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Keywords:

  • CO2 geologic sequestration;
  • deep saline aquifer;
  • numerical modeling;
  • multiphase flow;
  • methane;
  • temperature

Abstract

A large-scale carbon dioxide (CO2) injection pilot is ongoing at Cranfield, Mississippi, in a saline aquifer with high dissolved methane (CH4) content, employing one injection well and two observation wells. The breakthrough of CH4 and CO2 at the observation wells provides insights to phase partitioning and the multipath nature of flow through the formation. Injected CO2 is cooler than the formation temperature, making temperature another useful observation. Simulations of the first year of CO2 injection were conducted with the numerical simulator TOUGH2 and the equation of state module EOS7C, which includes CO2, CH4, and H2O, using an axisymmetric model with layering based on well logs from the injection well. Although the simplification of an axisymmetric model precludes study of formation dip or lateral heterogeneity, its simple structure enables a focus on physical processes involving the phase partitioning of CH4 and CO2, and temperature effects. Field observations that the model reproduces include the arrival of a bank of free-phase CH4 ahead of the main CO2 plume at each observation well, and non-monotonic changes in CH4 and CO2 mole fraction as a function of time, suggesting that multiple distinct flow paths exist between the injection well and the observation wells, each with its own bank of free-phase CH4 leading the CO2. Model results are compared with temperature observations made in the field with a Distributed Temperature Sensor (DTS) system, suggesting that a well-defined thermal response reached the near observation well within the seven-month monitoring period, but not the more distant observation well. © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd