Study On Fracture Features Of No.3 Coal-bed In Sorthern Shizhuang Area Of Qinshui Basin

Coal-bed fractures are the main factors affecting coal-bed permeability,and the permeability is the key factor restricting to the coal-bed methane exploration and development,so how to forecast coal-bed fracture parameters quantitatively has important significance.3# coal-bed in sorthern Shizhuang Area of Qinshui Basin is thickness and stable distribution and high gas content,so the research on the characteristics of 3# coal-bed fractures is certainly helpful for the coal gas mining and production increasing.This study of the characteristics of coal-bed fracture parameters is based on the ancient and modern stress field numerical simulation,the paleo tectonic stress field created the coal-bed fracture,but the current stress field is only transformation of coal-bed fracture,based on the principle of conservation of energy,the strain energy density and surface energy can connect the paleo tectonic stress field with the fracture parameters,then,the original parameters and the spatial distribution of fractures are determined by the paleo tectonic stress field,and considering the factors of the present stress field,later analysing the transformation of the present stress field for the pre-existing fractures,and at the end,Making the aperture and density of fractures as a link to establish the quantitative calculation formula between the modern stress field and the fracture parameters,then quantitative predicting the fractures' density,aperture,porosity and permeability parameters.Based on the structural features in the study area,combining with the micro seismic monitoring and crack array acoustic logging method to determine the direction of the current stress area;and the present stress field of well point in the area is calculated by using hydraulic fracturing;and then based on the previous analysis of the stress field in the region and the data of well stress field distribution in the area,with the help of ANSYS software,the numerical simulation method showed the current minimum principal stress(the minimum principal stress)is between 6.86MPa~24.3MPa,about NW direction;the current intermediate principal stress(the maximum horizontal principal stress)is between 7.25MPa~28.8MPa,about NE direction;the current maximum principal stress(vertical principal stress)is between 9.57MPa~42.4MPa,and the effect of gravity is very significant,mainly in vertical direction.Comprehensive utilization of fracture filling method,structural features and tectonic evolution history could determine the formation period of coal seam is Yanshan period.Then,actualizing the numerical simulation of the paleo stress field in Yanshan.Yanshan paleo stress field simulation results show that,during the period of Yanshan movement in the study area overall in the extrusion state,the fault zone stress value is higher than other parts,but the value of the area near the fault stress is low;the stress value of western low structure position is large,the stress value of eastern structural high parts is small.Equivalent minimum principal stress is between 6.83MPa~111MPa,vertical direction;Yanshan period equivalent intermediate principal stress is between 6.86MPa~155MPa,nearly SN direction;equivalent maximum principal stress of Yanshan stage is between 88.5MPa~398MPa,nearly EW direction.At the end of this paper,it has established the calculation model between the stress and strain and fracture parameters,then combining the calculation model and simulation results of stress field,and the results show that the fracture line density is between 15~87 bar/m,the high value distribution area is near the fault and the low tectonic area like the Syncline;The aperture is mainly between 0.004mm~0.7mm,the distribution is opposite to the density.The porosity except a small part of high value area,is between 0.025%~2.3%,the distribution is similar to the aperture;The permeability is different in three directions,but the distribution is similar to the aperture,influenced by the aperture.In general,the permeability of the eastern high part of the structure is high,and the lower part of the western structure is small.Comparing the simulated fracture parameters with the well test data,the results are consistent.