Steamdrive Stability in High Porosity Rock

Abstract

Abstract Steamflood displacement is considered to be stable, as compared to other gasflooding techniques. Stability on the microscopic and core scale is thought to lend stability to steam movement on the reservoir scale. High mobility displacing agents tend to "finger" beyond the front, but this effect is suppressed for steam by condensation within the finger as steam contacts lower temperature reservoir. Heat transfer from the steam to the surrounding connate water, oil, and rock is the primary mechanism to ensure stability. Because most reservoirs have a significant proportion of rock (70%-80%), there is sufficient thermal mass to cool any steam fingers. However, when the porosity is high, there is less rock to dissipate heat; therefore, less suppression of viscous instabilities. Depending on the porosity and other reservoir properties, the typical assumption of stable steamflood displacement may not be valid. Initial aspects of this work use analytical methods to examine frontal stability in high porosity rocks. Analytical methods indicate stable steam fronts for typical reservoir properties, but they also predict unstable displacement in high porosity rock. The second portion of this work uses pattern level thermal simulation to study the stability of steam drives in high porosity rocks. Two different permeability models (an isotropic model and a thief model) are compared at three porosities (25%, 50% and 70%). Low rock thermal conductivity is shown to reduce the suppression of the steam fingers. More importantly, the simulation results show that as the porosity increases the stability does decrease. This is exemplified by shorter breakthrough times for the higher porosity models. While the stability of a steam front does decrease, the difference is likely not very significant, around a 20% reduction in breakthrough time, compared to other aspects. For example, reservoir heterogeneity has a much greater impact on breakthrough time than the relatively unstable displacement front associated with high porosity rock.