Development and Application of a Simplified Naturally Fractured Numerical Reservoir Model

Summary

In this work, a simplified numerical model for naturally fractured reservoirs (NFRs) is presented, which considers three-phase flow of oil, gas, and water in the vertical direction and accounts for the vertical distribution of the pore volume contained in the primary and secondary porosity systems. A bottom aquifer associated with the reservoir, whose interaction is described through the Fetkovich (1971) model, is also considered. This simplified 1D vertical model applies to NFR of moderate to large thickness and/or structural relief, like those found in Southeast Mexico. The underlying basis of the model is that the flow of fluids in the fractured system occurs under gravity-segregated flow conditions and that under certain conditions, viscous forces dominate the flow of fluids in the vicinity of the producing wells, causing the well-known water and/or gas coning phenomena.

The modeling of gas and water coning is introduced in the simplified NFR simulator through a correlation originally developed by Pérez-Martinez et al. (2012) for water coning, extended in this work to gas coning after reproducing results published by Rodríguez et al. (2012) for gas coning phenomena. As the permeability of the fractures strongly impacts the flow behavior of fluids toward wells and coning phenomena, this property is accounted for in the numerical model by considering the 3D distribution of this property in the reservoir, as defined by its static characterization model. Hence, even though the numerical model is 1D vertical, the flow of fluids toward wells is properly modeled in the 3D domain of the reservoir. Oil production constraints on the wells, related to water and gas coning, and minimum bottomhole pressure, are applied by monitoring the production behavior of the wells at each timestep of the modeling.

The validity of the simplified numerical model was verified and proven by comparing results with those obtained from 2D and 3D equivalent numerical models. The model was applied in forecasting the production performance of an NFR under the multiple scenarios required in solving the optimal development plan of a fractured reservoir by using an elitist genetic algorithm (EGA), which involves determining the number, placement, and the vertical position of the producing intervals, as well as the operating conditions of vertical wells in an NFR that prevent water and gas production due to coning phenomena.