Numerical modelling of the dynamic process of oil displacement by water in sandstone reservoirs with random pore structures

Abstract

In order to maintain the production rate of a reservoir and improve the displacement efficiency, it is crucial to have an in-depth understanding of the process of oil displacement by water. However, with respect to the conceptualization of porous media of a reservoir, very limited efforts have been made to the pore structures inside the reservoirs. In this paper, the pore structures of a sandstone reservoir were generated by using the method of random growth algorithm. Based on the randomly generated model, a theoretical model to describe the dynamic process of oil displacement by water in the sandstone reservoir was established, and then corresponding numerical modelling was performed. The effects of the displacement velocity, the viscosity ratio of oil-water phase and the porosity of reservoirs on the displacement performance were also analyzed. Results show that due to a great difference in the viscosity between oil and water phases, the moving interface of water phase is not uniform, and the viscous fingering occurs, tending to proceed along the direction with the minimum flow resistance. There is not a linear relationship between the displacement velocity and the displacement efficiency. Too high displacement velocities do not lead to much better displacement efficiency, while a higher pressure drop is caused. Choosing a proper displacement velocity is indispensable in practical engineering. A lower oil-water viscosity ratio is more favorable to obtain high displacement efficiency. Under the present simulation conditions, when the viscosity ratio is 1.2, the displacement efficiency reaches 96.2% at a moderate Reynolds number. The porosity is not a sole factor determining the displacement performance. Even for the same porosity, the shape and length of preferential flow paths are different and randomly distributed, causing a different displacement performance. A large tortuosity tends to result in a low hydraulic conductivity and displacement efficiency.

Keywords

• sandstone reservoirs
• random pores
• random generation-growth algorithm
• oil-water displacement
• displacement efficiency