Flow assurance challenges in gas storage schemes in depleted reservoirs

Gas storage and subsequent production is of great interest due to fluctuations in gas demand and the difficulty for large scale gas storage in surface facilities. However, the injection and production of dry gas into or from a depleted gas reservoir could result in serious flow assurance challenges. The injected gas will evaporate connate/formation water to satisfy thermodynamic equilibrium. This will also happen during production phase and may cause salt precipitation, and potentially pore throats plugging. In this paper we quantitatively describe and discuss the parameters involved in water evaporation/production and salt precipitation for a gas production/injection well in a field. In this work, the impact in terms of formation damage (skin) will be evaluated and some recommendations for prediction and mitigation will be proposed. In addition, the water present in the produced gas is a major flow assurance threat due to possibility of gas hydrate formation in the production system. Origin, description, prevention and mitigation methods are discussed and compared. The results show that an increase in the salinity of formation water (due to production) will result in a decrease in the water vapour pressure, hence water vaporization, hence a decrease in the amount of hydrate inhibitor required for preventing hydrate formations in the system. Another important factor observed in this study is capillary pressure. A high capillary pressure will increase the gas/water transient zone and therefore there could be more water production from the well. In addition to water production problem we also observed that due to salt precipitation the porosity and permeability in the vicinity of the wellbore can be reduced by more than 50%. The effect of salt deposition and vaporised water production are potentially serious challenges in gas storage projects. It is necessary to carefully address these challenges in order to make gas storage projects practical and commercial.