Simulation of multi-period paleotectonic stress fields and distribution prediction of natural Ordovician fractures in the Huainan coalfield, Northern China

Fractures are effective seepage channels and storage spaces for oil, gas, and groundwater. Understanding the distribution characteristics of fractures can improve the efficiency of water production and the control of water-related geohazards in fractured aquifers. The development and distribution of natural fractures are usually controlled by paleotectonic stress fields. In this study, the data obtained from outcrop, cores, and thin sections were used to determine the development characteristics of such fractures. By doing an analysis of burial history, tectonic evolution, acoustic emission test, rock mechanics test, and conjugate joint analysis, two-dimensional (2D) heterogeneous geomechanical models were established by using a finite element method (FEM) stress analysis approach to simulate paleotectonic stress fields during the Indosinian, Early Yanshanian and Late Yanshanian periods. The effects of the variations of paleotectonic stress fields, hydrodynamic fields, structures, and rock mechanical parameters on the development of fractures could then be identified. A calculation model for the rock rupture rate was established to determine the quantitative development of fractures in different periods. Favorable areas for natural fracture development were determined by examining the relationship between the fracture linear density and pumping capacity of eleven wells. The simulation results indicate that the distributions of the maximum and minimum principal stresses and the maximum shear stress were nonuniform during the Indosinian, Early Yanshanian and Late Yanshanian periods, and high stress and/or low stress were mostly distributed in the fault and fold belts. During the Indosinian period, the tectonic and dissolution fractures were not developed. During the Early and Late Yanshanian periods, the tectonic and dissolution fractures were primarily developed around the fault and fold belts in the NNW-SSE and NWW-SEE directions. Those fault and fold belts are the locations with the greatest development of the natural fractures, which are also the main channels for the migration and accumulation of karst groundwater in the Ordovician formation in the Huainan coalfield. This study provides an important reference for the prevention and control of the Ordovician karst water damage in the study area, and proposes a new method for the prediction of fractured carbonate reservoirs.