SPE Production & Operations
Volume 23, Number 2, May 2008, 210-214

Summary

Although carbon dioxide (CO₂) injection is one of the most common enhanced-oil recovery methods that is widely used in many reservoirs, it may change the reservoir-fluid properties and cause problems such as asphaltene precipitation that will result in reduction of injectivity and productivity of oil wells and plugging of wellbores and production facilities.

This paper describes the effect of a miscible CO₂ flood on asphaltene precipitation in the Bangestan reservoir of the Kupal field in southwest Iran. The amount of asphaltene precipitation in the mixture of oil and CO₂ gas was measured by means of a PVT (pressure/volume/temperature) test, and the mechanism of precipitation under reservoir conditions was obtained by a high-pressure coreflood test. Then, the experimental results were compared with those obtained from available asphaltene-precipitation models.

Introduction

Asphaltenes are highly condensed polyaromatic-structure molecules consisting primarily of carbon, hydrogen, and minor proportion of elements such as sulfur, nitrogen, and oxygen.

Asphaltenes are believed to exist in the crude oil partly dissolved and partly kept in finely dispersed colloidal-suspension form that may be stabilized by nonpolar resin fractions of the crude (Branco et al. 2001). These higher-molecular-weight components of crude oil are in equilibrium at reservoir condition and may start to precipitate once the stability of the colloidal suspension is disrupted by change in pressure or temperature during primary depletion or introduction of miscible gas and liquids or other oilfield operations.

When it contacts the reservoir oil, the injected CO₂ can cause changes in the fluid behavior and equilibrium conditions as well as alter the asphaltene/resin ratio of crude oil such that it favors precipitation of organic solids, mainly asphaltenes (Srivastava et al. 1999).

Once asphaltenes have been precipitated from the oil during CO₂ flooding, they may continue to flow as suspended particles, or they may deposit onto the rock surface causing plugging and wettability alteration of reservoir matrix that can reduce recovery efficiency severely and cause formation damage.

Deposition begins with adsorption of flocculated asphaltene particles onto active sites on the rock surface (particularly onto high-specific-area clayey minerals such as kaolinite). This is followed by a hydrodynamic retention or trapping process of particles at the pore throats (Minssieux 1997; Ali and Islam 1997). Deposition of solid asphaltenes causes a reduction of the pore space available for fluids. Other formation-damage mechanisms may include permeability reduction and alteration of rock wettability from water-wet to oil-wet (Kamath et al. 1993; Clementz 1982). It is also possible that deposited asphaltene particles may be retrained in the flowing oil stream because of a mechanical-erosion or ablation effect if the interstitial velocity of the fluids becomes high enough (Wang and Civan 2001). This effect also has also been observed in studies of wax deposition in pipelines (Bott and Gudmundsson 1977).

Reservoirs with even minute asphaltene content are susceptible to asphaltene precipitation not only through pressure depletion during primary recovery, but also through composition change in fluid during gas injection (Taver et al. 1989; Nagel et al. 1990; Srivastava et al. 1999; Leontaritis et al. 1988; Mansoori 1997). For instance, Leontaritis et al. (1992) call attention to the Boscan crude of Venezuela that has not created any asphaltene problems although it has more than 17% by weight of asphaltenes (Lichaa 1977).Whereas, the Hassi- Messaoud oil has created severe asphaltene problems, although it has only a small fraction (0.1% by weight) of asphaltenes (Haskett and Tartera 1965). In fact, de Boer et al. (1995) have concluded that light-to-medium crudes containing small amounts of asphaltenes may create more asphaltene-precipitation problems during primary production. Nghiem and Coombe (1997) explain: Heavier crudes that contain a larger amount of asphaltene have very little asphaltene precipitation problems as they can dissolve more asphaltene.

© 2008. Society of Petroleum Engineers

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History

• Original manuscript received: 19 June 2006
• Meeting paper published: 12 June 2006
• Revised manuscript received: 24 April 2007
• Manuscript approved: 9 October 2007
• Version of record: 20 May 2008