CO₂ sequestration into the Wyodak coal seam of Powder River Basin - preliminary reservoir characterization and simulation.

摘要

Injection of carbon dioxide captured from flue gas into coal beds is regarded as one of the value-added options of CO₂ sequestration as the cost of injection can be partially or fully offset by the revenue generated through release of additional methane. The Powder River Basin is one of the major coalbed methane producing areas in the world. The paper presents findings of a preliminary reservoir simulation study on the feasibility of CO₂ sequestration over a nine-section area (4.8 km x 4.8 km) of the Powder River Basin into the thick Wyodak coal seam, one of the two major coal seams in the highly productive Fort Union formation. The reservoir model was built on the basis of information available in the public domain. Gamma ray logs from 60 wells were utilized for developing a 3-D geological model of the coal seam and overlying rocks in the area by employing geostatistical techniques. Considerable variability in gas and water production was observed in the 65 wells. This variability was utilized for capturing the reservoir heterogeneity by Gaussian geostatistical simulation, which produced realizations of fracture porosity and permeability distribution throughout the reservoir. Results of fluid flow simulation indicated that it would not be feasible to place more than one injector per 1.6 km x 1.6 km (1 mile x 1 mile) section of the area due to geomechanical constraint. As a preliminary estimate, it may be feasible to inject 0.658 million tons of CO₂ through such injector over a period of 20 years. 12% more CO₂ can be injected over the same period by using a horizontal well but the loss of injectivity may be substantial due to reduction of permeability by coal matrix swelling. The loss of permeability can partially be overcome by intermittent injection for 6 months followed by a similar soak period. Placing one vertical injector each into all the nine sections would result in a scaled-up volume of 5.5 million tons of CO₂ injection. However, the nature of overlying rock could play a vital role in retention of injected CO₂ and up to 20% of the gas may migrate up by buoyancy.