Abstract
This study clarifies the seepage characteristics of complex fractured pressure-sensitive reservoirs, and addresses a common technological problem, that is the alteration of the permeability degree of the reservoir bed (known to be responsible for changes in the direction and velocity of fluid flows between wells). On the basis of a new pressure-sensitive equation that considers the fracture directional pressure-sensitive effect, an oil-gas-water three-phase seepage mathematical model is introduced, which can be applied to pressure-sensitive, full-tensor permeability, ultralow-permeability reservoirs with fracture-induced anisotropy. Accordingly, numerical simulations are conducted to explore the seepage laws for ultralow-permeability reservoirs. The results show that element patterns have the highest recovery percentage under a fracture angle of 45°. Accounting for the pressure-sensitive effect produces a decrease in the recovery percentage. Several patterns are considered: inverted five- seven- and nine-spot patterns and a cross-row well pattern. Finally, two strategies are introduced to counteract the rotation of the direction of the principal permeability due to the fracture directional pressure-sensitive effect.
Highlights
Based on Warren and Root’s single-phase seepage equation, in 1976, Kazemi et al [1] developed a tool for simulating the multi-well single-phase and two-phase seepage of fractured reservoirs
Zhao et al [24] revised the effective stress value of a coal mass after hydraulic fracturing by applying the principle of statistical damage mechanics; they built a model for describing the liquid-solid coupling of coal seams in hydraulic fracturing (Fig. 11)
Fu [26] considered the elastic deformation of reservoir bed mediums, and established a liquid-solid coupling mathematical model of the discrete fractured-vuggy model and related numerical simulation method (Fig. 12)
Summary
Based on Warren and Root’s single-phase seepage equation, in 1976, Kazemi et al [1] developed a tool for simulating the multi-well single-phase and two-phase seepage of fractured reservoirs. In 1987, Chen et al [3] coupled the mathematical equations of shafts, and developed a simulator for the numerical simulation of the dual-medium, three-dimensional, three-phase, and multi-component thermal recovery of fractured reservoirs based on the fully implicit solution. Ding et al [12] performed the sophisticated numerical simulation of late-stage reservoirs under development with spatially partitioned, temporally segmented characteristic parameters These methods have considered the differences in fracture parameters of different regions, as well as changes of physical parameters of reservoir beds (e.g., porosity, permeability, and phase seepage) with time in reservoir development. Zhang et al [14] built a two-dimensional, two-phase seepage mathematical model for fractured reservoirs that considers minimum and pseudo-starting pressure gradients, and solved them using the alternative direction implicit method. Improving the seepage theory of ultralow-permeability fractured pressure-sensitive reservoir provides vital guiding significance for improving the development effect of reservoirs
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