Interfacial Effects in Gas-Condensate Recovery and Gas Injection Processes

Abstract

Gas condensate production and gas injection processes are strongly influenced by the gas/oil (or condensate) interfacial tension and by the wetting behavior of oil on the porous substrate. Oil (condensate) recovery is favored by low gas/oil (condensate) interfacial tensions and by complete wetting of oil (condensate)on the water phase that often covers the porous rock. The relevant parameters are dimensionless numbers that measure the importance of the oil/gas interfacial tension relative to gravity (Bond number) and to viscous (capillary number)forces. Another important parameter is the oil/gas contact angle on the water that often covers the porous substrate: a zero contact angle corresponds to complete wetting of oil, while a finite angle corresponds to partial wetting. These parameters are increasingly used in modern reservoir simulators for estimating oil and gas residual saturations and relative permeabilities. We analyze how these parameters vary with the operating conditions (e.g., pressure) and the nature of the injection gas: CH4, CO2 and N2. On approach to gas/oil complete miscibility, both the capillary and Bond numbers diverge, i.e., viscous and buoyancy forces dominate over capillary forces. For near-miscible conditions, these numbers obey simple scaling laws as a function of the distance to complete miscibility. The prefactors of these scaling laws depend on the particular oil+gas system, which allows to classify the efficiency of the different injection gases. For reducing capillary forces with respect to the viscous or buoyancy forces, supercritical CO2is more advantageous than off-critical CO2 or CH4, which themselves are more advantageous than N2. We also present a simple calculation scheme for predicting the conditions under which the transition to complete wetting of oil on water occurs, i.e., the oil/gas contact angle vanishes. The only ingredients needed are the composition and densities of the three coexisting phases (water, oil and gas), calculated by means of an appropriate equation of state. The essential result is that under typical operating conditions, oils or condensates spread on water, the transition to partial wetting being remote from complete miscibility conditions. CO2 is the most effective in promoting the wetting(or spreading) of the oil phase on water, followed by CH4 and then by N2. Introduction Interfacial forces play an important role in various oil recovery schemes, starting with primary oil recovery. The gas/oil interfacial tension and the wetting behavior of oil in the presence of gas control the distribution of the oil and gas phases within the pore space; therefore these quantities affect the two phase flow parameters: capillary pressure, phase permeabilities and the quantity of oil remaining after drainage with gas. These interfacial properties are strongly dependent on thermodynamic(e.g., pressure or composition) conditions. In the production of near-critical gas condensates or volatile oils and in near-miscible gas injection processes, variations of the gas/oil (or condensate) interfacial tension (IFT) by several orders of magnitude are not uncommon. Upon such variations, the flow regime changes from an emulsion-like flow at very low IFT to a capillary-dominated flow at high IFT. These changes are reflected in the multiphase flow parameters. Phase permeabilities are lower and residual saturations are higher for capillary-dominated flows (high IFT); for very low IFT residual phase saturations tend to zero and relative permeabilities tend to the corresponding phase saturations.1,2 The wetting behavior of oil changes with thermodynamic conditions as well, a transition from partial wetting (i.e., the oil forms lenses on the porous substrate) to complete wetting (i.e., a thick oil layer covers the susbstrate) being expected when the conditions for complete oil and gas miscibility are approached.3 This transition is referred to as the wetting transition. Important changes in the flow processes are expected at the wetting transition.4,5

Investigation of Underground Sour Gas Storage in a Depleted Gas Reservoir

“…Previous studies [6,9] confirmed this trend. Melean et al [9] studied the effects of thermodynamic conditions on the interfacial properties.…”

“…Previous studies [6,9] confirmed this trend. Melean et al [9] studied the effects of thermodynamic conditions on the interfacial properties. Relevant parameters which they studied are dimensionless Bond number and capillary number to relative interfacial properties to gravity and viscous forces, respectively.…”

“…Changes in the wetting behavior of reservoir rock and interfacial tension between the storage gas and reservoir fluid are other important issues that arise during UGS. Melean et al [9] investigated the influence of gas/condensate interfacial tension and wetting behavior of condensate on the porous substrate. They found that CO 2 is more advantageous than CH 4 and N 2 in reducing capillary forces and promoting the spreading of the condensate phase on water.…”

Laboratory Investigation of Enhanced Light-Oil Recovery by CO/Flue Gas Huff-n-Puff Process

“…The following mechanisms have been mentioned in the literature: (a) oil viscosity reduction; (b) oil swelling; (c) solution gas drive; (d) relative permeability hysteresis due to reduced water saturation, drainage/imbibition, and wettability alternation; (e) repressurization; (f) diffusion; and (g) interfacial tension reduction in the zone near the wellbore. (5)(6)(7) …”

Transition temperatures and contact angles in the sequential-wetting scenario of n-alkanes on (salt) water

“…In this state, the efficiency of oil recovery by drainage from a model reservoir is also similar to that from a system in the complete wetting state [3]. On grounds of dynamic arguments, however, one expects considerable differences to the flow in a system with a truly macroscopic wetting layer [14]. Therefore, accurate knowledge of both transition temperatures is highly desirable.…”