Abstract

The success of cold heavy oil production in Canada and elsewhere has greatly stimulated interest in foamy oil flow. It is generally accepted that, in Canadian heavy oil reservoirs, sand production is the trigger needed for inducing foamy flow during primary production under solution gas drive. Sand production results in the formation of a zone of high permeability surrounding the well-bore which in turn increases the pressure gradient further upstream in the reservoir. This higher pressure gradient beyond the zone of enhanced permeability induces foam formation. This process can be called enhanced solution gas drive (ESGD).

If this concept of foam generation by increased pressure gradient away from the well-bore is correct then it can be suggested that the same should happen in other situations that also result in shifting of the high pressure gradient zone away from the well-bore. For example, it should also happen in cyclic steam stimulation because the fluid mobility in the immediate vicinity of the wellbore becomes very high due to the increase in temperature and the zone of high pressure gradient is likely to be just beyond the heated zone.

The objective of this work was to examine whether or not foamy flow can be induced in the upstream section of a reservoir by increasing the fluid mobility in the downstream section by heating. A long sand-pack apparatus was designed to allow electrical heating of a growing zone starting at the production end during solution gas drive tests. The sand-pack was equipped with several intermediate pressure taps to measure the pressure profile within the sand during the depletion tests. The performance of depletion tests carried out with electrical heating was compared with similar tests carried out without any heating (isothermal tests). Results show that heating of the downstream section of the sand-pack enhances foamy flow behaviour in the upstream sections.

Additional tests were done under steady-state flow conditions to measure the change in pressure gradient in the cold segment due to the heating of the upstream segment. It was found that the effect of heating is analogous to sand dilation in the sense that both result in much sharper pressure gradients beyond the affected zone. These results show that foamy flow can be a significant mechanism in cyclic steam stimulation and should be included in predictive models.

Introduction

Heavy oil producers have learned from experience that many unconsolidated sand heavy oil reservoirs, exploited with vertical wells under primary depletion conditions, perform better when sand is allowed to freely flow into the wells(1). Production of sand along with the oil is now considered an integral part of the cold production technique as practised in Western Canada(2–4). It is also known that the solution gas drive recovery factors obtained in heavy oil reservoirs before sand production became popular used to be around three percent. The projected recovery factors with sand production are higher than ten percent(5). Therefore it is apparent that sand production not only improves the rate of oil production, it also has some bearing on the final recovery factor.

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