Tectonic stress estimation from ultrasonic borehole image logs in a coal bed methane well, northeastern Qinshui Basin, China

Abstract

Small-scale structures and tectonic stress are known to affect the production of coalbed methane. Ultrasonic borehole image logging (UBI) can be used to identify fractures and breakout zones around the wellbore and to analyze tectonic stress fields. In this study, a variety of image processing and automated fracture detection methods were used to process and interpret the ultrasonic logging data in a coal bed methane well. Combined with other logging data in the well, this study analyzed the ultrasonic logging response characteristics of the formation lithology and structure. Statistical analysis has been completed to identify the fracturing pattern. The results showed that the dip angle and orientation of the natural fracture were related to regional tectonics. The paleo-stress field, reflected by the natural fractures, was consistent with the geological conditions, while the current stress field, reflected by the breakout zones, was found to be different from the paleo-stress field. This study proposed a new method by using fractures with associated conjugate joints, which are obtained from UBI logs, to estimate the paleo-stress orientation.