Although a number of studies have reported significant effects of temperature on relative permeabilities no consensus has emerged on the generality of such effects nor on possible mechanisms causing such effects. Some of the recent studies have found relative permeability to be independent of temperature and have suggested that most of the reported temperature effects can be attributed to artifacts of the unsteady-state technique. The objective of this study was to critically examine the use of the unsteady-state technique for measuring relative permeability in heavy oil systems and to experimentally determine the effect of temperature on relative permeability curves for a clean silica sand/heavy crude oil/deionized water system.
Unsteady-state measurements were carried out in a 45 cm long, 5.6 cm diameter sand core at five different temperatures ranging from room temperature to 200 °C using a heavy crude oil and deionized water. It was found that the unsteady-state technique when employed in heavy oil systems is more susceptible to experimental artifacts (compared to its use in light oil systems), however, a careful analysis of the displacement data can provide meaningful relative permeability curves in spite of the inevitable artifacts.
The relative permeability curves derived from production and pressure drop histories of the displacements at different temperatures showed that, in this system, relative permeability curves vary with temperature. The endpoint water permeability as well as the effective water permeability at intermediate saturations increased with increasing temperatures. The endpoint oil permeability was found to be independent of temperature. While the shape of the oil relative permeability curve displayed a complex dependence on temperature, its significance remains uncertain due to the presence of several artifacts.
Production of oil from petroleum reservoirs usually involves simultaneous flow of two or more immiscible fluids through a porous rock. Multiphase flow in porous media is a complex process that depends on a number of factors including the absolute permeability, pressure drop, capillary pressure, fluid viscosities, and relative permeabilities of each phase. Of these, the relative permeability is probably the most important parameter in determining reservoir performance. For modelling thermal recovery processes for heavy oil recovery, one needs to know not only the relative permeabilities at the original reservoir conditions but also the effect of increasing temperature on the relative permeability curves.
A number of studies have discussed the temperature effects on oil/water relative permeabilities. Unfortunately, no consensus has emerged on the generality of reported temperature effects nor on possible mechanisms causing such effects. A brief summary of the recent literature on the effect of temperature on relative permeability is presented in Table 1. Three out of the nine experimental studies listed in this table show relative permeability to be independent of temperature while the other six report significant temperature effects. Most of the studies that report significant temperature effects suggest that irreducible water saturation increases and residual oil saturation decreases with increasing temperature.
There is obvious disagreement among researchers on the existence of temperature effects and this disagreement appears to be independent of which measurement technique was employed or whether or not crude oil was used in the tests.