Coalbed Gas System Description And Reserve Parameters Analysis In The Shaerhu Sag Of The Tuha Basin

Coalbed Gas (CBG) resources was estimated to be 14.3TCM in low rank coal, about 38.9% of total CBG in China based on the latest CBG resources estimation by the Ministry of Land and Resources in 2006. But, it is at the very beginning to explore and study CBG in low rank coal in China. Therefore, there are some technical or science aspects to remain obscure, such as how to prevent from oxidation and bio-degradation while measuring desorption gas, what caused big difference between density measurements, what made CBG reservoirs forming and preserving.This dissertation proceeded integrative study and worked out new conclusions and new points on low rank coal reservoirs in the Shaerhu Sag of the Tuha Basin. It tested different ways on well site and in lab for improving desorption measurement procedure, established mathematical models for finding out the causes of density difference, studied burial and geo-thermal histories to work out mechanism of gas reservoir forming and preserving.First of all, the coal in Shaerhu sag is sub-bituminous with following characteristics: thick (average of 77m), low inorganic content (less than 7%), low gas content, high moisture content (average of 29%).Secondly, a new desorption procedure was worked out, by which average nitrogen content is measured as 7% that is at same level as de-watering gas composition testing. Therefore, the new procedure does work well for low rank coal. The technology with the procedure is firstly implemented in China.Thirdly, based on density principle model study, moisture loss and porosity damage made density measured 17% bigger in lab than from logging, and log density is close enough to model results. So, the log density is recommended to use for GIP assessment, not lab density.Fourthly, in Shaerhu Sag, the eastern can preserve adsorbed gas and form a reservoir because it is a complete trap, while the western was different due to the seal layer was down-cut and filled by gravel that was providing a path for methane escaping and air invading. If there were sufficient groundwater, hydrostatic pressure in gravel formation could darn the pressure hole to keep methane adsorbed on coal inner surface. But, Shaerhu Sag has been seriously short of water since the end of Tertiary, which caused formation water evaporating and replacing by air once they communicated with air. So, the coal traps should be explored at first in those basins without sufficient groundwater, like Tarim basin, Junggar Basin, etc.