The present-day in-situ stress state and fracture effectiveness evaluation in shale oil reservoir: A case study of the Yanchang formation Chang 7 oil-bearing layer in the Ordos Basin

The Ordos Basin is an important area in China for producing unconventional oil and gas resources. There are large volumes of shale oil resources in the Yanchang formation Chang 7 oil-bearing layer, serving as a great potential in the Ordos Basin. Knowledge of the present-day in-situ stress field and fracture effectiveness in the Yanchang formation Chang 7 oil-bearing layer are of great significance for shale oil exploration and development in the Ordos Basin. In this study, several approaches are used to analyze the present-day in-situ stress field and its effect on fracture effectiveness in the Chang 7 shale oil reservoir. Generally, a dominant normal stress regime is determined in the Chang 7 shale oil reservoir. Fracture effectiveness is greatly influenced by present-day in-situ stress field, and fracture opening pressure can be affected by factors including burial depth of fractures, fracture orientation and present-day stress state, etc. The investigated results indicated that the present-day stress orientation shows a dominant ENE-WSW-NE-SW direction. The stress magnitudes indicate a positive linear relationship with burial depth, and the magnitudes of vertical stress (Sv), horizontal maximum (SHmax) and minimum (Shmin) principal stress follow Sv>SHmax>Shmin in the Chang 7 shale oil reservoir. Under the present-day in-situ stress field, all natural fractures in the Chang 7 oil-bearing layer are noneffective. When fluids are injected into the Chang 7 shale oil reservoir, those fractures having low angles between stress orientation and fracture strike, high fracture dip angles tend to become effective firstly. The critical pressure for this change of fracture effectiveness is about 14.1-36.5 MPa. The calculated opening pressure for natural fractures in the Chang 7 shale oil reservoir ranges 29.7-77.2 MPa with an average of 49.9 MPa. © 2020, Editorial Board of Journal of CUMT. All right reserved.