A Comparative Experimental Study of Huff-n-Puff Gas Injection and Surfactant Treatment in Shale Gas-Condensate Cores

Experimental and simulation studies have thus far demonstrated that gas injection is an effective way of recovering condensate in shale gas-condensate reservoirs. It is economically feasible and relatively easier to be injected into the tight reservoir as compared to most of the other injection fluids. To the best of our knowledge, surfactant treatment has not been tested in shale cores for gas-condensate fluid systems. In this study, the experimental performances of huff-n-puff gas injection and surfactant treatment are compared in recovering condensate from shale cores. A nonionic fluorocarbon surfactant is screened for experiments based on dynamic contact angle measurements for suitable use in core flooding experiments. The principle of fluorocarbon surfactants is to alter the core from liquid wetting to favorably gas-wetting condition, thus reducing the average condensate saturation trapped in the core. First, an ideal experiment is used to set up two-phase gas and oil flow similar to a gas-condensate reservoir. In this ideal experiment, methane is flooded through surfactant treated and untreated shale cores, which are both initially saturated with n-decane. The oil saturation thus established was reduced by 28% in the surfactant treated core, as compared to the untreated core. On the basis of the success of surfactant treatment in the ideal experiment, we designed realistic experiments for a gas-condensate system in a shale core. In these experiments, the shale core is flooded with a gas-condensate mixture to mimic the dynamic flow of gas-condensate fluid in the reservoir. The performance of surfactant treatment is compared with huff-n-puff gas injection to recover accumulated condensate from different zones mimicking those in the reservoir ranging from high condensate build-up zones (fracture and near fracture regions) to low condensate build-up zones (matrix) occurring further into the reservoir. Results indicate that wettability alteration of the core achieved due to surfactant treatment is capable of enhancing recovery only from the zones where the developed condensate saturation high. A large volume of treatment solution is required for the wettability alteration to be effective in the shale cores, but it is difficult because the penetration is shallow. Also, the effectiveness of injected surfactant is not found to be long lasting. This could considerably increase the cost of surfactant application in the field if frequent treatments are required. The main problem of surfactant treatment is observed to be the difficulty of injection and flowback of treatment solution in the tight shale cores and thus limits its application to the high permeability fracture network of a reservoir. However, a large pore volume of the reservoir consists of ultralow permeability nanopores, which will have a considerable volume of trapped condensate as the reservoir pressure is depleted. In comparison, huff-n-puff gas injection proved to be more efficient in terms of higher recovery in all of the studied zones, relatively easier penetration of gas into the core, and shorter recovery time.