Underground Gas Storage in a Partially Depleted Gas Reservoir

In this work, underground gas storage (UGS) was studied on a partially depleted gas reservoir through compositional simulation. Prediction of reservoir fluid phase behavior and history matching was done by utilizing detailed reservoir information. The performance of UGS with different scenarios of reservoir depletion, gas injection, and aquifer strength was analyzed. The injection capacity and deliverability of reservoir was set to 350 MMSCF/D (6 months) and 420 MMSCF/D (5 months), respectively. Based on different scenarios and the anticipated target rate, the optimum pressure for converting this reservoir to UGS was found to be about 1600 psia. Also, it was found that if the reservoir is depleted to a lower pressure, it contains insufficient base gas reserve and may not meet the target withdrawal rate. Results showed that this problem can be overcome by injecting higher volume of gas in the first cycle. Furthermore, it was shown that an active aquifer can lead to irreversible reservoir shrinkage, increase in water-gas ratio, and pressure rise in reservoir. Another source of pressure rise during the UGS operations is the difference between z-factors of injected and reservoir fluids. It was found that injecting lean gas with high z-factor into a reservoir containing fluid of lower z-factor results in pressure rise at the end of each cycle. At successive cycles, composition of reservoir fluid approaches that of the injected gas because of continual mixing. Theoretically, composition of reservoir fluid will be near the injected fluid after infinite cycles, provided complete mixing occurs in reservoir. Under these conditions, difference between z-factors of injected and reservoir fluids become smaller, and reservoir pressure stabilizes. Dans cet article, on étudie par simulation compositionnelle le stockage souterrain de gaz dans un réservoir de gaz partiellement déplété. La prédiction du comportement du fluide de réservoir et le calage d'historique ont été effectués en utilisant des informations détaillées de réservoir. La performance du stockage a été analysée avec différents scénarios de déplétion de réservoir, d'injection de gaz et de force d'aquifère. La capacité d'injection et la productivité de réservoir ont été respectivement fixées à 350 MMSCFD (6 mois) et 420 MMSCFD (5 mois). Sur la base de différents scénarios et d'un débit ciblé anticipé, la pression optimum pour convertir ce réservoir en un stockage souterrain a été évaluée à environ 1600 psia. En