Experimental and analytic studies to model kinetics and mass transport of carbon dioxide sequstration in depleted carbonate reservoirs

Abstract

There is undeniable evidence that concentration of carbon dioxide in the atmosphere is rising at an increasingly rapid rate primarily as the result of burning fossil fuels. Although the debate continues, most of the scientific community believes that higher levels of atmospheric CO2 will lead to a significant warming of the Earth’s climate and that there is already evidence that this is occurring. There are two ways to ameliorate this problem. One is to significantly reduce production of CO2, which is primarily a political-economic problem, and the other is to remove CO2 from emissions and/or the atmosphere and find some way to sequester it. Several possible ways to sequester CO2 are under investigation or have been suggested. These include removal by chemical reaction, deep seabed disposal, and pumping supercritical CO2 into various subsurface environments. Sequestration of carbon dioxide in depleted gas reservoirs appears to be a viable option, with a possible economic spin-off from the recovery of significant gas reserves. At the elevated temperatures and pressures encountered in reservoirs, carbon dioxide behaves as a supercritical fluid. Under these conditions, little was known regarding the, diffusion of carbon dioxide in natural gas, and displacement of natural gas by carbon dioxide. A major objective of this research was to obtain the necessary data to model these processes. Also, the added CO2 will react with reservoir waters that are often chemically complex high ionic strength brines making them more acidic. This can result in the dissolution of calcium carbonate (calcite) that is a common host rock or sandstone cement in reservoirs and lead to potentially serious problems for CO2 injection and the integrity of the reservoir. It was consequently a second major objective of this project to determine calcite solubility and dissolution kinetics in solutions representative of subsurface brines and produce a general dissolution rate equation. Both objectives were accomplished. Reservoir simulations indicated a large amount of CO2 would be sequestered, with the amount depending on reservoir water saturation. Simulation results also indicate a significant amount of natural gas could be produced. For an 80-acre pattern, natural gas production was calculated to be 3.2 BSCF or 63% of remaining gas-in-place for 30% reservoir water saturation. Gas revenues would help defray the cost of CO2 sequestration. Therefore, CO2 sequestration in depleted gas reservoirs appears to be a win-win technology. Considerable effort went into testing and refining the ability to predict calcite solubility in brines using a Pitzer-equation based computer model, with particular difficulties being encountered in solutions with high dissolved calcium concentrations. After that was accomplished, calcite dissolution kinetics were determined a wide range of brine compositions both including and not including potential inhibitors from 25 to 83 oC and a CO2 partial pressure from 0.1 to 1 atm. The reaction was found to be first order for undersaturations of 0.2 to ~1 and was surface controlled. The rate constant was fit to a multiple regression model, thus making it possible to predict calcite dissolution rates over a wide range of solution compositions, partial pressures of CO2 and temperature. Results indicate that equilibrium is likely to be reached relatively quickly in front of an advancing supercritical CO2 fluid.