A numerical study on the regional in situ stress field of the Keshen section and interface dislocation in the composite salt-gypsum layer

Salt rock found in the stratum of the sedimentary basin is considered to be exemplary in terms of quality among the class of cap rocks, and it engulfs a certain proportion of the world’s hydrocarbon resources. In recent years, approximately 40% of newly discovered hydrocarbon resources in China have been detected under deep salt layers. Statistics suggest that nearly 40% of drilling accidents and 50% of casing damage incidents occur near the layer interfaces within a composite salt-gypsum formation. For the in-depth characterization of processes such as deformation and damages occurring at the interfaces, analysis of the regional in situ stress field characteristics and distribution in salt structures is vital. For this purpose, the structure of the Keshen section of the Kelasu structural belt under the Tarim Oilfield was studied. Our study establishes a geomechanical model, which tends to be mainly based on constitutive elastic and rheological models (applicable to different layers, i.e., the upper salt layer, salt layer, and presalt layer). Furthermore, the stability of the composite salt-gypsum layer and prediction of formation stress were evaluated. Investigation of drilling accidents and wellbore integrity problems revealed that the perturbation at the interfaces was not earnestly contemplated. The results show a discontinuous pattern in the regional in situ stress distribution in all of the salt layers. The salt layer is characterized by creep behavior with differential stress of less than 1.0 MPa. The interface between the upper layer and the salt layer tends to bear inconsistent deformation of approximately a few centimeters along the wellbore wall.