Tectonic fracture prediction and its implication for hydrocarbon development with a case study in ultra-deep reservoirs of Kuqa Depression, Tarim Basin, NW China

Abstract

Tectonic fractures can be important reserving space and the main contributor to permeability in ultra-deep reservoirs of Kuqa Depression. In this study, a workflow based on geomechanical method is presented. Firstly, a 3D heterogeneous mechanical field is constructed by core-logging-seismic data; then the finite element in-situ stress simulation is carried out, which is established using heterogeneous mechanical parameters instead of uniform parameters. Based on strain energy theory and energy conservation principle, the relationship between tectonic fracture parameters and in-situ stress field is established by a suitable rock fracture criterion for ultra-deep tight sandstone. Characteristics of tectonic fractures are predicted and revealed by the above stress field simulation and the established relationship between stress and tectonic fracture parameters. The results show that the maximum horizontal principal stress (SH) orientation is generally N-S-trending in Bozi-1 gas reservoir. The present-day in-situ stress magnitudes are generally high with the minimum horizontal principal stress (Sh) is about 110∼160MPa, and the stress difference is over 35MPa. The characteristics of present-day in-situ stress largely vary among different structural styles. The in-situ stress magnitude and burial depth generally indicates a linear relationship. The in-situ stress gradient is stratified with burial depth and is consistent with fold stress stratification. Controlled by the effect of fold bending and protection of salt layer, the development of tectonic fracture in pop-up structures is better with larger fracture aperture and higher permeability, and fracture strikes are generally consistent with the SH orientation. Tectonic fractures in imbricated structures indicate great heterogeneity because of the development of faults and folds, and the rebuilding effect (permeability reduction) under the present-day in-situ stress field. Optimization of favourable zone for in-situ stress and tectonic fracture is of practical effects during well drilling and completion works.