Introduction

Heavy organics (asphaltene, resin, etc.) deposition is a common problem in all sections of the oil industry, including oil production, transportation, and processing [J. Escobedo, et al., 1997; G., A., Mansoori,
1997; G., A., Mansoori, et al., 1985]. One of the most important aspects of this problem is asphaltene deposition, which causes major difficulties in petroleum mixtures, such as petroleum reservoir fluids.

Deposition of these materials may cause a number of difficulties including wettability reversal, permeability reduction, increased pressure drop, well and pipeline plugging and finally production rate reduction[Nancy, E. Burke et al., 1990; S.J.Park et al.,1988; J. Escobedo, et al., 1997; G. A. Mansoori,1988].
Considering pressure drop due to continued production from reservoirs and higher viscosity of depleted reservoirs necessitates the need for pressure maintenance and gas injection. Pressure variation and composition change, like during gas injection, are the major causes of asphaltene deposition [Nancy, E. Burke et al., 1990;
Sirvastava, R. K., 1995]. Prediction of deposit formation needs a full study including inspection of different factors affecting that.

A petroleum reservoir fluid is a complex polydisperse mixture consisting, mostly, of light and heavy paraffins, aromatics, resins and asphaltenes. Asphaltene is recognized as the heavy fraction of petroleum fluids, which is insoluble in some species such as paraffins but soluble in polar/aromatic species such as toluene, xylene, etc) [Leontaritis,K. J.,et al., 1992; Kawanaka, S., et al., 1989; Hirschberg,A., et al.,1984; Pan, H., et al., 1997a].

Changes in such environmental parameters as pressure, temperature and composition (like during production, depletion, gas injection/addition of solvent or dispersant injection) can change characteristics in an oil mixture so that the oil will become unstable and finally heavy organics will flocculate and deposit. Surface activity and self-association of asphaltene particles and their tendency for growth during deposition, could lead to problems during production, transpiration and processing of petroleum.

Asphaltenes are defined as the fraction separated from crude oil or petroleum products upon addition of hydrocarbon solvents such as n-heptane (Speight, 1999). Resins are defined as the fraction of the desasphalted oil that is strongly adsorbed in surface-active materials such as Fuller’s earth, alumina, or silica, and can only be desorbed by a solvent such as pyridine or a mixture of toluene and methanol. Asphaltenes and resins are aromatic heterocompounds with aliphatic substitutions and they form the most polar fraction of crude oil. Resins have a strong tendency to associate with asphaltenes. Such association determines, to a large extent, their solubility in crude oil (Koots and Speight, 1975).

According to the field experience (De Boer et al., 1995; Kokal and Sayegh, 1995) and experimental observations (Andersen, 1994; Fotland et al.,1997; Hammami et al., 2000; Thomas et al., 1992) asphaltene stability depends on a number of factors, including the composition of the surrounding fluid, pressure, and temperature. The effect of composition and pressure on asphaltene precipitation is generally believed to be stronger than the effect of temperature. Addition of paraffinic compounds shifts the solubility of asphaltenes in the bulk oil because its solvent power affects interactions among asphaltenes and resins. If the paraffinic compounds are good solvents for resins but not for asphaltenes, as the volume of diluent increases both the interaction between resins and asphaltenes and the capacity of the former to stabilize the asphaltene molecules as small aggregates becomes weak, causing asphaltenes to precipitate. Pressure depletion alone can destabilize asphaltenes and is likely the major reason for asphaltene deposition in well-bore pipes. As the density of the crude oil decreases (because of depressurization), the screening effect on asphaltene interactions arising from the presence of oil components decreases, causing the interactions between asphaltenes to become stronger, which in turn induces the precipitation.

Asphaltenes are the least understood and the most problematic organic deposits. }They are high molecular weight, non-crystalline, polar compounds which exist in crude oil. Asphaltenes consist of polyaromatic condensed rings with short aliphatic chains and heteroatoms such as nitrogen, oxygen, sulfur and various metals. Asphaltene molecules carry a core of stacked, flat sheets of condensed (fused) aromatic rings linked at their edges by chains of alipathic and/or naphthenic-aromatic ring systems. The condensed aromatic rings exist in the form of a non-homogeneous flat sheet.

Asphaltenes: The asphaltene fraction, like the resins, is defined as a solubility class, namely the fraction of the crude oil precipitating in light alkanes like pentane, hexane or heptane. This precipitate is soluble in aromatic solvents like toluene and benzene. The asphaltene fraction contains the largest percentage of heteroatoms (O, S, N) and organometallic constituents (Ni, V, Fe) in the crude oil. The structure of the asphaltenes has been the subject of several investigations, but is now believed to consist of polycyclic aromatic clusters, substituted with varying alkyl side chains.  Figure 2-2 shows a hypothetical asphaltene monomer molecule. The molecular weight of asphaltene molecules has been difficult to measure due to the asphaltenes tendency to self aggregate, but molecular weights in the range 500-2000 g/mole are believed to be reasonable. Asphaltene monomer molecular size is in the range 12-24.

