In situ Stress-Coal Structure Relationship and Its Influence on Hydraulic Fracturing: A Case Study in Zhengzhuang Area in Qinshui Basin, China

In situ stress (comprised of minimum horizontal principal stress (sigma(h)), maximum horizontal principal stress (sigma(H)) and vertical principal stress (sigma(v))) and coal structure are the key control factors of hydraulic fracturing. In this work, 80 coalbed methane wells data were selected to explore the in situ stress-coal structure relationship and its influence on hydraulic fracturing. The results showed that the coal structure identification method based on principal components analysis is an effective tool in simplifying logging data and improving the recognition accuracy. Fracturing pressure (P-f) and sigma(H) are correlated positively with the content of undeformed coal, correlated negatively with the content of cataclastic coal and have a weaker relationship with the content of granulated coal. The coal seams in the southwest of the study area, which belong to I stress field (i.e., low vertical stress anisotropy (VSAI) and high horizontal stress anisotropy (HSAI)) are composed mainly of undeformed coal, and easily form planar fractures with long major fracture length and shorter major fracture height. The coal seams in the northwest-southeast study area, which belongs to II stress field (high VSAI and low HSAI), are composed mainly of cataclastic coal, and easily form complex plane fractures with second minister major fracture length and long major fracture height. The coal seams in the northeast study area, which belongs to III stress field (low VSAI and high HSAI), are composed mainly of granulated coal, and easily form complex fractures with shorter major fracture length and shorter major fracture height.

Automated summary

In situ stress and coal structure are the key factors that influence hydraulic fracturing. The principal components analysis method is effective in simplifying logging data and improving recognition accuracy. Different stress fields result in different compositions and characteristics of coal seams and fractures.