Optimal Selection And Prediction Of Sweet Spots For CBM Development In Changgang Syncline,Northern Guizhou,China

The geological information of coalbed methane?CBM?in coal resources exploration is extracted.In combination with the study of CBM exploration and sample test data,the basic attributes of coal reservoir,CBM content and reservoir reconstructive ability can be studied,a systematic understanding of the sweet spots and its geological conditions of CBM exploration are obtained.Based on the data of coal field exploration and sample test,the characteristics of coal seam distribution and the basic properties of coal reservoir are studied.Changgang syncline seam is many and thin with high degree of metamorphism.The spacing of coal seam is small,and its buried depth is moderate,which are suitable for the present CBM exploration and development technology.The results show that the total pore capacity of coal sample in the study area is small,the specific surface area is large,transition pores and micropores account for more than 80%,and the pore connectivity is better.But the porosity of coal is low,the fractures are mostly filled by minerals,so they are medium and low permeability reservoirs.Also,the Langmuir volume of coal samples are big,but a low Langmuir pressure,which shows a better material basis of gas storage,nevertheless,the geological condition of CBM development is not ideal.The fluid characteristics of coal reservoir and its surrounding rock were analyzed by pumping test and coal samples desorption data.The study found that:Changgang syncline coal-bearing strata is poor in water,have variety water quality types and a good closure.The water quality type of surrounding rock is HCO₃-Ca,shows a strong hydrodynamic condition,but the closure is weaker than coal-bearing stratum.Furthermore,the main coal seam has high CBM content,and its distribution is controlled by buried depth and tectonic framework,showing the characteristic of“the Southern CBM content is higher than Northern parts,the Eastren CBM content is higher than the Western parts,the wing is lower than core,the shallow depth is lower than deep part,and local anomalies”.The characteristics of coal structure,tectonic curvature and rock mechanical properties are studied.It is found that due to the wrinkle of faults,large area of tectonic coal is developed in the North of the study area,and the original structure coal is mainly distributied in the South.The tectonic curvature of No.7 coal seam is between-219.89×10^-55 and 307.85×10^-5,and the tectonic curvature value in the Southern region is between-20×10^-5 and 20×10^-5,which indicates a week reconstruction.Also,the mechanical properties of coal seam and its surrounding rock are quite different,the mechanical properties of the top and floor of No.8 and No.9 coal are close to that of coal seam,which is not conducive to crack propagation along the coal seam.The optimal selection and prediction of sweet spots for CBM development in the study area were completed by using multi-level fuzzy mathematics and the method of hierarchical optimization of key factors.Considering the geological characteristics of CBM in the study area,the optimal index system of sweet spot is established,and the?and?class are optimized.Then based on the idea of production layer combination,the vertical sweet layers and combination are optimized.