Abstract
A main mechanism of oil production in naturally-fractured reservoirs is gravity drainage in the gas-invaded zone, where the pressure is below the saturation pressure and the matrix blocks are surrounded by the gas in the fractures. We investigated immiscible gas injection in a single-matrix block using a 2D glass-etched micromodel under the influence of gravity. Two different micromodels containing distinct patterns of the matrix-fracture system were constructed. The fractures were either parallel to the macroscopic flow or perpendicular to it. Extensive experiments on two-phase flow of oil and gas through the micromodels were carried out over a range of the capillary and Bond numbers. The displacement patterns were recorded by a camcorder equipped with a microscope. The images were then processed with an image processing software in order to estimate the initial and residual saturations of the two phases in each experiment. To study the effect of the fractures, we also performed the experiments in the same micromodels but without the fractures.
Our experiments indicate that the residual oil saturation is minimum in the case of natural depletion without gas injection, if the macroscopic flow is aligned with the gravity. However, when the system is inclined at 45 degree (relative to the direction of gravity), or when gas is injected into the system at different angles with respect to the direction of the gravity, the residual oil saturation is more than twice that of the natural depletion. The behavior of the displacement by gravity drainage of oil during gas injection is between two extreme limits of displacements. One is invasion percolation with trapping in which fractal displacement patterns develop, and significant trapping of the displaced fluid takes place. The other limit is similar to diffusion-limited aggregation in which there is no trapping at all, and the displacement pattern is a branched structure.
