This paper addresses a study of nitrogen injection in naturally fractured reservoirs based on uncertain properties that impact the nitrogen distribution and the oil recovery under gravity drainage mechanism. This research focuses on sensitivity studies based on matrix block sizes and capillary continuity to access their impact on the gravity drainage mechanism. In addition, a study of the matrix grid resolution was performed to investigate the effects of vertical and horizontal subgridding on matrix oil recovery.
This study is a continuation of a research which considered a simulation of nitrogen injection through different scenarios by using the same conceptual model with isotropic and anisotropic matrix blocks, the injection of different gases and the construction of a thermal compositional case study to determine the variations of temperature that mainly occur in the gas cap.
ECLIPSE commercial simulator with compositional formulation and dual porosity option was used for the field simulation case studies. Simulation results considering variations in matrix block sizes with same shape factor indicated that gravity drainage mechanism depends not only on matrix block size but also on reservoir depth and nitrogen arrival time. For all cases studies, highest oil recovery was obtained for slabs with biggest matrix block height.
Furthermore, our sensitivity studies for capillary continuity were performed using several fracture apertures considering block size with 10 ft height. Simulation results indicated that oil recovery is affected by a factor that range from 1 to 2.2 by considering fracture apertures between 10 to 300 microns. Since fracture aperture histogram for the reservoir under study averaged 250 microns; discontinuity capillarity is a good approximation for the reservoir under study. Our simulation results suggest that for a naturally fractured reservoir study is necessary to reduce uncertainty of fracture apertures because it strongly impacts the oil recovery.
Subsequently, we investigate the effects of matrix subgridding in vertical and horizontal directions on total oil production by using a stack conceptual model of two matrix blocks surrounded by fractures. The matrix and fracture systems were divided into a number of Cartesian sub-grids and we used the compositional single porosity model of ECLIPSE 300 to perform our simulation runs.
Simulation results indicated that oil recovery for a grid refinement case study using 3×3×68 gridblocks was higher by a factor of 1.124 than the base case using 3×3×5 gridblocks. In addition, the total oil production for the case using 9×9×68 gridblocks was higher by a factor of 1.09 in comparison with the case using 3×3×5 gridblocks. Based on our simulation results; we concluded that subgridding is specially important when we have gravity drainage as the main oil recovery mechanism.