A flow assurance evaluation of gas storage in depleted reservoirs

Global demand for gas is seasonal, mainly due to fluctuations in its demand at different times of the year. Due to this seasonal variation in demand, gas is often stored during the off-peak period and produced during the peak period.

However, one of the major challenges associated with gas storage in depleted reservoirs is salt precipitation around the wellbore. The dry gas in contact with connate water in the reservoir causes some of the connate water to evaporate to satisfy thermodynamic equilibrium and as a result, salt precipitates when the solubility product of the dissolved ions is exceeded. This deposition of salt potentially leads to pore throats plugging, hence a reduction in the original properties of the reservoir such as porosity, permeability etc over time.

Conventional reservoir simulators do not take this dynamic change in the reservoir properties into account, which leads to inaccurate estimation of the reservoir performance over time. This also results in wrong estimation of the amount of chemical inhibitors required to mitigate flow assurance problems such as gas hydrates, corrosion, scales etc.

This study shows that these changes in the reservoir parameters have an effect on the prediction of conventional models. Hence, reservoir performance should be monitored during the entire field life and the injection/ production data obtained from one cycle should be used to re-define the main reservoir parameters and refine the model for successive cycles.

This study proposes a method of modelling the reservoir performance by taking into account the changes in reservoir and fluid properties over time. It also proposes that in the long term, commercial simulators should be integrated to account for changes in reservoir and fluid properties over time in order to obtain better predictions of reservoir performance.