Impact of Mixed Gas Stream on CO2 Plume Characteristics during and after Carbon Storage Operations in Saline Aquifers

The goal of this short study was to explain the effects of CO2 stream impurities (CH4 and N2) on (1) plume spread, (2) rate and extent of major trapping mechanisms, (3) CO2 storage capacity, and (4) well injectivity. The injection-stream base case consists of a 95% CO2 stream with 2.5% CH4 and N2. We varied the CO2 fraction from 75% to 100% (on a mole basis), defining three bounding cases: CO2BC, CH4BC, and N2BC containing 100% CO2, 75% CO2 and 25% CH4, and 75% CO2 and 25% N2, respectively. In a parametric study of the stream composition, we defined a simple generic reservoir with a uniform permeability of 300 md, a dip of 2°, and porosity of 25%. The model contains 120 300-ft-long cells in the dip direction and also includes four baffles with no permeability parallel to its top and bottom. The gas was injected for 30 years at a depth of about 6,000 ft and at a rate of 26 MMSCFD (equivalent to 0.5 Mt/yr of pure CO2) in a single well located in the downdip section of the model and perforated in the lower third of the 1,000-ft thickness of the injection formation. Temperature is constant at 135°F. Results are numerically monitored for 1,000 yr after start of injection. The modeling was done using CMG-GEM software, and we used a user-defined set of PVT properties. A sensitivity analysis on important model parameters was also done to assess their importance relative to the parametric-study results. The study considers only the two trapping mechanisms (residual saturation and brine dissolution) largely impacted by injection-stream composition. Plume spread, or maximum extent, is a strong function of composition. The maximum extent ranges from 10,350 ft for CO2BC to more than twice the distance for CH4BC (22,250 ft) and N2BC (24,250 ft) and varies approximately linearly for intermediate values. Similarly, time for the plume to reach the top of the formation varies from 14 yr (N2BC) to 18 yr (CH4BC) to 60 yr (CO2BC). The main difference between gas components is solubility in brine—CO2 is approximately 10 times more soluble than CH4 and N2 on a mole basis. The buoyant driving force, expressed as the ratio of gas-brine density difference to gas viscosity, is also approximately four times higher in the CH4BC and N2BC cases, and the ratio keeps increasing because the fraction of CH4 and N2 increases as CO2 dissolves.