Coalbed-Methane Evaluation Techniques - The Current State of the Art

Abstract

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Summary Because of the complexity of coal reservoirs, formation-evaluation techniques are extremely important for determining the commercial viability of coalbed-methane prospects. This article focuses on the reservoir evaluation techniques that are used to assess prospects for coalbed-methane development. A variety of reservoir-evaluation methods specific to coal reservoirs have evolved over time, including core evaluation, well logging, seismic, transient-pressure testing, and production analysis. This article addresses current methods and newer, state-of-the-art methods that are under development and becoming more accepted for use in evaluating coal reservoirs. As Fig. 1 shows, coals are complex, naturally fractured reservoirs. Coalbed-methane production is characterized by a high degree of variability. This variability in well production exists when comparing wells within discrete production fields, across regions within producing basins, and between coal basins. Lateral changes in reservoir permeability appear to be the primary cause of well-to-well production variation, although changes in coal-seam thickness, sorbed gas-content, water saturation, and reservoir pressure also affect well productivity. Many coalbed-methane prospects are under evaluation in the U.S. and throughout the world. In the U.S., higher gas prices have accelerated exploration of prospects in areas that previously were thought to be uneconomical for development. These areas include relatively unexplored new basins as well as step-out areas in developed production areas. In addition, significant prospect evaluation is occurring in international areas, including Australia, China, India, and Europe. Objectives of the Reservoir Evaluation From the reservoir-evaluation standpoint, two main questions must be addressed: What is the resource in place? And what are the production characteristics of the reservoir? Before discussing these questions, it is worth noting that reservoir evaluation for coal reservoirs must be performed on a field-by-field basis to develop a proper understanding of the production characteristics and reserves for each area. This requirement results from the inherent variability of the coal reservoir, which is born out by historic producing trends in mature development areas, such as the Black Warrior basin in Alabama (Fig. 2)and the San Juan basin in Colorado and New Mexico (Fig. 3). Also, it is important to understand this variability when evaluating pilot-project or exploration-test data while attempting to evaluate the commerciality of prospective development areas. Because of the inherent variability in production, it is often difficult to evaluate commercial viability on the basis of a few random data points. Often, multiple exploration wellbores and/or pilot tests are needed to understand fully the expected average production and reserves for a prospective development area. Volume of the Gas Resource in Place. Estimating the resource in place depends upon the following reservoir-parameter determinations.Reservoir geometry (depth, thickness, lateral extent, number of seams, and structure).Gas content (both sorbed and free gas).Coal composition (chemical, maceral, and mineralogical). Three methods are used to determine these properties: well testing, core testing, and seismic. Well logging is primarily used to evaluate reservoir geometry and coal thickness. The basic logging suite for evaluating coal reservoirs includes gamma ray, density, resistivity, and caliper tools. Neutron logs also are run in many cases. The density log (especially when run in a high-resolution format) is most useful for evaluating reservoir thickness because of the large inherent differences in density between coals and the surrounding rock layers. However, similar results can be obtained by use of the appropriate resistivity and neutron logs. Importantly, the high-resolution density log can be evaluated to determine the volume of noncoal material(primarily mineral-matter) in the target coal seam. Resistivity tools can be useful in some cases for estimating relative differences in reservoir permeability in multiseam reservoirs because of fluid invasion into the naturally fractured coal reservoir. Image logs are used sometimes to assist in this evaluation of cleat and fracture characteristics. Newer state-of-the-artchemical logs are beginning to be used to identify basic chemical components in the coal reservoir, which then can be reconstructed to give a log-based coalcompositional analysis. When calibrated with core data, these tools can be used to evaluate the relative quality of multizone reservoirs and assist indesigning well completions. Importantly, these tools are permitting a more detailed analysis of coal seams present behind pipe in wells that had previously targeted other hydrocarbon reservoirs. Fig. 4 shows a samplewell-log evaluation.