Summary
Increases in carbon dioxide (CO2) levels in the atmosphere and their contributions to global climate change are a major concern. CO2 sequestration in unmineable coals may be a very attractive option, for economic as well as environmental reasons, if a combination of enhanced-coalbed-methane (ECBM) production and tax incentives becomes sufficiently favorable compared to the costs of capture, transport, and injection of CO2.
Darcy flow through cleats is an important transport mechanism in coal. Cleat compression and permeability changes caused by gas sorption/desorption, changes of effective stress, and matrix swelling and shrinkage introduce a high level of complexity into the feasibility of a coal sequestration project. The economic effects of CO2-induced swelling on permeabilities and injectivities has received little (if any) detailed attention.
CO2 and methane (CH4) have different swelling effects on coal. In this work, the Palmer-Mansoori model for coal shrinkage and permeability increases during primary methane production was rewritten to also account for coal swelling caused by CO2 sorption. The generalized model was added to a compositional, dual-porosity coalbed-methane reservoir simulator for primary (CBM) and ECBM production. A standard five-spot of vertical wells and representative coal properties for Appalachian coals was used (Rogers 1994). Simulations and sensitivity analyses were performed with the modified simulator for nine different parameters, including coal seam and operational parameters and economic criteria. The coal properties and operating parameters that were varied included Young's modulus, Poisson's ratio, cleat porosity, and injection pressure. The economic variables included CH4 price, CO2 cost, CO2 credit, water disposal cost, and interest rate. Net-present-value (NPV) analyses of the simulation results included profits resulting from CH4 production and potential incentives for sequestered CO2. This work shows that for some coal seams, the combination of compressibility, cleat porosity, and shrinkage/swelling of the coal may have a significant impact on project economics.
Introduction
In recent years, primary production of natural gas from coal seams has become an important source of energy in the United States. Proven CBM reserves have been estimated at 18.5 Tscf, representing 10% of the total natural-gas reserves in the United States. CBM production started in the early 1980s as a small, high-cost operation but reached 1.6 Tscf in 2002. This was more than 8% of the total US natural-gas production that year (Kuuskraa 2003).
The production of CBM reservoirs begins with the pumping of significant volumes of water to lower reservoir pressure and to allow CH4desorption and flow (Stevens et al. 1998). The fraction of the original gas in place typically produced by primary depletion seems to be somewhat controversial. However, according to recent publications, recoveries are often between 20 and 60% of the original gas in place, so that considerable amounts of gas are left behind (Gale and Freund 2001; Stevens et al. 1999; Van Bergen et al. 2001).Because of this, and because of concerns about global warming caused by accumulations of CO2 in the atmosphere (National Energy Technology Laboratory 2003, 2004), new technologies for ECBM production based on the injection of carbon are being investigated in the US, Europe, China, and Japan (Coal-Seq Forum 2004, 2006).
In the CO2-ECBM/sequestration process, injected CO2 flows through the cleats in the coal by Darcy flow, diffuses into the coal matrix, and is sorbed by it; CH4 diffuses from the matrix into the cleats, through which it flows to production wells (Sams et al. 2005). The injection of CO2 into coalbeds has many potential advantages: It sequesters CO2, it reduces the production time for CBM, and it increases reserves by improving the recovery of CBM. However, the improved CH4 recovery is accompanied by an increase in costs for CO2 supply, additional drilling, and well and surface equipment. Thus, CO2-ECBM and sequestration are accompanied not only by promised benefits but also by new technical challenges and financial risks.
The Role of Coal Mechanical Characteristics on Reservoir Permeability Evolution and Its Effects on CO2 Sequestration and Enhanced Coalbed Methane Recovery
