THE ALLISON UNIT CO{sub 2}-ECBM PILOT: A RESERVOIR MODELING STUDY

In October, 2000, the United States Department of Energy, through contractor Advanced Resources International (ARI), launched a multi-year government-industry research & development collaboration called the Coal-Seq project. The Coal-Seq project is investigating the feasibility of carbon dioxide (CO{sub 2}) sequestration in deep, unmineable coalseams by performing detailed reservoir studies of two enhanced coalbed methane (ECBM) recovery field projects in the San Juan basin. The two sites are the Allison Unit, operated by Burlington Resources, and into which CO{sub 2} is being injected, and the Tiffany Unit, operating by BP America, into which nitrogen (N{sub 2}) is being injected (the interest in understanding the N{sub 2}-ECBM process has important implications for CO{sub 2} sequestration via flue-gas injection). The purposes of the field studies are to understand the reservoir mechanisms of CO{sub 2} and N{sub 2} injection into coalseams, demonstrate the practical effectiveness of the ECBM and sequestration processes, demonstrate an engineering capability to model them, and to evaluate sequestration economics. In support of these efforts, laboratory and theoretical studies are also being performed to understand and model multicomponent isotherm behavior, and coal permeability changes due to swelling with CO{sub 2} injection. This report describes the results of an important component of the overall project, the Allison Unit reservoir study. The Allison Unit is located in the northern New Mexico portion of the prolific San Juan basin. The study area consists of 16 methane production wells, 4 CO{sub 2} injection wells, and one pressure observation well. The field originally began production in July 1989, and CO{sub 2} injection operations for ECBM purposes commenced in April, 1995. CO{sub 2} injection was suspended in August, 2001, to evaluate the results of the pilot. In this study, a detailed reservoir characterization of the field was developed, the field history was matched using the COMET2 reservoir simulator, and future field performance was forecast under various operating conditions. Based on the results of the study, the following conclusions have been drawn: (1) The injection of CO{sub 2} at the Allison Unit has resulted in incremental methane recovery over estimated ultimate primary recovery, in a proportion of approximately one volume of methane for every three volumes of CO{sub 2} injected. (2) The study area was successfully modeled with ARI's COMET2 model. However, aspects of the model remain uncertain, such as producing and injecting bottomhole pressures, CO{sub 2} content profiles of the produced gas, and the pressure at the observation well. (3) There appears to be clear evidence of significant coal permeability reduction with CO{sub 2} injection. This permeability reduction, and the associated impact on CO{sub 2} injectivity, compromised incremental methane recoveries and project economics. Finding ways to overcome and/or prevent this effect is therefore an important topic for future research. (4) From a CO{sub 2} sequestration standpoint, the incremental methane recoveries (based solely on the conditions encountered at the Allison Unit), can provide a meaningful offset to CO{sub 2} separation, capture and transportation costs, on the order of $2-5/ton of CO{sub 2}.

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