Slick-water fracturing has emerged as the preferred technique for fracturing and proppant placement in ultra-low permeability unconventional gas reservoirs. However, significant proportions of all these completions fail to perform as well as expected. This could be due to poor reservoir quality, inefficient completions or non-optimal flow back controls. A major concern with using water as the fracturing fluid is that of fluid retention in the tight reservoir matrix due to high capillary pressures and presence of water sensitive clays. High water saturation in the invaded zone near fracture face reduces gas relative permeability and impedes gas productivity. Fracture cleanup in itself plays an important role in achieving maximum productivity. Historically it is assumed that as long as fracture is infinitely conductive, any water within the fracture is cleaned up readily. Most of the previous studies have ignored the effect of gravity in fracture cleanup. However, in this study a 3D numerical simulation approach is adopted to investigate the effect of gravity on frac-water recovery. It is shown that under scenarios with low gas flow velocities, the water may load at the fracture bottom and maybe difficult to unload for months. This will impact the early time fracture cleanup and therefore, the gas productivity.
Hydraulic fracturing of horizontal wells is instrumental in providing large reservoir contact area so as to achieve commercial gas recovery. The advantages of using water as the fracturing fluid include low cost and efficient proppant transport. However, only a small fraction of the total pumped fluid is recovered back. The mechanisms of fluid retention downhole and its impact on hydrocarbon recovery are subjects of intense debate. Previous work regarding fracturing fluid recovery has shown that a significant percentage of liquid may invade the matrix and get trapped at the fracture face due to capillary effects. This reduces the relative permeability of the hydrocarbon phase during flowback. Holditch (1979) studied water blocking issues in hydraulically fractured gas wells and demonstrated the role of capillary pressure, matrix permeability damage and relative permeability in fluid recovery. The study concluded that unless the reservoir rock permeability is significantly damaged by the fracturing fluid, a complete water block to gas flow will not occur. Multiple studies have investigated the impact of fracture cleanup and its impact on gas productivity since then. Gdanski and Walters (2010) simulated various scenarios to study the impact of fracture conductivity, matrix relative permeability and flow back conditions on load recovery and gas production. Wang et al. (2012) analytically modeled the relative impact of fracture fluid invasion, proppant embedment and gel residue in the fracture on shale gas production. Sullivan et al. (2006) have looked at impact of frac-fluid type (water or oil based) and its viscosity on cleanup. They noted that more viscous fluid cleaned up faster due to lesser matrix invasion. However, the above referenced works ignored the accumulation of liquid in the fracture itself due to gravity. In this study we show that liquid accumulation can occur within the fracture itself and is related to fracture face cleanup through counter-imbibition mechanism.