An Examination Of Steam-CO Processes
Authors: Stone T. , Ivory J. 
Published: March 2010

Carbon dioxide has been shown to enhance the recovery of bitumen/heavy oil from tar sands under certain experimental conditions. Some properties of CO2 as an additive are known, but an understanding of all the mechanisms behind recovery processes with this additive is incomplete.

A large, 150-cm diameter pressure vessel was used to conduct a number of un-scaled steam-carbon dioxide experiments. These experiments demonstrated the utility of using CO2 as an additive in order to improve the recovery a bitumen from the Athabasca oil sand.

Three of the experiments are discussed. These experiments employed (i) CO2 presoak followed by steam injection; (ii) steam-CO2 co-injection; and (iii) steam, CO2 and solvent co-injection. All three experiments were cyclic and employed a drive followed by a drawdown.

These experiments were analyzed using a multi-component numerical simulator which incorporates equation of state predictions and also contains accurate fluid property data. This analysis has suggested mechanisms by which the performance of carbon dioxide presoak, co-injection and slug processes were enhanced in the experiments being discussed.

Introduction

Considerable effort has been devoted to understanding the physical mechanisms behind the action of carbon dioxide in a bitumen or heavy oil recovery process from tar sands. The ability of CO2 to dissolve in the oil, thus reducing its viscosity, and the ability of CO2 to transport in a gas zone within a reservoir and research a bitumen/heavy oil rich area at the edge of the zone, are promising properties that warrant research into ways to best utilize these properties.

Experiments discussed in this paper were previously analyzed by Ivory et al. (1985). This analysis has now been extended by the use of a numerical model which permitted a better understanding of the physical mechanisms behind these steam-CO2, based processes. The experiments were carried out in a large, 150-cm diameter pressure vessel which was capable of holding approximately four tons of oil sand, in which stimulation processes could be studied at reservoir conditions. The oil sand was considered to represent an element of a reservoir adjacent to a horizontal fracture. All experiments employed a communication path with a permeability that was higher than the oil sand, in order to physically simulate a high permeability flow path under field conditions. The effectiveness of carbon dioxide as an additive for improving bitumen recovery under these conditions is the subject of this paper.

Firstly, the configuration of the experiments is discussed. This is followed by a discussion of experimental operating procedures. The numerical model used in this study is then described, including important physical properties. Following this, numerical simulation of the experiments is considered. Results of these tests are then used to ascertain the mechanisms underlying the efficiency of the presoak, co-injection and slug processes.

Experimental Facility

The 150-cm diameter simulator system (Fig. 1) consists of a large pressure vessel (150-cm diameter by 270-cm high) designed to allow steam and gas to be injected simultaneously into the test bed under controlled flow rates. Oil sand is packed into this vessel and maintained at field overburden conditions.