ conditions, thermal fracturing or geochemical alteration along grain boundaries causes a discontinuous change in sample physical properties.