Discussion of Interrelationship of Temperature and Wettability on the Relative Permeability of Heavy Oil in Diatomaceous Rocks

Abstract

Schembre et al. (2006) interpret the effects of temperature on diatomite wettability and relative permeability for a specific set of laboratory conditions. These researchers imply that countercurrent imbibition (CCI) of hot water and oil-driven by capillarypressure forces-is the dominant mechanism in the thermal recovery of heavy oil from diatomite. While their results are appreciated and of theoretical interest, additional experimental work would seem to be necessary before conclusions can be drawn about fieldscale key mechanisms and achievable-oil-displacement efficiencies.Several features of the testing procedures used by Schembre et al. (2006) prevent scaling their results from the laboratory to the field: extracted rather than preserved core material was used, deadoil rather than live-oil was used, and the cores were not stressed during testing. Bennion et al. (1985) demonstrated the importance of measuring relative permeabilities using preserved core material, overburden stress conditions, and live crude oil. These types of relative permeability functions-rather than those measured using dead-oil and extracted cores that were unstressed-were very similar to those developed empirically by Coats et al. (1977) and Dietrich (1981) to reproduce cyclic-steam field performance. Bennion et al. (1985) found that laboratory-measured relative permeability curves from preserved core, when performed at reservoir conditions, do not need to be adjusted downward to match low water production typical of cyclic-steam response.Regarding the effects of stress, heating amorphous Opal-A diatomite has been shown to be capable of causing a sample compression of 25% or more and a severe reduction in permeability (Dietrich and Scott 2007). Heating in the absence of a change in effective stress from the initial equilibrium condition causes compaction; heating at elevated effective-stress levels causes much more compaction (Fig. D-1). Sample compression of 25% or more is expected to severely alter the primary pore structure and, hence, the relative-permeability and capillary-pressure characteristics of the diatomite. Wilson (1956) showed that an overburden stress causing only a 5% reduction in porosity of a sandstone core can produce a sufficiently large change in pore size distribution to affect the relative permeability of the core.These facts not withstanding, Schembre et al. (2006) and Tang and Kovscek (2004) purposely use experimental equipment and procedures designed to minimize the effects of compaction, presumably to decouple the relative-permeability and capillarypressure effects caused by changes in wettability from those caused by compaction-induced changes in pore geometry. On the basis of laboratory results interpreted from the evaluation of a decoupled system, the authors provide confident remarks on how thermal reservoir simulation should be performed at the field-scale.Modeling at the field-scale supports a view that the CCI process evaluated by Schembre et al. (2006) plays a minor role in the recovery of heavy oil f...