Proposed Screening Criteria For Gas Injection Evaluation
Authors: Thomas B. 
Published: March 2010

One of the most important processes for revitalization of mature reservoirs is enhanced oil recovery (EOR) by gas injection. However, some engineers do not understand the science and, consequently, target reservoirs for gas EOR which do not possess appropriate character.

Six parameters which can be used to screen reservoirs for gas injection potential are proposed in this article. Using the screening criteria, three reservoirs have been evaluated. The results of the evaluation are consistent with observed reservoir performance on EOR.

Displacement Paradigms

Dullien(1) described the features of importance in multiphase flow in porous media. Whereas many practitioners of reservoir engineering have tended to employ global observations to infer small-scale response, Chatsis(2), Diaz(3), Kwiecien et al.(4), Kantzas et al.(5), and Ionnadis et al.(6,7,8) have focussed on the pore level to describe macro-scale performance. A host of other researchers have also contributed to the body of insightful literature which helps us to better comprehend flow in porous media. Indeed, Feder(9), Mandlebrot(10) and others have begun to see flow in porous media as a complex phenomenon to which fractal mathematics may hold an insightful eye.

The academics and the practitioners agree on which parameters govern fluid flow in porous media. Although it may not be exhaustive, the set of parameters to consider is:Phase Behaviour,Interfacial Tension (IFT),Mobility Effects,Pore Size Distribution,Gravity,Wettability.

These are not in any order of priority, but every reservoir is dominated by one or more of these parameters. Each will be discussed briefly.

Phase Behavior Effects

Some important considerations with respect to phase behavior are the extent of gas evolution with depressurization, organic solids deposition tendencies and the presence/absence of an initial gas saturation.

Interfacial Tension Effects

To displace oil from a pore the applied pressure drop must be at least as large as the capillary pressure. As shown in Equation 1, as the interfacial tension (IFT) decreases the capillary force decreases and as the pore throat diameter decreases the capillary force increases.

Equation 1 (available in full paper)

If Figure 1 portrays pore volume versus diameter and if accessibility equates to displacement then all oil contained in porous features greater than Dw/o will be produced, where Dw/o corresponds to the IFT of water and oil. From this model, if the gas/oil IFT were lower than the water/oil IFT, the gas should then access porous features of a smaller diameter than did the water, thus recovering more oil. Equation 1 only provides the relationship between three parameters which must be considered in understanding gas injection strategies. Parameters causing a deviation from accessibility/ displacement proportionality are many. Mobility effects will be discussed first.

Mobility Effects

Another parameter, generally recognized as being of extreme importance, is mobility.(11,12,13) The common fractional flow equation (data) describes the flow of oil and displacing fluid as

Equation 2, 3 (available in full paper)

Collins(14) showed that if dispersion is neglected, the length of a "viscous finger" is

Equation 4 (available in full paper)