Simulation of multi-period tectonic stress fields and distribution prediction of tectonic fractures in tight gas reservoirs: A case study of the Tianhuan Depression in western Ordos Basin, China

Hydraulic fracturing technology, which is emerging in low- or ultralow-permeability oil and gas and tight gas reservoirs, has become a key technology for improving the development efficiency and economic benefit of unconventional gas reservoirs. The success rate of hydraulic fracturing is closely related to the development and distribution of natural fractures. Data obtained from outcrops, cores, thin sections and image logging were used to determine the developmental characteristics of tectonic fractures in the study area to obtain a quantitative forecast of the distribution of gas reservoir fractures in the Tianhuan Depression. With the aid of burial history analysis, geologic interpretation, seismic inversion, logging evaluation, rock mechanics and acoustic emission tests, 3D geomechanical models were established using the finite element method in Tianhuan Depression to simulate the paleotectonic stress fields during Indosinian, Yanshanian and Himalayan periods. The effects of different rock mechanics parameters on the development and distribution of tectonic fractures were identified. A fracture density model was established to determine the quantitative development of tectonic fractures in the upper He8 Member during the production of multi-period tectonic stress fields. The validity of the fracture density model was verified by the fracture statistics of drill core density and gas well productivity tests. Favorable areas for hydraulic fracturing were also determined. Simulation results indicate that the maximum and minimum principal stresses and the maximum shear stress during the Indosinian, Yanshanian and Himalayan periods are controlled by the distribution of the sand body. Rock mechanics parameters play important roles in the development of tectonic fractures in the sedimentary basin with less faults and folds. The 3D models of the multi-period tectonic stress fields and density calculation modelling are not only significant for tight gas sandstone reservoirs but also for predicting of other reservoir fractures.