Effect of pressure and saturating fluid on the thermal conductivity of compact rock

Regional differences in heat flow of about 15% are often considered to be significant. However, it is shown here that the observed thermal conductivity for dry rock samples may differ by more than 15% from the conductivity that the rocks had in place—saturated and under hydrostatic pressure. Error in the apparent conductivity arises because cracks, which are present in most rocks, form effective barriers to the flow of heat when rocks are dry but have a negligible effect when the rock is saturated or when the cracks are closed by stress. Several formulas are presented for estimating the possible error for low-porosity crystalline rocks. The theoretical expressions show reasonable agreement with measurements made on dry and saturated samples of a granite under axial compressive stress, up to 930 bars.