Reservoir-scale study of oil shale hydration swelling and thermal expansion after hydraulic fracturing

Abstract

Volumetric expansion is a factor limiting oil shale exploitation via in situ pyrolysis because it affects the permeability of a reservoir. Most of the existing research on hydration swelling and thermal expansion involves core-scale experiments; however, it is also important to systematically study this topic at the reservoir scale. In this study, relevant field experiments between two wells were conducted to analyze the effects and mechanisms of hydration swelling and thermal expansion on reservoir permeability. The results showed that, after full hydration, the estimated permeability decreased by nearly 24% according to Darcy's law, depicting that hydration swelling had a weak effect on reservoir permeability. The microcracks induced by hydration were considered to weaken preexisting closed fractures. However, after heating the formation in a nitrogen atmosphere, the results showed that the estimated permeability decreased by nearly 52%. Even worse, after heating in an air atmosphere, the estimated permeability decreased by nearly 68%, and the high temperature produced by the combustion was considered to intensify the degree and range of thermal expansion in the oil shale. This study serves as a reference for follow-up oil shale exploitation via in situ pyrolysis.