Gas permeability and fracture compressibility for proppant-supported shale fractures under high stress

Abstract

Proppants hold fractures open and increase fracture conductivity but must survive and remain functional during pressure drawdown. The shale reservoir usually suffers a high effective stress during gas depletion whilst most previous experiment works are conducted under a relative low stress level. In this work, permeability evolution was explored in a proppant-supported natural fracture of Longmaxi shale from the Sichuan Basin, China under a large effective stress range (1.5–59.5 MPa). Proppant performance was examined via continuous permeability measurements and by optical microscopy and laser-classifier measurements of particle size distributions (PSD) recored both pre- and post-loading. The permeability of the propped shale fracture is two orders of magnitude higher than that of the non-propped fracture and strongly controlled by the proppant behaviour. Surprisingly, overall permeability of the proppant pack decreases with an increase in thickness of the enclosed proppant. The decrease in the permeability with high stresses is largest for unpropped fractures and decreases with an increase in the number of layers. Most important, the mean compressibility of the non-propped and propped fracture is not constant but reduces with an increase in confining stress. This indicates that the compaction, crushing, embedment and repacking of the proppant particles, because of high effective stress, resulting in a decrease in the porosity of the proppant pack further reducing the compressibility and permeability of the supported fracture.