ENHANCED COAL BED METHANE RECOVERY WITH CO2 SEQUESTRATION Report Number PH3/3 August 1999 This document has been prepared for the Executive Committee of the Programme. It is not a publication of the Operating Agent, International Energy Agency or its Secretariat. Title: Enhanced Coal Bed Methane Recovery with CO2 Sequestration Reference number: PH3/3 Date issued: August 1998 Other remarks: Background to the Study This study considers the global potential for using CO2 to enhance coalbed methane recovery, with simultaneous CO2 sequestration in the coal. Field tests indicate that carbon dioxide can be injected successfully into deep unminable coal seams and achieve these twin objectives. This process, called CO2 enhanced coalbed methane recovery (CO2-ECBM), has the potential to sequester large volumes of carbon dioxide while improving the efficiency and profitability of gas recovery. This report defines the criteria which can be used to classify coal fields in terms of their potential for coalbed methane production and the extent to which that production would be enhanced using CO2 injection to enhance methane recovery. It assesses the geological properties of coalbeds which will determine their ability to release methane and adsorb CO2. It catalogues the world’s major coal areas in terms of their potential for enhanced methane recovery using CO2. From this and from previous analyses of the potential global capacity for coalbed methane production, an assessment is made of the global potential for enhanced gas recovery (EGR) using CO2 and for the carbon sequestration potential of the process. Also, the study assesses the broad economics of enhanced recovery of coalbed methane for a range of carbon dioxide storage costs. As with previous studies by the IEAGHG covering sequestration, the CO2 is delivered to site and power generation and capture are not considered. The study was carried out by Advanced Resources International, Inc. from Arlington, Virginia, USA. Results and Discussion The following areas are described in the report • Technical background • American and Canadian demonstrations • Burlington Resources CO2 - ECBM pilot • Sources of carbon dioxide • Reservoir screening criteria for CO2 - ECBM • Costs and sequestration potential for CO2-ECBM Technical Background Coal forms by the compaction of plant material. Gases, including methane, are generated during this process and are either adsorbed onto the coal surface or are dispersed into the pore spaces around the coal seam. The amount of gas formed depends on the rank of the coal. In addition, the maturation process releases large amounts of water so that the coalbeds formed are often water saturated. The surface area of the coal on which the methane is adsorbed is very large (20-200 m2/g) and, if saturated, i coalbed methane reservoirs can have five times the amount of gas as that contained in a conventional sandstone gas reservoir of comparable size. A number of patents have been issued during the past twenty years relating to the process of injecting carbon dioxide into methane-bearing coal seams. Each of these patents is based on the principle that CO2 adsorbs more readily onto the coal matrix compared to methane. Injected CO2 is preferentially adsorbed (and remains sequestered within the seam) at the expense of the coalbed methane, which is simultaneously desorbed and thus can be recovered as free gas. Because laboratory isotherm measurements demonstrate that coal can adsorb roughly twice as much CO2 by volume as methane, the working assumption is that the ECBM process stores 2 moles of CO2 for every 1 mole of CH4 desorbed. However, the physical chemistry of this process has not yet been fully defined, and there remains the possibility that there are other physical processes active within the reservoir which could alter this ratio. Coalbed methane (CBM) is conventionally recovered by means of reservoir pressure depletion which is a simple, but inefficient, process recovering typically only about 50% of the gas in place. Hydraulic fracture stimulation is used to assist recovery but, even so, because permeability is normally low, many well drillings are required to achieve adequate gas flow. New technologies have been proposed for enhanced coalbed methane recovery (ECBM) to recover a larger fraction of gas in place. The two principle variants of ECBM are • inert gas stripping using nitrogen injection and • displacement desorption employing carbon dioxide injection. Simulation and early demonstration projects by Amoco indicate that nitrogen injection ECBM is capable of recovering 90% or more of gas in place (N2-ECBM works using a different physical process - by lowering the partial pressure of methane to promote desorption). The CO2-ECBM process is less well documented but likewise shows significant promise for enhanced coalbed methane recovery. For the past three years, Burlington Resources along with their partner Amoco have been operating a 13-well CO2-ECBM demonstration in the San Juan Basin in the Southwestern United States. Initial results show improvement in methane recovery in some