Visualization and Simulation of Imbibition in Matrix-Fracture

Abstract

To predict the ultimate oil recovery more realistically from naturally fractured reservoirs during water displacement, the knowledge of flow behavior in the matrix-fracture systems at pore level is very important. The main purpose was the visual assessment of some unclear theories that have been put forward to explain the production behavior of fractured oil reservoirs with water drive or during water injection. Two series of experiment were implemented on transparent glass-etched micromodel; one without the gravity effects at various capillary number values (Nc) and mobility ratios (M), and another under the gravity effects at different Nc and constant M. In the experiments without effects of gravity; it was observed that at low Nc the wetting fluid preferentially invaded the matrix. Two critical capillary numbers were identified, one for the start of penetration in the fracture when the viscosity ratio was much less than one, and another for which the rate of propagation of the front in the fracture is the same with that in the matrix, when the viscosity ratio was greater than one. These critical capillary numbers were well matched with the results of a pore network simulation. Free imbibition in fractured system was also investigated and compared favorably with pore network simulation. This process first involves the rapid invasion of the matrix, followed by the subsequent penetration of the fracture. A simplified theory was also developed for the experiments. All experiments under gravity effects were implemented on three types of micromodels with different fracture orientations and gravity effects. The amount of oil recovery from the matrix-fracture systems at various capillary numbers was determined and the results were compared with the model with no fracture. The results suggest that the amount of oil recovery by water displacement under the water-wet condition of our model is not very sensitive to the fracture orientation.