The Reservoir Characteristics And Tectonic Influence Of High Rank Coalbed Methane In Qinshui Basin, Shanxi Province

Main limitations of reservoir property of coal-bed methane (CBM) in China include as following: Generally coal-bearing basins have complex history of evolution and tectonism; Type of coal metamorphism is complex and coal undergone through multi-periods metamorphic evolution and superimposed metamorphism of multi-thernal source; Physical property of different coal reservoir is diverse and the porosity, permeability and gas saturation are usually low. On the basis of collecting large amount of geological data of coal field, natural gas and CBM, samples of coal, sand-shale, limestone and magmatite, this paper selects typical districts (Yangcheng, Huoshan and Qinyuan) of Qinshui basin in the southeast of Shanxi province and sets up model of sedimentary and burial history, determined key period of CBM reservoir formation. The great deals of test analysis and experiment simulation have been carried out and function of tectono-thermal event in CBM reservoir formation has been determined, in which it also studies the control factors of permeability and looks for valid prediction method of high-permeability area. At the same time comprehensive simulation of reservoir formation of high rank CBM has been carried out to determine characteristic and predominance of reservoir formation. A set of simulation experiment device of CBM reservoir formation with absolute intellectual property right was established and two patents were applied including an invention patent and a practical patent. A lots of physical simulation experiments were carried out and several new understandings about mechanism of CBM exploration and development were obtained and won good application effect in production and scientific research, such as dualism of pressure-relief and matrix expansion in the process of CBM production, breakthrough parameter and pressure difference of migration and accumulation of CBM in place, dissolving function of water to CBM. Definition of "Key moment"was presented that is the moment when buried depth i.e. effective thickness of overlying strata is minimum in geological history after last time of hydrocarbon formation in coal bed. The forward, backward consequence and modeling experiments show that geologic time when evolution degree is highest determines geochemical feature of CBM. This paper discussed condition of temperature and pressure in "Key moment"in combination with forward and backward consequence, and pointed out definitely that quantity of gas accumulation is correlative to "Key moment"of CBM reservoir evolution combined with gas accumulation history. This study shows that high coal rank will accelerate the large amount of CBM generation and improve physical properties of reservoir and to form CBM enriched area, influenced by abnormal thermal event. Different variable laws of physical properties were proposed: For high rank coal, with the reduction of system pressure, fissure pressure will decrease; overlying pressure will increase comparatively; and closure of fissure leads to the decreasing of permeability. On the contrary, for low rank coal, with the reduction of system pressure, matrix will expanse continuously in the state of pressure-relief, which results to the increase of pore diameter and matrix permeability, that is, with the production of CBM, physical properties of coal reservoir will become better. Prediction of high permeability area was developed to use the coupling relationship between the orientation of maximum principal compression stress, differential stress magnitude of recent tectonic stress field and permeability in test hole. The characteristics and predominance of reservoir formation for high metamorphic anthracite in China were summarized. Predominances include following three parts: 1. If metamorphosis degree is high, then amount of generated gas is large, adsorption capacity of coal is strong and gas content is high; 2. The tectono-thermal event and stress field will greatly affect on physical properties of coal reservoir; 3. The preserved condition of CBM, existed occluded water and high salinity water in the area, will be good, in which is beneficial to the preservation and production of CBM with pressure-relief methods by water draining.