Coal Measure Mechanical Stratigraphic Characteristics And Coal Measure Gas Development Potential Evaluation Of A Block In The Eastern Ordos Basin

The development of coalbed methane in China has gone through 40 years,and a large number of low-yield wells have seriously restricted the process of coalbed methane industrialization.Compared with coalbed methane development,coal measure gas development has multiple advantages,and the early engineering practice has proved that transforming coalbed methane development into coal measure gas development is an effective way to promote the production of coalbed methane wells.The low production problem of coalbed methane wells in a block in the eastern margin of Ordos Basin is serious,and the development potential of coal measures gas in this area is huge.It is of great significance to solve the low production problem of some wells by implementing coal measures gas development.However,the early geological research on coal measures gas development in this area is relatively shallow,and the main control factors affecting coal measures gas development are not clear.At the same time,it has not formed a supporting development potential evaluation system,and cannot provide strong support for technical implementation.Therefore,this paper takes the block as the study area,using the basic theory and method of mechanical stratigraphy to study the mechanical stratigraphy characteristics of coal measures in the area,combined with the analysis of the main control factors of coalbed methane well production,select the main geological factors affecting the development of coal measures gas,and construct the sweet layer evaluation system by multi-level fuzzy mathematics method.On this basis,the evaluation model of sweet spot area is established and the development potential of coal measures gas in the study area is comprehensively evaluated.The following conclusions are drawn :(1)The developed coal measures in the study area are the Taiyuan Formation No.8 coal seam section and the Shanxi Formation No.5 coal seam section of the Permian.The lithology of the strata mainly includes coal,mudstone,carbonaceous mudstone,argillaceous sandstone,fine sandstone,limestone,etc.The spatial distribution of the coal measures as a whole is influenced by the monoclinic structure and is inclined to the northwest direction.The petrological characteristics show that the burial depth gradually deepens from south to north,and the thickness gradually becomes thinner from south to north.The strata of coal section 5are mainly mudstone and fine sandstone,and the coal-rock combination relationship is mainly mudstone-coal interbed and sand-mud interbed.And the strata of coal section 8 are mainly fine sandstone,limestone and mudstone,and the coal-rock combination relationship is mainly:upper ash lower coal,upper ash lower mud,upper ash lower mud and coal.The mechanical characteristics of rock mass show that the numerical values of Young’ s modulus,shear modulus,compressive strength,tensile strength,brittleness index and fracture pressure in different types of reservoirs are in the order of tight rock layer,shale layer and coal seam from high to low,while Poisson’s ratio and critical strain energy release rate are in the opposite order with the former,and the reservoir reconstructive ability is in the order of tight rock layer > shale layer > coal seam.The fluid mechanics characteristics are characterized by good gas content.The gas content of coal seam is high.The shale and tight rock also have a certain gas content and a high value near the coal seam,and the gas content increases with the increase of burial depth of the reservoir.(2)In the sweet layer evaluation system,the weight of various types of reservoirs is the highest,the resource abundance is second,and the weight of recoverable is relatively low.Among them,the gas content,the difference coefficient of in-situ stress,the brittleness index and the pore pressure coefficient have a great influence on the development of coal measures gas.According to the calculation results of the sweet layer evaluation system,it can be divided into the sweet layer when the sweetness value is higher than 0.67,the middle layer when the sweetness value is between 0.33 and 0.67,and the acid layer when the sweetness value is lower than 0.33.Taking well MXQ-01 as an example,the sweet layer evaluated is mainly composed of coal and mudstone source rocks,followed by sandstone.The resource conditions of coal seam and carbonaceous mudstone in the sweet layer are good,and the transformability of mudstone,argillaceous sandstone and sandstone is strong,and the sweet layer generally has good recoverability.The acid layer is mainly limestone aquifer and nearby strata with high water inrush coefficient(3)The evaluation value of the dessert area is the result of the coupling of sweetness and thickness of the dessert layer.The dessert area of No.5 coal seam is mainly located in the eastern part of the study area,and the dessert area of No.8 coal seam is mainly distributed in the northwest,northeast and eastern regions of the study area.The 5th coal section sweet spot area is located in the northern part of the study area,and the 8th coal section sweet spot area is located in the western and eastern edge of the study area.The coalbed methane resources in the sweet spot area of No.5 coal seam should be selected first,followed by the coalbed methane resources in the sweet spot area of No.5 coal seam,and finally the coalbed methane resources in the sweet spot area of No.8 coal seam.There are 20 figures,9 tables and 64 references.