Abstract

In the enhanced coalbed methane production (ECBM) option involving injection of CO2 in a coal-gas reservoir, the injected CO2 induces incremental swelling of the coal matrix. This is believed to result in a significant reduction in coal permeability, and thus the CO2 injectivity. This paper discusses the results of a study where swelling of the coal matrix was first measured with injection of methane and CO2. This was followed by injecting CO2 in the methane saturated sample, and measuring the incremental swelling for increasing CO2 concentration. The measured swelling with individual gases was then used to model the incremental swelling when CO2 is injected in a CBM reservoir.

The experimental results showed that the amount of swelling produced by CO2 is more than three times that produced by methane. The single-gas swelling data was fitted to a model based on the Langmuir Sorption Theory, and the agreement between the modeled and experimental data was excellent. Finally, the Extended Langmuir (EL) Theory was used to compute the incremental swelling for increasing concentration of CO2. However, the measured and modeled incremental swelling with increasing concentration of CO2 did not match very well. These results are in agreement with the reported finding that the EL Theory fails to accurately predict the sorption behavior of methane/CO2 mixtures. However, the technique serves as a means to estimate incremental swelling until improved methodology is developed to do so.

Introduction

Natural gas stored in coal seams, known as coalbed methane (CBM) has grown in importance as a source of energy worldwide. Production of CBM in the US has increased from almost zero in 1980 to 1,732 billion cubic ft (BCF) in 2005, accounting for almost 10% of US natural gas production.

The commercial CBM production occurs mostly via depleting the reservoir pressure. In this method, large volume of formation water is first pumped off from the CBM reservoir to decrease the reservoir pressure, and resulting in desorption of methane from coal (1). Although the method is simple, it is rather inefficient since the estimated total methane recovery amounts to around 50%. Consequently, operation of CBM wells from coal seams with low gas content becomes uneconomical, or marginally economical. Hence, enhanced CBM recovery techniques (ECBM) have been developed as a means to recover a larger portion of gas-in-place (GIP). Typically, these techniques involve injecting a second gas into the coal reservoir to enhance the process of recovery of methane stored in situ. Depending on whether the injected gas exhibits a greater or lesser sorptive affinity for coal than methane, the process can either be CO2-ECBM, where CO2 displaces methane from sorption sites within the coal matrix blocks, or N2-ECBM, where N2 strips it from the coal matrix by reducing the partial pressure of methane in the cleat system. However, the former option is more attractive as it offers the added benefit of storing large amounts of CO2 in coal, sequestering it permanently.

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