Influence of Pore Geometry on Water-Oil Relative Permeability

Abstract

Rock properties that control pore geometry can also affect the relative permeability characteristics of a reservoir. If variations in rock types permeability characteristics of a reservoir. If variations in rock types are ignored, laboratory measurements for predicting fluid flow may be misleading; a common binocular microscope can be helpful in detecting these important variations. Introduction A curve of laboratory-derived water-oil relative permeability data that appears to be anomalous permeability data that appears to be anomalous frequently is suspect and raises questions regarding sampling procedure, core preservation and the method of analysis. It also prompts second thoughts about the pore geometry or the lithologic character of the pore geometry or the lithologic character of the reservoir sample. Hewitt and others have demonstrated that lithologic characteristics control reservoir characteristics. Several investigators have pointed out that relative permeability is a function of pore geometry. It is the permeability is a function of pore geometry. It is the purpose of this paper to illustrate that those rock purpose of this paper to illustrate that those rock properties controlling pore geometry in a reservoir properties controlling pore geometry in a reservoir can also affect the relative permeability characteristics. The use of a common binocular microscope to observe such rock properties can be helpful in establishing the validity of laboratory water-oil relative permeability measurements. permeability measurements. The relative permeability data discussed here were obtained from a commercial laboratory that uses the unsteady-state method. Their calculations employ equations from Johnson et al. Unless otherwise stated in the text, the rock samples were extracted and restored. Air permeabilities have been corrected for slippage (Klinkenberg effect). The photomicrographs are of thin-sections from the ends of the relative permeability plugs. Before being made into thin-sections, these samples were cleaned of oil and impregnated with a colored plastic to emphasize the natural pores. General Relationships of Rock Properties, Pore Geometry, and Water-oil Pore Geometry, and Water-oil Relative Permeability Some rock properties that affect pore geometry are listed in Table 1. They are closely interrelated and affect such reservoir properties as permeability, porosity, irreducible water saturation, and surface area. porosity, irreducible water saturation, and surface area. Because of such a large number of variables, one particular rock property seldom has the same particular rock property seldom has the same magnitude of effect from one reservoir rock type to another. This relationship implies there is an element of risk in using such readily available data as air permeability or grain size alone in attempting to characterize a reservoir. That is, although generalizations about these two can often be made, variation in sorting, cementation, or other factors can obscure their significance. However, the variation of any of these properties and their effects can be evaluated and properties and their effects can be evaluated and perhaps predicted if the reservoir rock is examined perhaps predicted if the reservoir rock is examined microscopically. Maximum use of this elementary approach requires an understanding of the geology of the reservoir. Any variations in rock types within a reservoir must be recognized prior to sampling. If rock variations are ignored, laboratory measurements cannot be expected to characterize or predict the fluid flow of the reservoir adequately. JPT P. 1199