Learnings from a failed nitrogen enhanced coalbed methane pilot: Piceance Basin, Colorado

Abstract A nitrogen flood, ECBM micro pilot was carried out in a deep, low permeability Cameo coal zone of the Mesavede Group in the Piceance Basin. The pilot project entailed the injection of about 28 mmcf (7.9E5 m 3 ) of nitrogen over a period of 15 days at pressures about 650 psig (4482 kPa) above the initial reservoir pressure, followed by a soak period of 15 days, and subsequently 11 days of flow back. The nitrogen flood was a failure inasmuch as there was no pressure change nor nitrogen or methane breakthrough in three closely adjacent monitoring wells, which were considered necessary to meet the metrics required for a field scale economic ECBM project. The results of the pilot test, in conjunction with subsequent laboratory field simulation tests and numerical modeling, are consistent in that they show injection of nitrogen, of necessity above reservoir pressure, neither displaced methane nor promoted methane desorption from the low permeability Cameo coal. The results suggest the injection of nitrogen above reservoir pressure increases the total adsorbed gas resulting in swelling of the coal, which increases the effective stress and hence decreases permeability, the reverse of what is intended. The field, laboratory, and numerical results indicate that for a nitrogen ECBM flood to initiate, in coal such as studied here, the in situ permeability must be high enough that nitrogen can displace, and thus reduce the partial pressure for methane. The results thus indicate that ECBM production by nitrogen flooding will only work in coals where the pre-existing permeability is high enough that conventional coalbed methane production is possible or has already been carried out. Results of our studies provide learnings relevant to carbon dioxide sequestration and ECBM for coals of similar properties to those of this study. An initial high permeability is critical, particularly for carbon dioxide injection, due to its higher volumetric strain coefficient and greater adsorption affinity than methane or nitrogen. Such high in situ permeability exists mainly in shallow coals, which also have the potential of being exploited in the future as a resource and hence may not be viable sites for long term carbon dioxide sequestration.