Experimental investigation of the effect of temperature on thermal conductivity of organic-rich shales

Abstract

Experimental data on thermal conductivity of hydrocarbon reservoirs and surrounding formations at elevated temperatures are important for a variety of purposes: determining heat flow density; designing and optimizing thermal methods of enhanced oil recovery (EOR); basin and petroleum system modeling; exploitation of geothermal reservoirs; and design of radioactive waste disposal. However, there is an acute shortage of such data at present, particularly for unconventional reservoir rocks. We helped to fill this data gap by using a new, sophisticated technique to carry out thermal conductivity measurements on 28 oil shale samples from various unconventional formations in a temperature range of 30–300°С. The formations studied were Bazhenov and Abalak (West Siberia, Russia), Mendym, Domanic, Sargaev and Timan (Volga-Ural, Russia). Divided-bar and optical scanning methods were combined in the thermal conductivity measurements in order to account for thermal anisotropy and heterogeneity of the rocks and improve quality of the experimental data. Application of the optical scanning technique enabled us to select representative full-diameter or split-core samples from the continuous thermal core logging data and to choose an optimal location for drilling of core plugs from the core samples for measurement of thermal conductivity at elevated temperatures, with due account for anisotropy and heterogeneity inherent to oil shales. This approach enabled us to reveal and overcome systematic errors in previous measurements of oil shale samples using the divided-bar method and to develop an approach for the correction of measurement results. The measurements of thermal conductivity of our samples at elevated temperatures found a 5–27% decrease of rock thermal conductivity with temperature increase from 30 to 300°С. New methodology of the rock thermal conductivity determining at elevated temperatures is reported that includes a combination of two instruments: Thermal Conductivity and Thermal Diffusivity Scanner and DTC-300. It allowed to account for the contact resistance, non-ideal form of rock samples prepared for study (nonparallelism of rock samples surfaces), and changes in rock samples structure after heating.