wells with minimal breakthrough of CO2. American and Canadian demonstrations Substantial CBM activity has taken place in the US over the last 15 years with current annual CBM production of 14 Mt CH4, 90% of which comes from the San Juan Basin in Colorado and the Black Warrior basin in Alabama. Estimates of CBM resources in the US range from 6-13 Gt CH4 and Canadian resources are of the same order of magnitude. Except for an experimental demonstration scheme in the USA (see below), all CBM production in the world uses primary, natural drive technology which is capable, at current economics, of recovering only a fraction of the potential CBM resource. However, development of proven US gas recovery systems, to incorporate ECBM techniques, is a clear route forward. An economic analysis and ‘proof of concept’ study is currently taking place of ECBM in Alberta, Canada; this involves use of CO2 both for more efficient release of methane and also as a means of sequestering CO2. A practical research project has been established under the Implementing Agreement of the IEA Greenhouse Gas R&D Programme to enable international collaboration to support this work. Burlington Resources CO2 ECBM demonstration Burlington Resource's Allison Unit field contains the world's first (and to date only) experimental ii CO2-ECBM recovery demonstration. The Allison Unit is located within the northern portion of the San Juan Basin, in northern New Mexico close to the Colorado border (Figure 1). The San Juan Basin is by far the most prolific coalbed methane development, currently accounting for over 75% of total worldwide CBM production and nearly 5% of the USA’s natural gas production. It is also the most thoroughly studied from a reservoir standpoint. Prior to CO2 injection, the Allison Unit had been considered a sub-average performer, with gas production rates less than half that of San Juan Basin Fairway wells but it was still economically viable. Another reason for selecting the demonstration location was its proximity to a major carbon dioxide pipeline that crosses the basin (Figure 1). Figure 1. Location of the Burlington Resources CO2ECBM demonstration in the San Juan Basin, USA. The Allison Unit demonstration comprises four CO2-injection wells and nine methane production wells. The report suggests that the enhanced production achieved by Burlington, in its best well, is illustrative of "best practice" CO2-ECBM, at least during the current preliminary development of this technology. Future R&D and operational experience should be expected to lead to further improvements in recovery. CO2 Sources A variety of CO2 sources, both natural and anthropogenic, may be used for CO2-ECBM recovery operations. Naturally occurring, high-pressure CO2 from underground reservoirs is currently the lowest cost source. The Allison demonstration utilises naturally occurring CO2 produced at the McElmo Dome field in southwestern Colorado and transported by pipeline across the San Juan Basin 3 to the Permian Basin of West Texas. Delivered cost is approximately $0.012/m ($0.50/Mcf) of CO2. A second option is to utilise anthropogenic sources of CO2that currently are being vented to the atmosphere. For example, in the San Juan Fairway, the natural CO2 concentration of produced coal 3 seam gas is 6 to 12%. Approximately 4.3 million m /day (150MMcfd) of CO2 is currently separated from produced CBM and vented to enable the gas to meet pipeline specifications in the San Juan Basin. Finally, and of particular relevance to the control of potential greenhouse gas emissions, industrial CO2 may also be used as injectant in ECBM operations. Potential industrial CO2 sources include primarily coal- or gas-fired power plants and other large industrial plants. Industrial CO2 is not readily available in the San Juan Basin, but could be a viable source in other coalbed methane basins. Unlike relatively pure natural formation CO2 sources, however, industrial emissions require processing to remove water, SOx, and other constituents. Industrial CO2 also requires compression. Nevertheless, iii potential future restrictions on emissions could make industrial CO2 a cost-effective source. For example, an industrial emitter may find it economically attractive to pay a CBM operator to sequester CO2. Under this scenario, handling and disposing of CO2 injectant could actually became a revenue stream for a CBM operation, rather just than a cost. Reservoir Screening Criteria Reservoir screening criteria are essential for locating favorable areas for the successful application of CO2-ECBM. This study has developed a preliminary list of reservoir characteristics that are likely to be important for CO2-ECBM application. These are: • Homogeneous Reservoir: The coal seam reservoir(s) should be laterally continuous and vertically isolated from surrounding strata. • Simple Structure: The reservoir should be minimally faulted and folded. • Adequate Permeability: Although a minimum permeability cannot be specified, preliminary simulation indicates that at least moderate permeability is necessary for effective ECBM (1 to 5 mDarcy).