Abstract

Previous economic studies of the recovery and utilization of methane from coalbeds using vertical wells were based on drainage in advance of mining where a single seam is drained with well spacing designed for rapid predrainage. A production economics study has been conducted which shows that the lower overall investment costs for wider spacing and increased production from multiple seams result in gas recovery at costs well below current prices for new gas imports. When applied to some of the thicker, deeper Western coals, the method also results in a favorable ROI.

Introduction

A study conducted by TRW in 1977 for the U.S. Department of Energy's Morgantown Energy Technology Center (METC) examined the recovery of the methane contained in coalbeds using various extraction techniques and utilization options. The extraction options considered stimulated vertical wells and horizontal wells drilled into the coalbeds from within the mine or directionally drilled from the surface. The holes drilled from the mine considered both holes drilled immediately in advance of mining and from a shaft sunk into the mine several years in advance of when it was required for mine operations.

The economics of vertical wells to extract methane in advance of mining did not compare favorably with in-mine drilling, principally due to the higher costs of the wells and the close well spacing typical of the short time desirable for drainage for mine safety purposes (approximately 3–5 years). The wells were typically spaced at 20 acres or less and drained only the coalbed associated with the mining. For typical installations associated with mine predrainage the required gas selling price for a predrainage the required gas selling price for a favorable return on investment was 2–3 times the price for horizontal wells and was in excess of the market price for the gas by 30 to 100 percent. price for the gas by 30 to 100 percent. This paper extends the earlier work to the economics of multiple vertical wells, each producing from several coalbeds with spacing producing from several coalbeds with spacing based solely on economic effectiveness. Several utilization options are also considered to provide a more complete picture of the economics of this widely applicable recovery technique for this marginal resource.

RESOURCE CHARACTERIZATION

The specific methane content of a particular coalbed can generally be related to the type of coal, the depth, the geological structure and the geographical area. While the coal rank and the depth are the major factors, the structure in which the coalbed lies can provide paths of migration which result in loss of the gas throughout crops or along faults. The geographical variance in methane content suggests that the depositional environment may have affected the original formation and retention of the methane. The methane content has been measured directly in many coalbeds in the eastern U.S. and determined from indirect measurements from mine ventilation data. The wide variation in methane content is shown in Figure 1 for selected coalbeds. The primary factors considered in this study are the amount of gas in place in the coalbeds and the depth of vertical wells to extract the gas.

A second resource characteristic to be considered in the effectiveness of vertical wells is the number of coalbeds that occur with a single well. As shown in Figure 2, while the gas in place can be treated as the total for all the place can be treated as the total for all the coalbeds in a single well, the number of separate zones directly affects the completion and stimulation costs. Similarly, the very thick coalbeds which occur in the West may have to be treated as multiple zones for the purpose of stimulation and completion and their attendant costs. Since the production of the methane is permeability limited, production of the methane is permeability limited, multiple zones could provide a higher flow rate than if all the gas was contained in a single coalbed with high specific gas content.

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