The Poroperm Characteristics Of Coal Reservoirs In Southern Qinshui Basin And Their Geologic Factors

The production base in southern Qinshui Basin has been the first one to achieve large-scale commercial development of coalbed methane(CBM)in China,breaking the restricted area of CBM exploration and development in high rank coal in the world.The main coal seams of southern Qinshui Basin are No.3 coal seam in Shanxi formation and No.15 coal seam in Taiyuan formation.However,the development of CBM is mainly focusing on No.3 coal seam and there are a lot of technical problems in the development of CBM in No.15 coal seam.Therefore,from the perspective of poroperm characteristics of coal reservoirs,a systematic study on comparing the two coal seams and exploring the geological factors of poroperm characteristics of coal reservoirs is necessary and then provide scientific basis for solving the technical problems of CBM development of No.15 coal seam.The pores of coal reservoirs of southern Qinshui Basin are mainly filled with kaolinite,calcite,pyrite and other minerals.The mineral filling effect on the pores of No.15 coal seam is more serious than that on the pores of No.3 coal seam.This may be related to the different roof lithologies and sedimentary environments of the two coal seams.The pores of No.3 coal seam are mainly open micropores,which benefit the adsorption,desorption and diffusion of CBM.The pores of No.15 coal seam are mainly semi-closed micropores and fine pores,easily causing "bottleneck problems"for seepage.The macrofractures and microfractures of No.15 coal seam are more supermature than No.3 coal seam,but the mineral filling degree of the fractures of No.15 coal seam is more serious than that of No.3 coal seam.The distribution of No.15 coal seam's microfractures in different scales is not reasonable,which is not favorable to the diffusion and migration of CBM.The influencing factors of the development characteristics of the pore system include the buried depth,the metamorphic grade,the maceral and minerals.The influencing factors of the development characteristics of the facture system contain the current tectonic stress field,the maceral,the metamorphic grade and minerals.The permeability of No.3 coal seam is higher than that of No.15 coal seam.The pore system has limited contribution to the permeability of coal reservoirs.The permeability increases exponentially with the increase of the fracture density and the fracture permeability can be greatly reduced by the filling of minerals.The permeability increases exponentially with the increase of the principal stress difference of current tectonic stress field.The influence of tectonic stress on the original permeability of coal reservoirs is essentially achieved by controlling the opening degree of natural fractures.When the buried depth is over 800m,the permeability of coal reservoirs is basically at a low value range.The relationship between the permeability and the buried depth is actually the control effect of crustal stress on the permeability.The permeability of coal reservoirs can be influenced by the coal body structure which is obviously controlled by the tectonism.It can be known that the current tectonic stress field is the main controlling geological factor of the permeability of the coal reservoirs in southern Qinshui Basin.Natural fractures,minerals and coal body structure are the secondary factors.