Abstract

The efficiency with which oil is displayed by water during a water flood can be defined as the ratio of the volume of oil displaced to the volume of oil originally in place. Recovery efficiency is affected by the pore geometry of the rock as well as by fluid-rock and fluid-fluid interactions. In this presentation, water is considered as the wetting phase and oil as the non-wetting phase.

Little attention appears to have been given to identifying and quantifying those aspects of pore systems in rocks which affect the recovery efficiency of reservoirs. Four aspects of pore systems appear to be particularly important:

1. pore to throat size contrast

2. throat to pore coordination number

3. non-random heterogeneities

4. surface roughness of pores

Large pore to throat size ratios affect non-wetting phase displacement efficiency adversely. Pore systems with large pore-throat ratios commonly form as a result of dissolution during the development of secondary porosity in both sandstones and carbonates.

Throat to pore coordination number is a measure of the connectivity of a pore system and large coordination numbers are thought to favour high recovery efficiency. However, the lack of reliable information on coordination number for pore systems in rocks makes it difficult to assess the effects of this variable on recovery efficiency.

Correlations between total porosity and recovery efficiency have been demonstrated and it is thought that an increase in connectivity or coordination number of pore systems, with increase in porosity, contributes to this relationship.

Porosity and pore throat ratio provide a better indication of recovery efficiency than does permeability. Permeability is controlled by the size of the larger conducting pathways in a rock and size may vary independently of the geometric and topologic variables which affect recovery efficiency.

Some types of non-random heterogeneity affect recovery efficiency adversely.

Pore-throat size ratio, throat-pore coordination number, total porosity and type and extent of non-random heterogeneities appear to be the principal attributes of a pore system which affect recovery efficiency. Surface roughness of pores affects wettability and also the displacement process, but the effects are less well known.