Abstract

Occurrence of asphaltene deposition in producing formation constitutes one of the most serious problems currently encountered in the petroleum industry in many areas of the world.Reversibility of asphaltene deposition represents crucial argument and controversy in laboratory research of the petroleum industry. Deep understanding of this phenomenon is the key for treatment the problem of asphaltene deposition.

The major goals of this study were to investigate (1) asphaltene adsorption rate on carbonate rock surfaces under static condition, and (2) asphaltene deposition and its reversibility under dynamic flow conditions. For the sake of achievement these goals, two groups of experiments were undertaken.The first one measured asphaltene adsorption rate under static condition while the second group was devoted to study reversibility of asphaltene deposition under dynamic flow condition through actual porous medium.

The results of the study indicated that the increase of aging time increases asphaltene adsorption on carbonate rock surfaces under static condition. However, the major part of asphaltene is adsorbed during the early thirty hours of the first contact of oil with rock surface. The results of dynamic flow experiments showed that asphaltene deposition is a continuous process causing permeability damage and is also partially reversible. Furthermore, the asphaltene deposition causes more damage in low permeability rock than that one in higher permeability. The obtained results are expected to have important implications for better formulation of treatments of asphaltene deposition.

INTRODUCTION AND REVIEW

Asphaltene is one of the major compound responsible for various and unwanted blockage cases in the petroleum industry.Chemically asphaltenes represent complex molecules of polar, polyaromatic hydrocarbon or non-hydrocarbon but high molecular weight fractions of crude oils. However, no clear chemical formula has been reported for asphaltenes yet. Speight et al. (1984) concluded that asphaltenes are difficult to define even when a standard method of precipitation is applied. By definition of asphaletene is that the portion of the crude oil that is insoluble in low molecular weight of n-alkanes but soluble in benzene or toluene (Long, 1981; Speight et al., 1981).Asphaltene is considered partly dissolved and mostly kept in finely dispersed collodial suspension particles (Leontaritis, 1989). The other high molecular weight fraction namely non-polar resins is surrounding suspended asphaltene particles and hence stabilizes its suspension. The common belief is that asphaltenes stabilized by resins are small enough in size so that their transport do not pose any threat to permeability. The ratio of polar to non-polar components and the ratio of high to low molecular weight components control the solubility of asphaltenes and resins within the crude oil.If asphaltene flocculate in situ they deposit on the rock surfaces through either adsorption or absorption. The deposition of such large particles constitutes the real threat of naturally occurring formation damage.

Asphaltene adsorption on rock surface has been documented and attributed to the polarity of asphaltene molecules (Clementz, 1982; Collins et al., 1983). The most significant factors influencing asphaltene deposition are considered to be composition of the crude oil (Mansoori et al., 1988; de Boer et al., 1995), pressure and temperature (Dubey and Waxman, 1991; Leontaritis, 1989) properties of asphaltene (Leontaritis and Mansoori, 1987) and conditions of wellbore hole and pipeline into which the reservoir fluid is flowing (Leontaritis, 1989).

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