Meitan kexue jishu (Mar 2023)
Research on the quantitative method of fault sealing to coal seam gas
Abstract
Fault has the dual function of sealing and channel to coal seam gas, which mainly depends on the sealing ability of the fault. Aiming at the quantitative characterization method of fault sealing ability to gas, combined with the multi-element theory of fault zone structure development, the influence of fault zone structure parameters ( two-wall coal seam docking coefficient, the shale content, the tightness coefficient, etc. ) on fault sealing are analyzed. Considering the correlation between the width of the induced fracture zone and the permeability, a formula for calculating the equivalent thickness of the mudstone in the fault zone and a permeability compensation algorithm is proposed. According to the low-velocity seepage theory, the calculation of the limit sealing pressure of the fault is realized, and then the fault sealing is quantitatively characterized. The results show that:①The fault sealing is mainly controlled by the internal structure of the fracture zone and the stress state of the fault plane, considering the sealing capacity of the roof, the floor and the coal reservoir pressure, the fault sealing ability would be quantitatively evaluated by using the structural parameters of the fault zone and the fault sealing pressure; ②The permeability of the coal seam in the induced fracture zone is negatively correlated with the distance from the fault plane, the farther away from the fault plane, the lower the permeability, and approaches the permeability of the original coal seam finally; ③ The greater the equivalent mudstone thickness, the higher the ultimate sealing pressure of the fault, and the better the sealing ability of the fault; ④The calculation results of Sitou fault sealing in Zhengzhuang-Fanzhuang Block of Qinshui Basin show that, the fault sealing pressure is higher than 4.67 MPa, which is greater than the reservoir pressure of 3.83 MPa. However, there are differences in the sealing ability on the fault strike, with good sealing ability on the northeast fault plane and relatively poor sealing ability on the southwest fault end. The research results have important technical support for scientifically judging the gas sealing ability of the fault plane, guiding the prediction of coal and gas outbursts, and serving the development of coalbed methane.
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