Fault Development Characteristics and Their Effects on Current Gas Content and Productivity of No. 3 Coal Seam in the Zhengzhuang Field, Southern Qinshui Basin, North China

摘要

Faulting is one of the most common geological features that impacts current gas content (CGC) of coal seams, which is an important parameter for coalbed methane (CBM/gas) production. However, the detailed faulting effects on CGC and productivity have not been thoroughly investigated, because of inadequate methods for quantitative evaluation of the effects. In this study, we carry out comprehensive analysis of diverse geological coring data and experimental results and production data of exploration wells as well as the three-dimensional fault interpretation in the Zhengzhuang field. We introduce a parameter fault scale (iF ) to quantify the scale of faults, which is described by fault length, fault throw, and the investigated area. Then we propose a horizontal grid method to determine the faulting influence on CGC. The relationships between the CGC and the fracture density of coal core, the mechanical properties of the roof and floor of coal seam, the average gas production in a faulting system (including the upthrown side, downthrown side, and fault plain) are analyzed. In the study area, the well-developed normal faults have significant influence on CGC, but the locally developed thrust faults have negligible influence. The square grid with length of 1.5 km is probably the most effective grids for analyzing faulting effected distance on CGC. Thus, the normal faults can be divided into small- (iF _(1.5) 0.05), moderate- (0.05 iF _(1.5) 0.15), and large-scale (iF _(1.5) 0.15). The evident effect of faulting on CGC can be observed up to ∼1.5 km away from the normal fault. When the coal seam is further away from the normal faults (i.e., 1.5 km), the CGC is higher or closer to 20.7 m~(3)/t. In contrast, when the coal seam is 1.5 km from the normal faults, the CGC decreases with increasing value of iF . In general, the CGC is higher on the downthrown side, with better gas preservation conditions than the upthrown side. Relatively high CGC (20 m~(3)/t) is necessary for good gas production of CBM wells. The CBM wells with low ADGP (500 m~(3)/d) and low CTGP ( 1.5 km) from the normal faults is the most favorable productivity area, and only the small-scale normal faults developed is the secondary favorable productivity area. These places have the average gas production as high as 1000 m~(3)/d and cumulative total gas production as high as 4 × 10~(5) m~(3).