Abstract

There are abundant clay occurrences in the oil sand reservoirs in the Clearwater Formation at Cold Lake. It has been suggested that hydrothermal reactions of the clay minerals may cause permeability damage during thermal recovery. To quantify such the damage, two corefloods (BCF1 and BCF2) were conducted at 250 °C. During the corefloods, three periods of permeability damage were recognized. The first period of permeability damage occurred during and shortly after the core was heated to 250 °C. Experimental evidence suggests that thermally activated grain crushing and subsequent fines migration were responsible for this initial permeability decrease. The second period of permeability damage was a gradual process but resulted in 65% (for BCF1) to 78% (for BCF2) permeability loss. Hydrothermal reactions (berthierine to Fe-saponite) are considered to be responsible for this significant permeability damage. The third period of permeability damage occurred when fresh water was injected into the core. Osmotic swelling of the Fe-saponite (the hydrothermal clay formed during the coreflood) was considered responsible for this damage.

A recent study by McKay and Longstaffe (1994) indicated that berthierine was still present in a post-steam core 20 m away from a injection well in Cold Lake area. The inconsistency between laboratory experiments and field observation may be due to the kinetics of the reactions. The rate of silicate/aqueous solution reaction is 20 times slower at 150 °C than at 250 °C. Thus, the lack of field evidence for the berthierine reactions may be due to that the cores sampled so far are not close enough to the well bore where conditions most favor such a reaction. There is some field evidence for the disruption of berthierine grain coats and permeability damage due to subsequent fines migration. Regardless of the mechanism by which berthierine impacts permeability, field and experimental results suggest that completion intervals should avoid berthierine-rich horizons.

Introduction

It has been widely recognized that the clay minerals can cause formation damage to hydrocarbon reservoirs. Berthierine is among the most abundant digenetic clay minerals in the Clearwater Formation at Cold Lake (Abercrombie, 1988; Longstaffe, 1990; Hornibrook and Longstaffe, 1996). Petro graphically, the berthierine in the Clearwater Formation is present as coatings on the sand grains. Due to its surface coating nature and Fe-rich composition, the Cold Lake berthierine may react with injection fluids and thus affect reservoir quality (Zhou et. al., 1995).

To demonstrate the effect of hydrothermal reactions on reservoir permeability, two hydrothermal corefloods were performed. This paper describe methods and results of the corefloods, and discusses their implications. Field evidences were gathered to compare with the experimental observations. The cause for the discrepancy between the laboratory and field observations are discussed.

COREFLOOD EXPERIMENTS
Sample Preparation

The oil sand sample used in this study was from Imperial Oil's Cold Lake lease (434–442m, 6–21–65–4W4). The oil sands are semi-consolidated, berthierine-rich, and bitumen-poor. Bitumen in the sands was extracted using methylene chloride. Prior to bitumen extraction, almost all sand grains are coated by berthierine, a Fe-rich clay (Figure la); after bitumen extraction, the grain coating clays are mechanically disturbed and the fines form aggregates in the sands (Figure 1b).

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