Influence of normal development faults characteristics on the exploitation of vertical coal-bed methane wells:A case study of the Shizhuang Block in the south of Qinshui Basin

To understand the influence mechanism of fault development characteristics on the development of coalbed methane (CBM) and reveal its production control mode,the No. 3 coal seam in Shizhuang Block,Southern Qinshui Basin was taken as the research object and the influence characteristics of normal faults of different scales and combi- nation forms on the physical properties,engineering effect and drainage of coal reservoirs were analyzed. Six factors such as gas content,coal structure,permeability,well diameter expansion rate,fracture pressure and daily water pro- duction under the control of normal fault were selected,and the primary and secondary relations of different factors on gas production were analyzed using grey correlation method,and then the main control methods of normal faults on gas production were revealed. The results show that the influence of normal fault on gas production is controlled by its own nature,scale and location,and there is a certain distance range. The control range of upper wall of Sitou fault is about 300 m,and the control range of the lower wall of Sitou fault is about 250 m. The control range of the upper wall of F11 fault is about 250 m,and the control range of the lower wall of the F11 fault is about 200 m. As the upper plate of the normal fault is active disk,the shear stress along the fault plane is greater than that of the lower disk,so that the influ- ence range of the upper disk is larger than that of the lower disk. With the decrease of the distance from the fault,the influence of the normal fault increases,which is characterized by the decrease of gas content and fracture pressure,the increase of pore fracture development degree and well diameter expansion rate,the more broken coal body structure, and the more complex fracture curve shape. It is considered that the normal fault is formed under the action of tectonic stress,and there is a stress concentration zone near the fault plane. Under the influence of stress concentration,the coal seam in this area is obviously deformed and ruptured,and the mechanical strength of the coal seam in this area decrea- ses greatly. With the increase of the distance from the fault,the stress action intensity decreases gradually,and the de- formation fracture also weakens gradually. At the same time,the tension cracks are generally developed around normal faults,and then change into the compression and torsional fractures with the prolongation of distance. The opening de- gree of tension cracks is low,and after reaching a certain distance,they become tensioned. Therefore,the closer it is to the normal fault,the more complex the geological conditions are,the more difficult it is to carry out the project,and the worse the gas production is. The normal fault mainly affects the daily gas production of coalbed gas wells by controlling per-meability,gas content and communicating aquifers. In order to achieve the efficient exploitation of CBM near the fault,some considerations should be paid to whether these parameters are abnormal or not.