Technical solutions, lifecycle phases and prospect development are the keys to unlocking CBM potential.

Globally, coal is estimated to contain from 2,976 Tcf to 12,640 Tcf of methane. Methane gas adsorbed onto and within the coal macerals desorbs through a complex flow path of micro-pores and cleats. This gas migration is caused by diffusion at various scales, depending on the degree of interconnection of the flow paths. To extract the methane, a concentration gradient must be created within the coal by removing free water or gas from the cleat system through reducing reservoir pressure.

Conventional concepts can quantify reservoir mechanics after gas is released from the coal matrix, but coalbed methane (CBM) projects require virtual-based evaluations that are performed earlier and are more thorough than conventional projects. Therefore, CBM technologies, when viewed from a development lifecycle perspective, must depart from a conventional oil and gas approach. Because CBM prospectors rarely understand the controlling factors that make a good or bad well, a regional view and multiwell approach must be adopted early in the program so the economically viable acreage can be identified as early as possible.

CBM prospects are classic technology plays. Such plays require innovative applications of enabling technologies to allow prospectors and operators to reduce cycle time before first production. Technology also can enable screening of potential projects, add value to pre-production knowledge gathering, and validate economic forecasts that often must project further into the future than those of conventional assets.

Lifecycle concept

Five distinct lifecycle phases can occur simultaneously within many contiguous acres of coal rights. For example, some areas may be mature or declining, while other areas have not yet been evaluated for production potential.

Phase 1: Regional resource reconnaissance. An operator determines whether a property has adequate production potential to justify acquisition and exploration.
Phase 2: Local asset evaluation. Evaluations are conducted to determine whether a specific area should be exploited and the most economic development methodology for exploiting it.
Phase 3: Early development. This phase consists of initiating development drilling in potential areas and attaining targeted project production. This stage is critical for capital investment.
Phase 4: Mature development. This phase involves maintaining project production/economic targets through the development of marginal areas, infill drilling and remediation.
Phase 5: Declining production. This phase can result in secondary recovery efforts as a means of extending economic viability. Declining production requires plugging unproductive wells, removing equipment, and restoring the site while attempting to maintain a positive cash flow.

"Top ten" enabling technologies and matching to lifecycle phase

Ten specific enabling technologies have been shown to offer the best chance for CBM projects to reach their full lifecycle potential and help monetize gas assets. Some are suited to a discrete phase of the lifecycle while others span multiple lifecycle phases.

Geospatial well-pattern optimization. This requires an understanding of CBM production mechanics and reservoir simulation for production and economic forecasting. Optimized well patterns are determined by reservoir characteristics, completion effectiveness, well-stimulation effects, drilling and completion costs, operating costs and outside factors. For minimal cost, virtual simulation permits economic assessment, well-pattern comparisons and completion options for hundreds of virtual wells.

Core and core analysis. Scientific analysis of coal core can be critical to the success of Phases 1 and 2. Calculating gas in place from direct core measurements is a major first step in assessing methane gas reserves trapped in the coal. Gas content determination is largely independent of the core porosity and permeability, but is a function of methane adsorption within the coal macerals. Although gas content estimates using assumed isotherms, reservoir pressures, and gross seam thickness are appropriate during early prospect evaluation, they must be validated through direct-core measurement in Phase 2. A complete coal seam "anatomy" can be obtained and applied to the macro-scale reservoir.

Well logging. One of the greatest technical breakthroughs in CBM well logging involves log-processing methods and core-log integration. As one example, electric micro imaging provides the closest thing to a continuous core as currently possible. It can be integrated with the whole core so that grayscale levels can be correlated to discrete lithology. Such integration can be performed in one well and applied across the field or basin that lacks core information.

Cleat permeability determination. Three primary technologies are available for cleat permeability determination, predominantly in Phases 2 and 3. These include openhole discrete-seam drillstem testing (DST) applied to high-grade seams during the corehole process; interference testing and injection fall-off to enable the acquisition of far-field cleat permeability anisotropy; and G-function derivative analysis.
Reservoir engineering software tools. Coal-reservoir engineering has two essential software tools: gas-in-place calculations and reservoir-simulation models for CBM. Reservoir simulation utility is directly proportional to model capability and data collected during Phases 1 and 2. Entering incorrect input data, not using coal-specific simulation models and ignoring certain parameters can result in erroneous simulation results.

Prefracture diagnostics. To acquire data for a 3-D simulator and determine optimal hydraulic fracturing parameters, small injection/breakdown tests, stepdown-rate tests, minifrac tests, and "feeler" slugs of proppant slurry are part of prefracture diagnostics. These tools are most applicable on initial pilot production wells to estimate the stress level, leakoff mechanisms and values, and to determine causes of near-wellbore pressure restrictions.

Primary hydraulic fracture stimulation. Globally, hydraulic fracturing remains the most effective technology for enhancing commercial production rates of CBM reservoirs. In the early phases, an optimal fracturing system is critical for acquiring valid gas and water producibility data for subsequent reservoir simulation analysis. Phase 3 fracture design simulation can be used to match the outcome of the Phase 4 treatments and provide dependable predictions for future post-fracture treatment production.

Multiseam coiled-tubing hydraulic fracturing. Another significant enabling technology involves hydraulic-fracturing multiseam completions where individual discrete seams can be treated and flow-tested in a cost-effective, rapid procedure. Coiled-tubing fracturing allows the placement of 300 to 1,000 sacks of proppant per coal seam. The fracturing tool can be relocated and reversed clean within 30 min. Pump time is approximately one hour per seam, and up to seven stages have been treated in a single day. Treatment pressures of up to 7,500 psi can be achieved with current coiled-tubing technology.

Secondary production enhancement. During the final two lifecycle phases, technologies focusing on secondary production enhancement are increasingly important for extending the life of the CBM field. Technologies enabling the extension of Phases 4 and 5 include the following:
• hydraulic refracturing of previously fractured wells, including proppant tackifiers;
• hydraulic fracturing of previously cavity-completed wells; and
• chemical-enhancement processes designed to mitigate specific water or methane production impairment mechanisms.

Infill drilling. Infill drilling is a proven method for extending Phases 4 and 5. Because infrastructure investments have been capitalized, combining this approach with secondary production-enhancement methods offers a solution to stemming field-wide production declines. Reservoir modeling and history-matching cumulative production can help identify infill-drilling candidates and, when combined with geospatial well-pattern optimization, can add value to the CBM asset.

Environmental technologies

Until recently, environmental issues and enabling technologies for CBM have been mostly ignored in the early phases of a project. When environmental issues are identified and anticipated throughout all phases of a CBM project, the probability of success is enhanced. For example, in exploration-oriented prospects, identifying water-production issues in Phases 1 and 2 can allow operators to evaluate water-management options up-front. Additionally, produced water rates, volumes, composition, and disposal should be tested. If these variables are underestimated during economic analysis, an otherwise viable CBM project can fail. Environmental technologies are rapidly becoming critical for CBM throughout all lifecycle phases.

Conclusions

The many enabling technologies developed in recent years can help make evaluating the potential of CBM plays more efficient and reliable. The top ten enabling technologies described may be the most viable technologies for providing accurate insight and optimum asset management to prospectors, developers, engineers, consultants, and investors. Using the proper enabling technology in the proper phase can provide the information necessary to successfully develop a CBM project.
Outside of the United States, most CBM reserves remain unproven or have not yet reached early development status. Proper application of the key technologies and strategies presented in this article may help global CBM projects reach commercial development or may allow evaluation of potential projects with a higher degree of certainty.

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