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. It is predicted that different surfactants in the hydraulic fracturing fluid play an important role in changing shale wettability and capillary pressure, eventually, enhancing natural gas production by influencing imbibed water to migrate away from the fracture face to the shale pores. To validate this hypothesis we have selected two commonly used surfactants in hydraulic fracturing fluids, cationic octadecyl trimethyl ammonium chloride (OTAC) and anionic ammonium dodecyl sulfate (ADS), and studied their role in altering the pore accessibility and water uptake rate in shale fractures using neutron imaging and small / ultra-small angle neutron scattering. Neutron imaging results demonstrate a substantial reduction in water uptake rate due to the synergistic interaction of the ADS/OTAC mixture into the shale fractures as the surfactant concentration is increased. The results also indicate that surface tension alteration due to surfactant addition plays a predominant role in controlling water uptake rates during initial injection, whereas wettability alteration via surfactant adsorption is more likely to impact water uptake rates after shut-in periods following initial fracturing completions. Further studies are underway to examine the change in frac fluid accessibility into nanometer range pores in shales exposed to varied concentrations of ADS/OTAC 1:1 mixtures. This SANS work will provide an improved understanding regarding the pore range impacted by hydraulic fracturing fluids and the role of surfactant additives in promoting natural gas production in shale reservoirs.<br/>

Effect of surfactant adsorption on pore accessibility and water uptake in organic-rich shales

253rd National Meeting (2017)

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