Pore Network Modelling of Heavy Oil Depressurisation: a Parametric Study of Factors Affecting Critical Gas Saturation and 3-Phase Relative Permeabilities

Summary

This study deals with the application of pore-scale network modelingtechniques to depressurization in heavy-oil systems. A three-phase simulatorhas been developed to account for the fundamental steps of such adepressurization process, from the nucleation of embryonic bubbles, to theirgrowth by solute diffusion and expansion, to the final stages of coalescence,migration, and production. The model is utilized to examine the impact of avariety of different physicochemical properties—including bubble density andgas-oil diffusion coefficient—and different depletion strategies upon criticalgas saturation and three-phase relative permeability. Results pertaining toheavy oils are also compared to those from light-oil depressurizationsimulations, and a number of important differences are highlighted.

The first sensitivity analyzes heavy-oil depressurization under conditionsof instantaneous and progressive nucleation (IN and PN). In the first scenario,the evolution of gas is characterized by embryonic bubbles that are nucleatedrandomly soon after the bubblepoint pressure is reached. In the case of PN, astochastic algorithm has been developed from related experimental observations.This models the progressive nucleation of additional bubbles as a function oflocal supersaturation (characterized by pore-scale variations in dissolved gasconcentration). It is shown here that IN and PN observations are notnecessarily contradictory—the model reconciles the two phenomena and shows howeach relates to the underlying experimental parameters.

The vast majority of depressurization studies reported in the literature todate have tended to focus upon light oils and binary systems. However, therather different characteristics of heavy oils---high interfacial tension, lowgas/oil ratio (GOR), and high oil density---suggest that such studies may notbe particularly representative of heavy-oil depressurization. In order toinvestigate this issue, a second set of sensitivity analyses is performed,whereby the network model is used to compare critical gas saturations andrelative permeabilities arising from the depressurization of light and heavyoils. It is found that substantial differences can arise in the two systems. Asa consequence, improved recoveries (with higher critical gas saturations) arepredicted for some heavy-oil systems.

Introduction

Heavy-oil solution-gas drive is not a well understood production mechanismbecause a wide range of different petrophysical parameters and experimentalfactors interact in a rather complex way. The work presented here attempts toclarify the situation by examining these interactions in the controlled settingof numerical simulation.

Heavy oils can be very viscous (viscosities of thousands of centipoise areoften reported) and are characterized by relatively low gas diffusivities.Their high asphaltene content, low dissolved gas/oil ratio, and high(essentially pressure-invariant) interfacial tension (IFT) are other well-knowncharacteristics.

At the production stage, additional characteristics becomeapparent---heavy-oil depressurization often results in higher-than-expectedrecovery (up to 15% of the original oil in place 1 ), and foamy oils are oftenobserved at the wellbore. This nonequilibrium foamy oil is composed of a largenumber of small bubbles dispersed in the oleic phase (approx 1 bubble per grain2 ; approx 1.5x10 13 bubbles per ml 3 ). This is thought to provide additionaldriving energy for oil production and consequently leads to increased recovery.Associated critical gas saturations can be very high (up to 40% reported byMaini et al. 4 ) as the formation of a continuous, free-flowing gas phase isdelayed. 5

The knowledge base relating to heavy-oil depressurization has been expandingin recent years, but the exact physical and chemical processes characterizingthese reservoirs are not yet entirely understood. Contradictory experimentalobservations and heuristic interpretations are often found in the literature.For example, some authors do not see any evidence of foamy oil behavior, andothers simply highlight very low gas mobility and high oil viscosity as themain reasons for better performance--- low critical gas saturations are alsoreported. 6--8

Micromodel experiments by Maini 1 and Bora et al. 9 show importantdifferences in the numbers of bubbles as the depletion parameters change. Fastdrawdown rates lead to a high bubble density (several orders of magnitudehigher than at the lower rates), although this is mainly as a result of bubblebreak-up caused by high viscous pressure gradients. Micromodel observationsalso report high supersaturations for bubble nucleation in heavy oils, oftenaccompanied by reduced bubble coalescence, thought to be caused by highasphaltene content. Bora et al. 9 also point out that nucleation within theirheavy oil is not instantaneous.

The high values of viscosity are believed to be a key factor in theheavy-oil depressurization process. Among the first to assess this effect wasHandy, 10 who obtained higher recoveries with viscous oils during solution-gasdrive. He also underlined how low diffusion coefficients---typical of highviscosity oils---may superficially produce the same effect as an increaseddepressurization rate. In their micromodel experiments with both heavy andlight oils, Kamp et al. 11 found higher levels of supersaturation for heavyoils caused by very low diffusion rates (once again, correlated to highviscosity).

Lago et al. 2 studied the depletion of a series of heavy oils and a lighteroil---more bubbles were seen to nucleate in heavy oils. An upper limit for thenumber of bubbles nucleated was estimated to be 1 bubble/grain, but this highdensity could again be caused by bubble fragmentation. Nucleation was observedto be progressive, with a continuous increase in the number of bubblesthroughout the depletions (although this was less marked for the lightoil).

Two different nucleation mechanisms were observed by Tang and Firoozabadi 12: IN for low-GOR heavy oils and progressive nucleation PN for high-GOR heavyoils. Increasing GOR would be expected to result in higher bubblepointpressures and lower IFTs. These, in turn, could lead to prolonged low levels ofsupersaturation and the possibility of progressive nucleation.