Cushion and working gases mixing during underground gas storage: Role of fractures

Underground gas storage can help realize sustainable energy supply. This process is based on two main elements, namely cushion gas (CG), which serves to retain the reservoir pressure, and the working gas (WG). In a depleted hydrocarbon reservoir, the required base gas constitutes a large portion of total stored gas. Given the high cost of the base gas for a gas storage project, attempts have been made to reduce this cost by replacing some of the base gas by some less expensive alternative. The main challenge arising from such a replacement is that the CG and WG may mix together, with the mixture exhibiting a lower heat value (HV). In this process, reservoir heterogeneity and fractures, in particular, affect the mixing of the two gases significantly. In this research, the effect of fractures and relevant mechanisms in the replacement of the CG and its mixing with the WG in natural gas storage were numerically simulated. The presence of fracture in the reservoir was found to result in three different behaviors in three different time windows. During early days of production, the fractured reservoir produces a gas of a higher HV than that retrieved from the conventional reservoir. Continuing the production process, however, sets the scene for ending up with a higher HV for the gas retrieved from the conventional reservoir rather than the fractured reservoir. The presence of fractures contributes to the sweeping phenomenon in early production stages but rather boosts the gas mixing effect (GME) in the late production stages, while the sequence is reversed when a conventional reservoir is concerned. A comparison between the CO2 and N2 as alternative CGs showed that the N2 tended to disperse in more layers of the reservoir in presence of fractures, resulting in higher degrees of GME across the reservoir.