Thermal Diffusivity, Heat Capacity, and Thermal Conductivity of Oil Reservoir Rock at High Temperatures

Abstract

The contact-free, laser-flash (LFA 457) apparatus was used to measure the thermal diffusivity ($a$) of oil reservoir rock samples over the temperature range from (303 to 723) K at atmospheric pressure. The measurements of the heat capacity ($C_{P}$) of the same oil reservoir rock sample were performed over a temperature range from (305 to 771) K using DSC 204 F1 technique. The combined expanded uncertainties of the temperature ($T$), thermal diffusivity ($a$) and heat capacity ($C_{P}$) measurements at the 95% confidence level with a coverage factor of k = 2 are estimated to be 20 mK, 3 % and 1 %, respectively. The measured thermal diffusivity and heat capacity data and their temperature dependence for oil reservoir rock were interpreted in terms of the damped harmonic oscillator (DHO) theory and modified multi-component Einstein model, respectively. Theoretically based correlations for the thermal diffusivity (DHO model) and heat capacity (multi-peak model based on vibrational spectra) were adopted to accurately represent the measured data. The measured values of $a$ and $C_{P}$ together with the density (ρ) data were used to calculate the derived values of thermal conductivity ($\lambda = \rho C_{P} a$) of the oil reservoir rock. The effect of phase changes (dehydration and thermal decomposition) in the intra pore fluids (oil and water) on thermal properties (thermal diffusivity, heat capacity, and thermal conductivity) of oil reservoir rock samples have been studied. We observed rapid increase of the heat capacity of the oil reservoir rock sample in distinct temperature ranges, around 323 K and 745 K. We attribute these irregularities in temperature dependence of thermal diffusivity and heat capacity to the dehydration (intensive vaporization of pore water) and the thermal decomposition of the residual heavy oil component (pyrolysis), which occurs at high temperatures. Based on the present measured thermophysical property data we have developed a model that simulated the heat transfer process in an oil reservoir where the thermal diffusivity of the reservoir media is considered as a function of temperature. The temperature variation at each point of the reservoir is calculated using a heat transfer equation with temperature dependent thermal properties of the reservoir media, i.e., the reservoir temperature profile, $T\left( {x,t} \right)$, was simulated with thermophysical property changes of the reservoir media. We observed heat transfer alteration of the oil reservoir media due to temperature dependence of the thermal diffusivity. It was shown that taking into account the temperature dependence of the thermal diffusivity, considerably affects the heat transfer alteration of oil reservoirs.