Fracture fluid filtrate in low permeability sandstones may severely reduce the effective gas permeability.
This work investigates the role that fluid filtrate from slickwater fracturing has on rock-fluid and fluid-fluid interactions and to quantify the resulting multiphase permeability evolution during imbibition and drainage of the filtrate by means of specialized core testing techniques. Three suites of experiments were conducted. In the first suite of experiments, fluid leak-off tests were conducted on selected core samples to determine the extent of invasion and leakoff characteristics. In the second suite, multiphase relative permeability measurements were conducted on sandstone plugs saturated with fracture fluid filtrate. Pulse decay permeability techniques were employed to measure liquid and gas effective permeability for both drainage and imbibition cycles. These experiments capture dynamic permeability evolution during invasion and cleanup of the fracture fluid (slickwater). The final suite of experiments consist of adsorption flow tests to investigate, identify and quantify possible mechanisms of adsorption of the polymeric molecules of friction reducers present in the fluid filtrate to the pore walls of the rock sample. Imbibition tests and observation of contact angles were conducted to investigate possible changes in wettability.
Results from multiphase permeability flow tests show an irreversible reduction in end-point brine permeability and relative permeability with increasing concentration of friction reducer. Our results also show that effective gas permeability during drainage of the imbibed slickwater fluid is principally controlled by trapped gas saturation rather than by changes in interfacial tension. Adsorption flow tests identified adsorption of polymeric molecules of the friction reducer present in the fluid to the pore walls of the rock. The adsorption friction reducer increases wettability of the rock surface and results in the reduction of liquid relative permeability. The originality of this work is to derive a set of multiphase and petrophysical parameters from laboratory experiments that adequately captures multiphase permeability evolution specific to slickwater fluid systems, during imbibition and flowback. This will be useful in diagnosing formation damage from aqueous phase retention and optimizing fracturing fluid selection.