All stages of oil and gas exploration and development involve the study of in-situ stress. Since the traditional two-dimensional and three-dimensional homogeneous models can no longer fulfil the requirements of research and production, numerical simulation of the stress field has become an effective study method. In this study, we took the Jia 2 member in Puguang area as a case to establish a geological model and a mechanical model based on the tectonic framework and the distribution characteristics of the rock mechanical parameters, respectively, and loaded the model with the present-day in-situ stress state calculated from the logging data as the boundary conditions. The simulation results show that 1) the orientation of the maximum horizontal principal stress in the study area is near E-W, and the in-situ stress orientation is locally deflected due to the influence of faults; and 2) the magnitude of in-situ stress is predominantly affected by the burial depth and lithology, and the minimum horizontal principal stress, maximum horizontal principal stress, and differential stress are mainly concentrated in the ranges of 30–60, 50–80, and 10–40 MPa, respectively. We also analysed the opening sequence of the multiple fracture systems during development, using the present-day stress field model. The analysis revealed that the E-W fractures will open first, and the continuously increasing operating pressure will lead to formation breakdown, producing a fracture network.
Natural fractures are identified as high-quality storage space and seepage channels for the Triassic tight sandstone reservoirs in the Dongpu Depression, playing an important role in the tight sandstones oil production. In this paper, outcrops, cores, thin sections and imaging logs were used to describe the growth of natural fractures at different scales. The correlation between fracture and crude oil production capacity was analyzed using production data. Results show that natural fractures are primarily distributed in fine sandstones and siltstones, which are mostly shear fractures of near E-W and NE-SW strikes. The natural fractures of near E-W strike are generally parallel to the present-day maximum horizontal principal stress with the biggest apertures and the highest permeability, which are the main seepage channels, next being the fractures of NE-SW strike. The natural fractures of near E-W strike are also the most important contributors to the crude oil production in the Triassic tight sandstones of the Dongpu Depression. The intensity, permeability and direction of natural fractures govern the crude oil productivity in the per unit sandstone thickness.
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