Mechanisms of how surfactants mitigate formation damage due to aqueous phase trapping in tight gas sandstone formations

Abstract

The aqueous phase trapping (APT) (often occurs when water-based fluids are used for drilling, completion and stimulation operations) is known to lower the productivity of the tight gas sandstone reservoirs significantly. The use of surfactants is commonly recommended for mitigating the formation damage due to ubiquitous aqueous phase trapping in these reservoirs. However, mechanisms of how surfactants help to remove the formation damage due to ATP are not fully understood. In this study, therefore, we have investigated the mechanisms of the trapped aqueous phase removal from the tight gas sandstone formations by considering the effect of surfactants on the wettability alteration, the work of adhesion, and the apparent water film thickness.Theoretical analyses suggest that further the larger the contact angle that the drop of water creates with the reservoir rock surface, the less energy is required for producing gas to displace the water from the reservoir. Therefore, the first part of the study was devoted to determining the optimum surfactant type and concentration, which would yield to the maximum water/rock contact angle. Within the experimental conditions of this study, the maximum increase in the contact angle was obtained when 0.05 wt% cationic surfactant solution was used.Results from further experimental work have shown that the 0.05 wt% cationic surfactant solution was conducive to reduce the work of adhesion, the water saturation and the apparent water film thickness, thereby, effective to remove the aqueous phase trapping damage and regain the rock permeability. Results have also demonstrated that surfactants become more effective in removing the trapped aqueous phase as the drawdown pressure increases.