The Geological Characteristics And The Production Effect Factor Of Deep Coal Seam In Southern Qinshui Basin

Significant amount of coalbed methane(CBM)has been found in the coal seam(depth below 1000m)in southern Qinshui basin,however,the exploration and exploitation of CBM is full of difficulty.Based on Shizhuang block in southern Qinshui basin,this dissertation takes the No.3 coal seam of Shanxi formation and the No.15 coal seam of Taiyuan formation as the research objects,analyzes the properties of deep coal seam and its influence factors,summarizes the geological characteristics of CBM in deep coal seam,and clarifies the CBM well productivity and its affecting factors.A transition zone in the deep coal reservoir is clarified.With the increase of buried depth,the pore structure becomes poor,the permeability shows exponential decline,and the gas content presents increase firstly and then decrease.By using the method of analytic hierarchy process and BP neural network,this dissertation studies the depth of the transition zone,which is 736?1100 m.The inner mechanism of the change of deep coal reservoir physical properties is revealed,which is geo-stress.There are three kinds of geo-stress state in deep stratum:shallow stress state,intermediate stress state and deep stress state.The transformation of these three stress states lead to the change of pore structure,fracture,and permeability,which causes the geological particularity of deep coal reservoir.The characteristics of CBM productivity and its influence factors are analyzed.The daily gas production is less than 500 m³in deep coal seams in northern Shizhuang block.The influence factors include geological conditions,engineering technical conditions and drainage management.The main reasons are the low permeability,the large water production caused by the joint exploration of No.3 and No.15 coal seam,the poor fracturing,and the discontinuity of drainage caused by the frequent downtime in the process of drainage.The countermeasures and suggestions of deep CBM exploration and development are put forward.There exists a transitional zone in deep coal reservoir,and its critical influence is geo-stress.We must select the dessert area firstly in CBM exploration and exploitation of deep coal seam,and then conduct effective fracturing to improve the coal reservoir permeability.At the same time,it is particularly important to establish a reasonable and continuous drainage system.In addition,the effective ways to improve CBM production of the deep coal seam include the injection of CO₂,which can enhance CBM recovery,and the combined exploration of CBM and tight gas.