Optimizing Injection Intervals in Vertical and Horizontal Wells for CO2 Sequestration

Abstract

AbstractStorage in deep saline aquifers is one of most viable options for sequestering anthropogenic CO2. The choice of completion for injection wells -- fully or partially completed vertical well, length of horizontal well -- has significant effect on the eventual trapping of CO2 as well as its potential for leakage. Thus optimization of the completion for a target injection rate can help maximize trapping. The density difference between CO2 and in-situ brine influences the optimization because it causes the pressure difference between wellbore and formation to vary with depth in a CO2 injection well. For a long completed interval in a vertical well, the flux distribution is therefore not uniform. Reinforced by relative permeability effects, this can cause deeper perforations to contribute less to the injection rate. In extreme cases only a fraction of the perforations contribute to injection. The degree of non-uniformity depends on the injection rate. The nonuniformity in turn reduces residual phase trapping because less volume of rock and brine comes in contact with CO2.Field scale compositional reservoir simulations are carried out to study the plume behavior and CO2 trapping under different injection strategies. A semi-analytical algorithm is discussed to determine the optimum interval of injection. For horizontal wells two competing effects, trapping along the well length vs. trapping along the transverse direction, determine the optimum well length. These effects can be correlated to the dimensionless gravity number during injection. Injection at bottom of aquifer results in better trapping and lower leakage potential. For a given rate, the perforation interval can be calculated so that all the perforations contribute throughout the injection period. In a horizontal well, longer well length increases the trapping in direction of well path but reduces trapping in the transverse direction. Consequently, the benefits of a strategy to maximize injectivity may be offset by the disadvantage of less CO2 entering secure modes of storage.