Abstract

Non-thermal enhanced oil recovery (EOR) techniques show a great potential for recovering oils from the thin and shaly heavy oil reservoirs of Saskatchewan. Among the non-thermal processes, immiscible carbon dioxide injection holds the most promise of accessing these reservoirs. This technique, however, is much less developed than thermal methods. The process, if proven applicable to Saskatchewan reservoirs between three and seven metres thick, will access approximately 90 percent of the total oil in- place.

The paper is divided into two sections. The first deals with the characterization of a Kintfersley area heavy oil. The characterization includes analysis of: stock tank oil with and without additives, recombined reservoir fluid, and reservoir fluid plus carbon dioxide. The second section describes a scaled physical model, and two displacement experiments conducted using a Lloydminster area heavy oil.

The laboratory phase behaviour data were generated to show lhe effect of pressure and temperature on carbon dioxide solubility, oil density and viscosity, compressibility, and swelling factors. The viscosity of the reservoir fluid at 25.5 °C was reduced from 819 mPa.s to 45 mPa.s with the addition of 73.1 sm3 m3/ of carbon dioxide al 7 MPa, an eighteenfold reduction. The same reduction in viscosity would require heating the sample to approximately 80 °C. The above oil, under similar conditions, increased in density from 963.0 kg/ m3 to 974.3 kg/ m3 and swelled approximately 15%.

Two scaled model experiments (secondary displacements) were conducted using a 10-cycie water-alternating-gas (WAG) process with a WAG ratio of4:1. In each run, the total mass of carbon dioxide injected was 1.41 g-mol (0.53 PV at 25 MPa, 0.30 PV at 4.1 MPa). The scaled model displacements indicated the immiscible carbon dioxide WAG process to be partially sensitive to the operating pressure in the range of study. More important is the relative volume of carbon dioxide, at experimental conditions, which dictates overall performance.

Introduction

Carbon dioxide flooding appears to be the only non-thermal recovery process that holds promise of allowing access to the typically-thin reservoirs in which most of Saskatchewan's heavy oil is found. Thermal methods are often inefficient and uneconomical due to excessive vertical heat losses, because of thin pay zones, and steam scavenging by bottom water zones.

Carbon dioxide may behave as a miscible or immiscible fluid when contacted with oil at reservoir conditions. Holm1 defines miscibility as follows: "For petroleum reservoirs, miscibility is defined as that physical condition between two or more fluids that will permit them to mix in all proportions without the existence of an interface. If two, or more, fluid phases form after some amount of one fluid is added to others; the fluids are considered immiscible and an interfacial tension exists between the phases."

Moderately viscous heavy oils (10–15 °API) lack the necessary extractable hydrocarbons (C5-C30) for miscible conditions to be economically attained. In some cases, moderately light oils (25–35 °API) are being displaced immiscibly because the pressures required to achieve miscible conditions would severely fracture the formation.

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