Permeability of Oil Shale Under In Situ Conditions: Fushun Oil Shale (China) Experimental Case Study

Abstract

Oil shale is a critical strategic energy source with very large reserves, and it is acknowledged as an alternative energy source to crude oil. Oil shale in situ retorting is the only technologically feasible method to achieve large-scale industrial development. In situ permeability is one of the most critical technical parameters for successful implementation of oil shale in situ retorting. In this study, the permeability of Fushun oil shale at different temperatures was investigated using a triaxial permeability testing machine under in situ conditions. The results show that the permeability of the oil shale under three-dimensional stress can be divided clearly into three stages from 20 °C to 600 °C. First, the oil shale remains almost impermeable from 20 °C to 200 °C. Second, in the temperature range from 200 °C to 350 °C, the permeability of the oil shale first increased and then decreased to almost zero at 350 °C owing to the dual constraints of the surrounding stress and additional expansion stress under in situ conditions. Third, from 350 °C to 600 °C, the permeability of the oil shale increased sharply and reached a maximum of 100 × 10−5 μm2 at 600 °C. Thermal cracking and solid organic matter pyrolysis were the main factors controlling the change in oil shale permeability in the test temperature range. The results suggest that the threshold temperature for oil shale permeability change was 350 °C under in situ conditions. In addition, under in situ conditions, the oil shale permeability decreased with increasing pore pressure at different temperatures. This work has direct applications for oil shale in situ large-scale exploitation.