This paper presents from the Lawrence Livermore Laboratory Hoe Creek No. 2 underground coal-gasification experiment. The experiment used an injection well and a production well 18.7 m apart and alternating periods of air and oxygen/steam injection. Both low- and medium-heating-value gas was produced. Fourteen days of reverse combustion linked produced. Fourteen days of reverse combustion linked the injection and production wells and was followed by 58 days of forward-combustion gasification. The first five days of gasification produced good-quality gas, with the higher heating value averaging 125 kJ/mol (140 Btu/scf). After this period, the burn was along the top of the coal seam, which sharply lowered the gas quality. The high burn probably resulted from the injection-well casing being broken or burned off at the top of the coal seam. Switching injection to the bottom of the seam resulted in good-quality gas until the end of the experiment. Forward gasification consumed 1310 m3(1952 tons) of coal. Air injection produced gas that averaged 94 kJ/mol (105 Btu/scf), produced gas that averaged 94 kJ/mol (105 Btu/scf), and a two-day oxygen/steam burn produced gas of medium heating value: 225 to 270 kJ/mol (250 to 300 Btu /scf), with no operational or safety problems. The same geometry that produced poor-quality gas with air produced good-quality gas with oxygen/steam.
Introduction
About half of the estimated 4 Eg (exagrams, 10(18)g) of U.S. coal resource lies in thick seams 300 m or more below the surface, deep enough that both strip and deep mining would be uneconomical. Moreover, in seams 30 m or more thick, conventional deep mining can recover only about 20% of the coal.
In situ coal gasification may make it possible to economically recover much more of this deep-lying coal while avoiding some of the environmental and personnel-safety problems of conventional mining. personnel-safety problems of conventional mining. In situ gasification of the coal can provide combustible gas without requiring underground mine workers, and the in situ process will avoid some of the surface disturbance caused by strip mining.
Of the huge resource of deep, thick coal beds, 170 Pg (petagrams, 10(15)g) have been definitely identified in the Rocky Mountain region as meeting the following requirements for in situ gasification: the deposits are at least 15 m thick and lie between 150 and 900 m below the surface. Current estimates indicate that this resource alone, if converted to synthetic natural gas at an overall efficiency of 50%, would produce some 4.2 × 10(13) m3 of pipeline-quality gas - six times the proven U.S. natural gas reserve. (In 1977, the United States consumed 6.2 × 10(10) m3 of natural gas). Alaska probably has at least as much coal in suitable deposits.
All in situ gasification processes burn some of the coal in place under controlled conditions and use the heat evolved to break down the remaining coal, generating combustible product gases. Air or oxygen pumped into the coal seam through an injection well pumped into the coal seam through an injection well maintains and controls the burning, and the product gases are conveyed to the surface through a production well drilled into the seam a short distance away. This process requires a reasonably free passage for gases from injection to production wells, passage for gases from injection to production wells, but solid coal is seldom permeable enough to provide such a passage.
In our first gasification experiment at Hoe Creek (located near Gillette, Wyoming) we used explosive fracturing to enhance the permeability of the coal. In Hoe Creek No. 2, we used reverse-combustion linking, a process developed in the U.S.S.R. more than 30 years ago and probably the most highly developed linking process available for in situ coal gasification. Reverse-combustion linking has been used in all the tests performed by the Laramie Energy Research Center (LERC). However, because the coal at the Hoe Creek is much wetter and naturally more fractured than that at the LERC site near Hanna, Wyoming, we were not sure if reverse combustion would work at our site.
Our experiment consisted basically of an injection well and a production well drilled 18 m apart and cased almost to the bottom of the coal seam. In the reverse combustion, we injected air, which percolated through the coal to a fire started at the bottom of the production well.
