Abstract

An experimental procedure for determining the effectiveness of CO2 injection into methane-containing coal samples for the purpose of enhancing methane production is described. Experimental results on production is described. Experimental results on both dry and water saturated samples reveal that CO2 injection greatly enhances both production rate and recover efficiency. Most effective is a cyclic CO2 injection-gas production technique which recovered essentially all of the adsorbed methane in the 3 1/2" core samples used in the experiments.

Introduction

The coal which lies buried beneath the United States at depths of less than 3000 feet is thought to contain nearly 300 trillion standard cubic feet of pipeline quality gas. This exceeds the current pipeline quality gas. This exceeds the current proved gas reserves in the U.S. and represents a proved gas reserves in the U.S. and represents a significant possible supplement to dwindling supplies. The natural gas found in virgin coal beds is predominantly methane, usually exceeding 80%. Very predominantly methane, usually exceeding 80%. Very small percentages of ethane, propane, butane and pentane have been detected. Carbon dioxide and pentane have been detected. Carbon dioxide and nitrogen may be as high as 15% in gas from virgin coal. Most of the gas present in coal beds is adsorbed on the coal surfaces and desorption is normally a very slow process.

In addition to the desirability of producing this gas as an energy source there is the added important advantage from a safety standpoint of demethanating a coal bed prior to mining. Attempts to produce the gas in coal through vertical well bores by pressure drawdown have generally not been commercially successful because of low production rates. Differing theories on the transport of gases through coal have been proposed by Cervik, Kissell, Skidmore and Chase and Kuuskraa, et. al to explain these low production rates. None of these has concluded that desorption rate is the mechanism which controls production rates from wells drilled into coal beds. production rates from wells drilled into coal beds. A key publication by Every and Delosso in 1972 showed that carbon dioxide proved to be very effective in displacing methane from crushed coal under laboratory imposed flow conditions at ambient temperature. This led to the proposal that competitive adsorption-desorption of methane by carbon dioxide might provide an efficient means for rapid degassification of coal y beds and thereby increased recovery rates of methane from vertical well bores. This paper describes a laboratory procedure for measuring the effectiveness of carbon dioxide in replacing methane from 3 1/2" diameter samples of Pittsburgh coal and also presents the experimental results.

EXPERIMENTAL PROCEDURE

The coal from the Pricetown mine in West Virginia was delivered in large lumps which were then stored under water until cored for use in the experiments.

The experimental apparatus is represented schematically in Figure 1 and pictorially in Figure 2. The pressure vessels used were 4 inch (10.16 cm) I.D. ant pressure vessels used were 4 inch (10.16 cm) I.D. ant 12 inches (30.48 cm) long. The vessels were designed for operating pressures of 100 psi (4.78 Pa) and 200 psi (9.56 Pa). The coal samples were cut to diameters of 3 1/2 or 3 3/4 inches (8.89 or 9.555 cm) and varying lengths between 2 to 4 inches (5.08 to 10.16 cm) and stacked in the vessels to a total height of approximately 11 1/2 inches (29.21 cm). The system was evacuated for several hours. Methane was expanded from a constant volume pressure cylinder into the vessel containing the coal resulting in methane adsorption on the coal surfaces. As methane was being adsorbed, the pressure in the system declined and the amount of pressure in the system declined and the amount of methane adsorbed was determined by material balance. At some arbitrarily selected time, usually six to eight days, the pressure in the system was noted, and the amount of adsorbed methane calculated. In some cases, in order to get a greater quantity of methane adsorbed, the pressure was increased and the process repeated several times. When the desired amount of methane had been adsorbed the excess gas in the vessel was vented to atmospheric pressure. At this point the natural desorption production cycle was begun and continued until no more gas was being produced, or was stopped after some arbitrarily chosen time interval.

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