Present-day in-situ stress field within the Yanchang Formation tight oil reservoir of Ordos Basin, central China

Abstract The present-day in-situ stress field has many important and practical applications during the exploration and development of hydrocarbon resources. The Ordos Basin is a significant petroliferous basin in China, within which, the Yanchang Formation serves as the most significant tight oil reservoir. However, an overview of previous studies indicates that little was known as to the present-day stress state in the Yanchang Formation. Therefore, the present-day stress field in the Yanchang Formation tight oil reservoir of the Ordos Basin is systematically analyzed in this study. The orientation of horizontal maximum principal stress (SHmax) within the Yanchang Formation of Ordos Basin ranges between north-northeast-south-southwest-trending (NNE-SSW) and east-northeast-west-southwest-trending (ENE-WSW) based on interpretations of borehole breakouts and drilling-induced fractures from borehole imaging logs. The SHmax orientation shows various tendencies in different wells, which may result from differential development in lithology, natural fracture and bedding plane over various regions. The estimated magnitudes of vertical stress (Sv), SHmax and horizontal minimum principal stress (Shmin) in the studied wells indicate that the normal faulting stress regime is dominant within the Yanchang Formation. In addition, the geomechanical assessments of wellbore stability and natural fracture reactivation are also performed. Natural fractures in the Yanchang Formation Chang 7 tight oil reservoir of Ordos Basin are developed striking mainly in the northeast-southwest-trending (NE-SW) and west-northwest-east-southeast-trending (WNW-ESE). Among them, those fractures with NE-SW-trending strikes are more conductive with reference to the present-day stress state, contributing more to tight oil production in the Ordos Basin. In addition, within the Yanchang Formation, if horizontal wells are drilled towards the NNE-SSW-trending to ENE-WSW-trending, they are more likely to have wellbore instability issues. The results in this study provide important geological information for tight oil production in the Yanchang Formation of Ordos Basin.