Numerical modeling of in-situ stress and prediction of favorable area of shale gas layer in Wufeng to Longmaxi formations, Nanchuan region, southeastern Chongqing

Abstract

Upper Ordovician Wufeng to Lower Silurian Longmaxi formations in Nanchuan region, as an important shale gas productive layer, is characterized by large shale thickness, deep burial depth, and complex in-situ stress with rapid direction changes. Therefore, the study of in-situ stress field is of significance for effective deve-lopment of shale gas in the study area. In order to clarify the characteristics and distribution of the in-situ stress field, the SHELLS finite element stress field modeling method, with faults, topography, heat flow, petrophysical parameters and boundary conditions as constraints, was used in the study of the stress field in the Wufeng-Longmaxi formations in the Nanchuan region.The results indicate that the maximum compressive horizontal principal stress is in thrust regime and that there are four principal stress directions and regions in general: NW-SE, NE-SW, near EW and near SN. The strain rate is in thrust regime, with three regions of low strain rate (magnitude ≤ -18), medium strain rate (magnitude between -18 and -17.6) and high strain rate (magnitude ≥ -17.6) and their corresponding NE-SW, NW-SE, SN and EW spreading directions. The fault slip rate regime is in thrust, the fault slip rates range from 0 to 0.001 2 mm/a. The modeled results of the maximum compressive horizontal principal stress, the strain rate and fault slip rate were compared with the measured data respectively, including the maximum compressive horizontal principal stress directions measured in drilled wells, the regime and magnitude of strain rate in Guizhou and Chongqing, and the properties of regional faults. The modeled results showed high agreement with the measured data, indicating the accuracy of the predicted results. Finally, based on fracture openness and fracture development revealed by modeled results, favorable fractured reservoir development zones for shale gas was evaluated, and class Ⅰ and Ⅱ zones for further exploration and development were predicted.

Keywords

• numerical modeling of shells finite element
• present stress field
• shale gas
• wufeng- longmaxi formations
• nanchuan region