CO2 injectivity into brine aquifers: why relative permeability matters as much as absolute permeability

For economic reasons operators of geologic storage projects are likely to inject CO2 at the largest possible rates into the smallest number of wells. Thus a typical CO2 injection well is likely to run at the largest bottomhole pressure that is safe. Operators will also tend to prefer thicker, higher permeability target formations. However, a constant-pressure well exhibits a varying rate of CO2 injection for two reasons: classical multiphase flow effects, and long-term injection of CO2 removes water from the near-wellbore region. Drying precipitates dissolved salts, so the permeability of the dry rock need not equal the initial aquifer permeability. Mobility of CO2 in the dried rock and mobility of CO2 and brine the two-phase flow region determine the variation of injectivity with volume of CO2 injected. We find a four-fold variation in injectivity when seven different CO2/brine relative permeability curves (Bennion and Bachu [1]) are used, holding all other reservoir parameters the same. Since the product of formation permeability and formation thickness is relatively easy to measure, once a well has been drilled, uncertainty in relative permeability will therefore be a large contribution to uncertainty in achievable rates in CO2 storage projects. We develop analytical expressions for the injectivity variation in terms of phase mobilities and the speeds of saturation fronts. Classical theory (Buckley-Leverett) does not account for the drying front; using only Buckley-Leverett yields both quantitative and qualitative errors. The expressions are consistent with detailed reservoir simulations using commercial software (CMG's GEM) that account for the full physics and complete phase behavior. The expressions can refine the estimated number of wells needed for a target overall injection rate. This analysis also enables an operator to assess the value of retrieving core and measuring relative permeability in a prospective storage target. (C) 2008 Elsevier Ltd. All rights reserved