Effect of water saturation on rock thermal conductivity measurements

Abstract

Abstract We studied the effect of water saturation on rock thermal conductivity measurements using a set of 67 samples covering igneous, sedimentary and metamorphic rocks. The samples were collected from both fresh outcrops and borehole cores, and belong to different geologic provinces of the Indian shield. Effective porosity computed from measurements of sample weights under dry and saturated conditions reveal that the samples fall in the low-to-moderate porosity range, from near-zero to 8%. Thermal conductivity was measured both in dry as well as water-saturated states. The salient findings of the study are as follows. (i) Thermal conductivity measurements on porous rocks confirm many previous studies that the water saturated thermal conductivity is greater than dry thermal conductivity. (ii) The effect of water saturation on thermal conductivity of igneous, sedimentary and metamorphic rocks increases as the porosity of the rock samples increases, which indicate that the effect primarily depends on porosity. (iii) The observations are consistent with theoretically computed effect obtained using a geometric mixing law prediction model for two phase system, i.e. pore fluid and solid rock. The study shows clearly and dramatically both theoretical and experimental evidence that ignoring a 5% porosity incurs a systematic thermal conductivity error of about 15%, which directly reflects into a similar error in heat flow determinations. (iv) Application of water-saturated thermal conductivity values of low-to-moderate porosity sandstones from Shivpuri area in central India to a recent heat flow determination from the same area shows that the reported heat flow was under-estimated by up to 12% due to the use of dry-state thermal conductivity measurements. This paper reinforces the importance of current practice always to measure thermal conductivity in the water-saturated state.