Pressure-dependent flow characteristics of proppant pack systems during well shut-in and the impact of fines invasion

Abstract

Near-wellbore screenouts during the shut-in stage due to the development of compact proppant clusters is an issue reported in several fracturing treatments for Coal Seam Gas (CSG) reservoirs. The usual solutions to counter this issue implemented in deep rocks such as Shale and Siltstone do not solve for coal reservoirs due to its geological characteristics. Therefore, the development of proppant packs is an expected outcome during CSG extraction, and the permeability in the proppant pack could decide the overall gas productivity. This study investigates the permeability of proppant packs prepared using the Under-compaction method to simulate compact proppant clusters experienced in near-wellbore screenouts. In this regard, two commonly used proppant types are selected; sand and ceramic, and a series of tri-axial permeability tests were conducted under various effective stresses relevant to CSG environments. According to the experimental outcomes, interestingly, both proppant packs exhibited non-Darcy and Darcy flow regimes, while the non-Darcy effect was more profound at low confining pressures due to its high packing porosity. Results showed that the permeability through the compact proppant pack depends on both the reservoir pressure and the type of proppants selected. The sand proppant pack (SPP) showed 1–2 orders higher than the permeability values of the ceramic proppant pack (CPP) until a threshold pressure is reached (10 MPa), after which the ceramic proppants produced higher permeability values. We also investigate the potential impact of the invasion of coal fines into the proppant packs and the corresponding pore blockings. The impact of fines invasion on CPP permeability was more potent compared to the fines induced SPP permeability. The shape of the ceramic particles, the polar effect of the α-Al2O3 atoms, and the attraction of the oxygen functional groups of the coal fines to these polar sites increase the likelihood of fines clogging in a ceramic proppant pack.