Imbibition of Mixed Charge Surfactant Fluids in Shale Fractures

Hydraulic fracturing of shale reservoirs often requires millions of gallons of water, but only a fraction of the injected water returns to the surface. Shale gas production has been demonstrated to be positively correlated with the amount of water imbibed by the shale. In this study, neutron radiography is used to evaluate the effect of two commonly used surfactants in hydraulic fracturing fluids on the rate of capillary water uptake in shale fractures. Cationic n-octadecyl trimethylammonium chloride (OTAC) and anionic ammonium dodecyl sulfate (ADS) were added to deionized water at a 1:1 molar ratio at different concentrations and imbibed into a 200 mu m Marcellus shale saw-cut fracture. The correlations between the surfactant concentration and fracture aperture with the capillary pressure and water uptake were examined in detail to understand water imbibition during hydraulic fracturing. The effect of wettability alteration during aging of the reservoir on the water uptake rate was studied by comparing water imbibition rates into shale fractures pre-exposed to the surfactant solutions to those of unexposed shale fractures. A 51% reduction in the rate of water uptake into the unexposed shale fracture was observed when the 1:1 mixed ADS/OTAC surfactant concentration was increased from 0.1 to 0.9 mM. This decrease in imbibition rate is attributed to changes in interfacial tension and not wettability alteration of the fracture surface. For the pre-exposed sample, surfactants have sufficient time to adsorb to the shale and alter the surface wettability. The rate of water uptake for pre-exposed shale fractures was reduced by 96% compared to that of unexposed shale fractures for 0.1 mM ADS/OTAC mixture. These experimental observations suggest that natural gas production may be improved after a well shut-in period when mixed-charge surfactants are included in hydraulic fracturing fluid formulations and have sufficient time to alter shale wettability toward a more oil-wet state.