Insight from MRI and Micro-Model Studies of Transport of Solvent into Heavy Oil During Vapex

Abstract

Mass transport of solvent into heavy oil during Vapex was studied using techniques of Magnetic Resonance Imaging (MRI) and visual glass micro-models aided by advanced image analyses. Whereas experiments involving MRI images of sand-pack provided detailed description of vapour chamber propagation and phenomena such as asphaltene deposition, diffusion and dilution of heavy oil occurring in the transition zone (on scale of several centimetres), micro-models revealed phenomena at pore level. These included episodic nature of de-saturation within transition region leading to mobilization and entrapment of heavy oil in the vapour chamber. It was seen that bulk of diluted/de-asphalted oil drains via sides of the vapour chamber. Episodes of capillary de-saturation within the transition zone cause not only a relatively long residence time of solvent into diluted oil within the reservoir but also provide large surface area for expediting such transport. Consequently, solvent content in the diluted oil within the transition zone could exceed the amount needed for onset of de-asphalting. De-asphalting also seems to be promoted by the presence of connate water. The diluted and de-asphalted oil, in turn, drains near the edge of the vapour chamber as continuous oil column or, within the transition region as discontinuous oil ganglia. With continued solvent transport, oil viscosity as well as, interfacial tension between diluted oil and vapour decrease. As a result, occasionally, interfacial forces holding up diluted oil ganglia are reduced below the prevailing hydrostatic head and episodes of de-saturation are initiated. These episodes may culminate in mobilized oil ganglia coalescing with other ganglia, or with continuous oil column to the production well. Alternately, upon encountering finer sized pores, they could break-up and some oil may again get held up. Propagation of vapour chamber and oil rates during Vapex, therefore, depends upon properties of diluted and de-asphalted oil within the transition region (between the vapour chamber and oil region unaffected by solvent transport). This paper presents evidence for these mechanistic insights and discusses practical implications to operation and design of Vapex projects. Introduction In the Vapex (Vapour Extraction) operations for heavy oil recovery, a condensable solvent (e.g. propane or CO2) is injected into the reservoir via a horizontal injector and mobilized oil is drained via a horizontal producer placed directly underneath it1,2. The solvent is chosen such that it is close to its dew point under reservoir conditions and resulting solvent-oil mixture in vicinity of the vapour chamber, has significantly lower viscosity as compared to the native oil. The main driving mechanism is gravity to help drain the oil thus mobilized (having reduced viscosity) as shown in Figure 1. The main motivation for this work was to investigate role of different factors on oil rates during Vapex operations. Specifically, it was to obtain further understanding of process mechanisms and to identify optimal operating and design factors, including solvent selection.

Optimal Solvent and Well Geometry for Production of Heavy Oil by Cyclic Solvent Injection

“…The simulation results of the three cases, using GEM (11) , were compared, on the basis of equivalent production period (10 years) and equivalent solvent injection rates, using monthly production profiles, cumulative production profiles, and GOR profiles. The aim of this study was to investigate the impact of soaking time on solvent recovery performance.…”

“…Because conventional solvent injection is a slow process, which involves molecular diffusion (10) and convective dispersion (11) within the porous medium, we propose that solvent soaking be applied, as an alternative, to promote more effective dilution. Through compositional simulations the optimal soaking time could be obtained based on the selected injection rate and the predetermined solvent mixture.…”

Compositional Changes During Vapex (Vapour Extraction) Operations in Heavy Oil Pools

“…The diluted and de-asphalted oil, in turn, drains near the edge of the vapour chamber as continuous oil column or, within the transition region as discontinuous oil ganglia 10 . These can be clearly seen in successive MRI images for Athabasca oil-propane system as shown in Figures 6 and 8 after 5 days of elapsed time, as was briefly mentioned earlier.…”

Effects of Waterflooding and Solvent Injection on the Solvent Vapour Extraction (VAPEX) Heavy Oil Recovery

“…The precipitated asphaltenes may deposit onto the rock surfaces and cause reservoir plugging in a low-permeability porous medium. In addition, it was reported that the presence of the connate water inside the porous medium results in a faster spreading of the solvent vapour chamber 13,14,15 , an increased initial heavy oil production rate 15 , and a decreased amount of deposited asphaltenes onto the porous media 16 . It was also found that in the presence of the connate water, the precipitated asphaltenes may be carried away from the porous medium by the solvent-diluted heavy oil during its gravity drainage process 16,17 .…”