Numerical Study on the Prediction of “Sweet Spots” in a Low Efficiency-Tight Gas Sandstone Reservoir Based on a 3D Strain Energy Model

The study of the construction of a micro-scale rupture parameter from the perspective of rock stress and strain is a frontier in geoscience. The strain energy density (U) can quantitatively characterize the probability of internal micro-scale ruptures in different types of rock. Based on this, in this paper, a systematic forecasting method for tight sandstone sweet spots in a low-amplitude tectonic zone based on U -value calculations was proposed. The specific steps are as follows. First, a geological model of the target layer was created, and a new rock mechanics parameter assignment method based on sedimentary facies control principle was proposed. Then, the palaeo-tectonic stress field of the target layer in the Yanshanian period was recovered through the boundary loading. Finally, the strain energy density distribution of the target layer was obtained based on energy conservation principle. The simulation results of the paleo-tectonic stress field show that, the distribution of horizontal stress is mainly affected by lithology and local structure, and the vertical stress is mainly affected by the burial depth. Stress diffusion occurs in some areas, which are mainly affected by lithologic mutations or complex structures. The U values of the target layers have a band-like distribution and are mainly distributed between 0.12 and 0.30 J·m
^-3
. The relationship between strain energy density and productivity of tight sandstone reservoirs was analyzed, and the criteria for distinguishing sweet spot areas based on U values were proposed. This method is applicable to strongly heterogeneous tight sandstone reservoirs in the low-amplitude tectonic zone of the Ordos Basin and have reference values for similar types of reservoirs around the world.