Effect of injection rate, initial water saturation and gravity on water injection in slightly water-wet fractured porous media

The objective of this work was to evaluate methods for improved water injection in fractured carbonate rocks by performing experiments on outcrop core. The experimental setup consisted of a composite of 3 limestone outcrop cores with a total length of 116 cm and average diameter of 6.5 cm. The composite core (matrix) was placed in the center of a plexiglass cylinder, with the annulus between the core and the cylinder wall serving as the fracture. Water injection tests were performed by injecting water from the bottom and producing from the top. This system experienced an advancing oil/water level in the fracture. The composite core was completely immersed for 5-7 days after finishing the experiment; creating changing boundary conditions from partially to fully water immersed. Immersion type experiments were also performed separately. The effect of water injection rate and initial water saturation on oil recovery by imbibition mechanism was investigated experimentally. Water injection tests were performed at three different rates and three different initial water saturations. Breakthrough oil recovery was found to be higher when the water injection rate was low, whilst final oil recovery was less rate dependent. Alteration of initial water saturation was observed to exert minimal effect on final oil recovery, and any observed effect could be classed as experimental error. However, the rate of oil production increased with increasing initial water saturation. The results confirm that oil recovery by water injection is low in this rock type, due to oil retention by capillary forces and wettability effects. Therefore, to overcome oil retaining forces, the effect of reduced inter-facial tension (IFT) on oil recovery was investigated experimentally, by choosing a low tension, equilibrated fluid system. This will increase gravity and decrease capillarity effects on the core, which in turn will result in improved recovery. The experimental results indicate that oil recovery is increased mainly due to the decreased ratio between capillary and gravity forces. Li and Home analytical model, which incorporates both capillary and gravitational effects, was used to scale IFT and dimension successfully. Scaling results indicated that IFT and core dimension, for all experiments, could be scaled by using height of the core as the shape factor. However, using characteristic length (L-a) as the shape factor, IFT scaling worked well but dimension could not be scaled. (c) 2007 Elsevier B.V. All rights reserved.