Discussion of the geological adaptability of coal-bed methane horizontal wells of high-rank coal formation in southern Qinshui Basin

The practical development experiences of CBM horizontal well for 10 years in southern Qinshui Basin shows that there is a big development difference among the different types of horizontal wells under the same geological conditions and among the same type of horizontal well under different geological conditions. Therefore, the geological adaptability of horizontal wells needs to further be investigated. This paper studies the effects of key geological parameters of horizontal well on the completion method and the stimulation method, based on present development situation of horizontal wells, coal mine observation and indoor experiments by statistics data in Fanzhuang and Zhengzhuang block. The results show that the size and direction of ground stress and the coal structure decide the completion method of horizontal wells. The completion method consists of open-hole completion, casing completion, screen completion and so on. The open-hole completion is the most economic method if the well bore is stable. When the buried depth is more than 600 m, the vertical stress of the coal and rock is greater than the compressive strength, which results in that the open hole of horizontal well is prone to collapse and the casing completion method and the screen completion method should be adopted. When the buried depth is between 600 m to 700 m, the daily production of screen completion horizontal well can be over 3 000 m3/d. However, when the buried depth is deeper than 700 m, the hydraulic stimulation is needed, so the casing completion method is suitable. The production of horizontal well decreases with the increase of the angle between the horizontal well borehole trends and the greater the maximum horizontal principal stress direction because the smaller the angle, the greater the effective stress on open hole, and the easier the deformation and collapse of open hole, where the casing and screen completion way should be adopted. When the borehole trend is perpendicular to the direction of the maximum horizontal principal stress, the production of the open-hole horizontal well is the highest, so the open-hole completion method should be adopted. When the horizontal well hole passes through the granule or mylonite coal development zone, the open-hole horizontal well is much easier to collapse, leading to daily production only about 1 000 m3/d. While the daily production is up to 4 500 m3/d, when the screen completion way should be adopted. The micro-fractures development degree and the vertical heterogeneity of coal seam determine whether horizontal wells need to be fractured. The micro-fractures development degree can be quantitatively characterized by the fracture index. When the fracture index is higher than 100, the daily production of screen horizontal wells is higher than 3 000 m3/d. When the fracture index is less than 100, the low permeability results in a small single-well controlling area and hydraulic fracturing is required, where the production of the screen horizontal well is lower than 1 000 m3/d while the production can reach 7 000 m3/d after hydraulic fracturing. If there are low-permeability layers with fracture index less than 100 in vertical direction of coal seams, the gas production is poor with screen completion and hydraulic fracturing is also required, where the gas production can be up to 8 000 m3/d. © 2019, Editorial Office of Journal of China Coal Society. All right reserved.