Effect of Composition on Waterblocking for Multicomponent Gasfloods

Abstract

Abstract During tertiary miscible gas injection direct contact between gas and oilcan be prevented by water surrounding residual oil. The principal aim of ourstudy is to assess the importance of this waterblocking phenomenon in multicomponent gas injection. We study this process using a multicomponentpore-scale model. Light components in the gas dissolve in the water and diffuse through the water to reach the oil. This causes the oil to swell. Eventuallythe oil swells sufficiently to contact the gas directly. However, components inthe oil can diffuse into the gas, causing the oil to shrink and preventing thecontact. We apply our model to a variety of first-contact and multiple-contactmiscible gas/oil systems from published field studies. Due to the lowsolubility of hydrocarbons in water, oil swelling and shrinkage can prevent direct contact for many days to years. We show that increasing the miscibilityof injected gas, by, for instance, moving from a multi-contact miscible to afirst-contact miscible displacement increases the time taken to achieve directgas/oil contact. This leads to an extended two-phase region in the reservoir, even for a thermodynamically miscible gas flood. Introduction Mass transfer across water barriers by molecular diffusion is an importantprocess during oil recovery by gas injection.1–5 Miscible gas injection is typically applied after a waterflood in which case significant volumes of oil may be trapped within the pore-space in the form of ganglia surrounded by water (Fig. 1). In this case the injected gas may not come into direct contact with the oil and recovery will be reduced, as the gas will notbe able to displace the oil trapped behind the water. This effect has been observed mainly in the laboratory6–9 and istermed waterblocking. Depending upon the relative compositions of the oil and gas, gas components will diffuse into the oil and oil components will diffuse into the gas. If the first mechanism dominates then trapped oil droplets will swell and may ultimately rupture their retaining water barrier, enabling them to be sweptaway by the gas. If the second mechanism prevails then the oil droplets will shrink and the gas will be enriched with oil, although inevitably some oil willstill be trapped by the water. Waterblocking during carbon dioxide (CO2) injection has beeninvestigated theoretically by a number of authors.1–5,10 This involves modeling the transfer of a single component (CO2) diffusingthrough the water into the trapped oil. These studies showed that the time-scales at the pore level over which CO2 diffuses through the water, swells the trapped oil and ruptures the surrounding water is relatively short (~1 day for a 100 m thick water film) mainly because of the high solubility of CO2 in water. Hence it appears that waterblocking is relatively unimportant for oil recovery via carbon dioxide injection over the time-scales of typical recovery schemes (~10 years).