In situ generated proppants for shale reservoirs

Abstract

A large fraction of the fractured surface created during hudraulic fracturing is not propped. Injected proppants often settle near-wellbore in low viscosity fracturing fluids like slick water and do not reach the end of fractures. Many microfractures are too narrow to accommodate injected proppants and close during production. Poor placement of proppant leads to a loss of fracture conductivity and undermines the productivity of shale wells. In this study, reactive chemicals are injected to generate hydroxyapatite crystals on calcite-rich shale surfaces to act as in-situ proppants to improve microfracture conductivity. Batch experiments were conducted with reservoir shale samples in both low salinity frac water and high salinity brine. Crystals were generated in both low and high salinity brines. The crystals grew to several hundred microns and tended to form on calcite-rich shale surfaces. The hardness data showed that properly designed formulations could avoid the shale softening and even increase the hardness of the fracture surface (from 230 to 280 MPa). Reactive flow experiments were performed to evaluate fracture conductivity change after the chemical treatment. Micro-CT and scanning electron microscopy were used to visualize in situ proppants. A 3–10 times increase in post fracture conductivity was observed for both reservoir and outcrop shale samples. The propping of the microfractures can decrease the production decline during hydrocarbon production in shale reservoirs.