Semi-analytical model to determine perforation interval for secure CO2 storage in saline aquifers

AbstractDue to density difference between injected CO2 and in-situ brine, the pressure difference between wellbore and formation varies with depth in a CO2 injection well. Consequently the flux distribution along a vertical well is not uniform during the early stages of injection. For injection rates below a certain threshold, this can lead to only a fraction of the perforations contributing to injection. Generally this reduces the efficiency of CO2 immobilization by dissolution in brine and by residual trapping because less volume of rock and brine comes in contact with injected CO2. Thus for injection rates below the threshold, optimization of the length of the perforated interval is required to maximize trapping. We describe a semi-analytical algorithm that finds the optimum interval of injection for a given flow rate so that all the perforations contribute throughout the injection period. Although bottomhole pressure rises while injecting in smaller perforation interval, the greater mobility of the CO2 phase upstream of the drying front reduces this increase and enables the use of smaller interval. In the case of a horizontal well, the length of well plays an important role in determining the CO2 trapping. The two competing effects, trapping along the well length and along lateral direction, determine the optimum well length required. Greater well length increases the trapping in direction of well path but reduces in lateral direction because of the ratio of gravity forces to viscous forces becomes larger. Thus dominance of either of these competing effects and cost of drilling determine optimal well length. This study illustrates the effect of different injection strategies on multiple objectives of CO2 sequestration including maximizing trapping and minimizing leakage potential. We find that the benefits of a strategy to maximize injectivity may be offset by less CO2 entering secure modes of storage