United States of America
There have been over thirty trials in the US divided into four main batches:
The first series, between 1973 and 1976 were commissioned to evaluate the methods being used in Russia. The projects were carefully researched and extensively monitored and vertical wells formed the basis of the process, usually drilled quite close together.
The second series was conducted by what is now known as the Lawrence Livermore National Laboratory (LLNL) and the Dept of Energy facility at Hoe Creek. The tests at Hoe Creek provided the basis for a number of developments which included the validation of the CRIP ignition scheme, the validation of subsidence models, and the first oxygen/steam injection experiments in the USA. A wide range of instruments and monitoring tools were used, including the use of chemical (gas) tracers.
The tests also gave rise to the first recognition of possible groundwater hazards. In Hoe Creek I, where explosive fracturing was used, the test continued for 11 days with using air injection. Approximately 7% of the gas was lost into the rock formation. Hoe Creek II used reverse combustion, and gasification lasted for 43 days. Water influx significantly lowered the gas quality. To decrease the water inflow the operating pressure in the burn zone was increased. This resulted in a significant amount of gas (approximately 20%) being lost into the rock formation. Much of this loss is thought to have occurred when the burn zone collapsed, exposing the overlying Felix No 1 coal seam, which was at a lower hydrostatic pressure. This event which happened in strata would now be classed as having high environmental risk (and would therefore not be considered for development of any kind), which gave UCG an unjustified reputation as a potential hazard.
The third series were conducted at Rawlins, in Wyoming, using steeply dipping seams and was successful in producing high quality gas, with lower oxygen demand than was the case in horizontal strata.
The fourth series (Centralia and Rocky Mountain trials) occurred from 1984 -1989 was a direct comparison between the CRIP method and the enhanced Soviet (vertical borehole) method. Previous to these series, different tests were run by the LLNL at Centralia, Washington State (Large Block Tests, Tono trials) to validate the CRIP concept. The CRIP method produced higher quality gas (over 10MJ/m3 as against 8MJ/m3), gasified about 20% more coal on a daily basis and was overall more efficient in extracting energy from the coal. The trial developed a technique, the ‘Clean Cavern technique’, to manage the decommissioning of UCG reactors by: (a) carefully controlling pressure; and (b) using water to quench and flush the reactor to minimise the risk of contaminant build up. The RM-1 trial demonstrated that with careful site selection and appropriate operating procedures, UCG can have minimal impact on the environment