Analysis Of Factors Influencing The Productivity Of Coalbed Methane In Different Geological Units In Shizhuangnan Block,Qinshui Basin

Coalbed methane(CBM)in the Shizhuangnan Block of Qinshui Basin has been produced for many years.However,the productivity of CBM wells shows significant difference,thus studying the productivity difference and its influencing factors have great effect on to the later development and reconstruction of CBM wells.In this paper,based on the seismic,logging,well testing,drainage and production data in the research area,coal reservoirs are divided into different structural geological units based on the fuzzy mathematical analytic method and analytic hierarchy process.In addition,productivity law of CBM wells and its geological influence factors are analyzed in detail.Some conclusions are presented as follow:(1)Based on the four evaluation indicators,i.e.fault strength,fault density,fault dip angle,and fold complexity factor,the complexity of the geological structure in the study area is quantitatively evaluated by using analytic hierarchy process.Then,the contours of the structure complexity of the study area are drawn.According to the structure outline map and structural complexity contour map of the study area,it can be concluded that the study area can be divided into 6 structural geological units,which is conductive to propose constructive deployment for the CBM wells.(2)Based on the gas production characteristics,the CBM wells in the study area can be divide into four types,i.e.ultra-high-yield,high-yield,medium-yield,and low-yield.By counting the distribution of various types of wells in each structural geological unit,the production characteristics of CBM wells in different units can be further analyzed.Next,the drainage curve was divided into water production fluctuation type,gas rise and water fall type,Same rise and fall type,stable water production type.(3)The geological factors such as permeability,gas content,burial depth,thickness,reservoir pressure,critical desorption pressure,and the ratio between critical desorption pressure and reservoir pressure have different influences on gas production in each unit.Study the relationship between geological factors and gas production in each unit by using gray correlation method.According to the correlation,it is found that the burial depth and gas content have great influences on the gas production in the entire study area,but there are differences in the correlation of each unit.(4)We can predict productivity of CBM wells and describe the dynamic change of reservoir pressure and gas content by numerically simulating typical wells in different units.According to the results,the formation process of inter-well pressure interference and dynamic expansion of the pressure drop funnel are obtained.By combining the dynamic change of reservoir pressure and gas content with the gas production curve,the time when CBM wells reach peak gas production is further predicted.