Journal of Canadian Petroleum Technology
Volume 49, Number 2, February 2010, 32-39

Abstract

Immiscible CO₂ injection is a potentially viable method of enhanced oil recovery (EOR) for medium oil reservoirs in southwestern Saskatchewan. The relatively high reservoir pressures could result in a large extent of CO₂ dissolution, significant oil viscosity reduction and oil swelling. Laboratory corefloods were used to compare the performance of different modes of CO₂ injection into a medium-gravity oil system. The following methods were compared: injection of a single CO₂ slug chased by water, simultaneous injection of water/CO₂, and different water-alternating-gas (WAG) cycles.

The results indicate that both a single CO₂ slug and the first WAG cycle in a series produced oil very efficiently, possibly owing to good gas/oil contact at the relatively high residual oil saturation at this stage. The simultaneous injection of water/CO₂ was not as effective as a single CO₂ slug, possibly because the co-injected water shielded the oil from being contacted by the gas. Among the four runs, a coreflood with four WAG cycles recovered the most incremental oil—20.58% initial oil in place (IOIP)—while the co-injection process produced the least (8.91% IOIP).

This experimental study suggests that the immiscible CO₂ EOR process is viable for medium oil reservoirs with relatively high pressures, and that proper process application is important for maximizing additional oil production.

Introduction

Miscible/immiscible CO₂ flooding is an increasingly popular EOR technique. So far, most of the CO₂ miscible/immiscible processes have been applied in light oil reservoirs, and there has been little application in medium oil reservoirs. The in-place resources of medium oils, defined as 20 – 30°API, in Saskatchewan are estimated at up to 1,338 × 10⁶ m³ (8.42 billion barrels)(1). Most of these reservoirs are characterized by thin pay (2 to 10 metres in thickness) and shaly sand, and they have been under production for over four to five decades. The estimated average recovery by conventional waterflood, as listed in Table 1, is only approximately 23% IOIP because this method has generally exhibited early water breakthrough and high water cuts owing to the unfavourably high water-to-oil mobility ratio. By adapting miscible/immiscible CO₂ flooding technologies that have matured in light oil applications, the immiscible CO₂ process is potentially viable for recovering additional oil from these medium oil reservoirs if we can improve its displacement and sweep efficiency(2).

© 2010. Society of Petroleum Engineers

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History

• Original manuscript received: 18 March 2008
• Meeting paper published: 17 June 2008
• Revised manuscript received: 17 December 2009
• Manuscript approved: 30 December 2009