EXPLORING THE ROLE OF SURFACTANTS IN CONTROLLING WATER LOSS IN SHALE GAS RESERVOIRS

It is known from industrial experience that the majority of the hydraulic fracturing treatment water used to stimulate gas production in shale reservoirs is lost to the shale and does not return to the surface. Water imbibition in low permeability reservoirs usually results in reduced reservoir gas production, however, shale gas production has been demonstrated to be positively correlated with the amount of water imbibed by the shale. This study presents results from a comprehensive analysis of the role of surfactants in altering shale surface properties and investigates the controlling factors effecting surfactant adsorption mechanisms in hydraulically fractured shale reservoirs. Contact angle & surface tension measurements were used to study the change in wettability of Marcellus shale due to interaction with commonly used surfactants in hydraulic fracturing completion fluids. Cationic octadecyl trimethyl ammonium chloride (OTAC) and anionic ammonium Dodecyl Sulfate (ADS) were selected for this study due to their regular use in hydraulic fracturing fluids. Contact angle measurements of mixed charge surfactant solutions on Marcellus shale samples demonstrate a synergistic interaction of cationic and anionic surfactants in modifying shale wettability toward a less water-wet state at surfactant concentrations below critical micelle concentration. Atomic force microscopy is used to investigate the role of shale mineralogy in controlling the adsorption mechanism for these mixed surfactant solutions and relate mineral spatial distribution to alterations in shale wettability. To further explore the impact of mixed surfactant solutions on water uptake in shale gas reservoirs, neutron imaging is used to quantify the rate of water imbibition in fractured shales exposed to a range of surfactant solutions and total surfactant concentrations. Results indicate that mixed charge surfactant solutions can serve to both reduce surface tension and shift shale wettability toward less water-wet conditions due to interaction with positive and negatively charged mineral surfaces in shales. Both of these effects can lead to enhanced natural gas relative permeability and may be associated with observed gas production trends when hydraulic fracturing fluids contain mixed charge surfactant additives.