Geological Characteristics Of Low-rank CBM And Optimum Selection Of Development Blocks Of Key Depression In Erlian Basin

This paper takes the key depression containing Huolinhe Depression,Bayanhua Depression and Jiergalangtu Depression in Erlian Basin as research object,and the coal reservoir basically development law,physical characteristic and gas content are analysed,at the same time the geological control factors are identified,finally based on the quantitative evaluation method,the favorable depression of the low-rank coalbed methane in the study area is optimized to the development unit.The main findings contain: the low-rank coalbed in study region has a large thickness and varies greatly in plane distribution;the coal type is mainly lignite,and the maceral is mainly composed of huminite;the coal quality is characterized by high moisture,high volatile matter and low sulfur;the reservoir temperature is normal to abnormal;the reservoir pressure is slightly under pressure to normal.The physical properties of coal reservoirs are generally characterized by high porosity,underdeveloped and partial filling fractures,low matrix permeability and large difference in rock mechanics parameters between roof and floor.Coal rank,coal petrology and quality,coal structure and paleotectonic stress field are the main factors affecting the characteristics of pore and fracture;pressure,temperature,coal rank and coal petrology and quality are the main factors affecting adsorption;fracture development degree and stress are the main factors controlling the permeability of coal matrix.Coalbed methane is mainly microbial with a small amount of mixed gas.The gas content of coal seam is low in this area,but due to the large thickness of coal seam,there is still high resource abundance.Coal rank,thickness and burial depth of coal seam,structure,sedimentation and hydrogeological conditions are the main geological control factors of gas content property.Among them,hydrogeology is the most critical factor,which is not only conducive to the preservation of coal-bed methane,but also affects the formation of secondary biogenic gas.The three-phase gas content prediction model in low-rank coal was establishment,and the calculation results shows: the gas is mainly composed of free gas and adsorption gas,and the gas content of each phase changes nonlinearly in the vertical direction.Based on the orthogonal experiment of coalbed methane production sensitivity and fuzzy comprehensive evaluation method,it is considered that the development geological conditions in Jiergalangtu Depression are the best comepared with the other depressions,and eight indexes containing resources quantities,reservoir and geological conditions and so on are further selected for evaluation of favorable development geological units.The comprehensive evaluation shows that the favorable development geological units are mainly located in and near No.J5 and J7 boreholes in the middle of this depression.