In the crude, asphaltenes tend to attract towards each other to form agglomerates. The most common theory of asphaltene stability in crude oil suggests that naturally occurring resin molecules form a steric repulsive layer around asphaltene particles. If the concentration of resins is insufficient to cover the surface of the asphaltene particles, perhaps due to a decrease in temperature, pressure or PH, then the asphaltene will precipitate out of the solution. Changes in the composition of the crude, such as adding solvent partially dissolve the resin molecules that cover the surface of the asphaltenes and disrupt the resin-asphaltene system leading to the flocculation of asphaltenes. In severe cases, this may cause an arterial blockage anywhere from the oil well to the production and processing facilities.

Structure and Chemistry of Asphaltenes and Resins

Our knowledge of the asphaltenes is very limited. Asphaltenes are not crystallized and cannot be separated into individual components or narrow fractions. Thus, the ultimate analysis is not very significant, particularly taking into consideration that the neutral resins are strongly adsorbed by asphaltenes and probably cannot be quantitatively separated from them. Not much is known of the chemical properties of asphaltenes. Asphaltenes are lyophilic with respect to aromatics, in which they form highly s cattered colloidal solutions.
Specifically, asphaltenes of low molecular weight are lyophobic with respect to paraffins like pentanes and petroleum crudes. There have been considerable efforts by analytic chemists to characterize the asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous sources.
A number of investigators have attempted to postulate model structures for asphaltenes, resins, and other heavy fractions based on physical and chemical methods (http://www.uic.edu/~mansoori)
 

Molecular structure of asphaltene proposed for Maya crude (Mexico) by Altamirano, et al. [IMP Bulletin, 1986]
 

Molecular Structure of Asphaltene Proposed for 510C Residue of Venezuelan Crude by Carbognani [INTEVEP S.A. Tech. Rept., 1992]

RIPI Contribution

Natural depletion of petroleum reservoirs and also gas injection for enhance oil recovery, are unavoidable processes in oil industry. Foremost, prediction of the problems due to these two processes is very necessary and important.

So many field and experimental experiences have shown that heavy organic depositions, especially asphaltene deposition, are principal results during these processes. Results of laboratory simulation of asphaltene deposition during natural depletion of petroleum reservoirs and also during gas injection and
enhanced oil recovery (EOR) processes are reported here. This is achieved through the designed experimental setup, for investigation of pressure and composition effects on asphaltene deposition in petroleum fluids at high pressure and high temperature conditions. Here asphaltene deposition during decreasing pressure, from pressures greater than reservoir pressure to pressures below the bubble point pressure (natural depletion) and also asphaltene deposition during natural gas injection in reservoir
condition, are studied for a wide range of reservoir fluid samples.

Laboratory Activities

Our activities in experimental section include asphaltene deposition in High-Temperature High-Pressure conditions (producing wells and miscible gas injection conditions). In other words natural depletion (primary recovery) of petroleum reservoirs and also miscible gas injection in enhance oil recovery processes are simulated in PVT laboratory.
 

Investigation of Asphaltene Deposition in Porous Media During Miscible Gas Injection

The pressure change effect on asphaltene deposition

The gas injection effect on asphaltene deposition:

Onset of Asphaltene-Resin flocculation

Theoretical Aspects (Simulation and Modeling)

Simulation and modeling of asphaltene deposition in petroleum reservoir during Natural depletion and miscible gas injection in enhanced oil recovery are the other activities in this department.
 

Current Projects (related topics)

•  

•  Investigation of South Pars Gas Injection Effects on Agha-Jari Oil Reservoir From Asphaltene  Deposition Point of View

• Quantitative and Qualitative Study on Asphaltene Deposition in Bangestan Formation
        (Shadegan and Kupal reservoirs)

•   Asphaltene-Wax formation at Well column and Porous media of Zelaee-Gas Condensate
        Field in Bangestan Formation

•  Thermodynamic Inspection of Asphaltene-Wax formation at Wellcolumn and Porous media
        of Zelaee-Gas Condensate Field in Bangestan Formation

•  Thermodynamic Inspection of Asphaltene-Wax formation at Wellcolumn and Porous media
        of Zelaee-Gas Condensate Field in Bangestan Formation

•   Inspection of Asphaltene Formation in Ramshir Oil Field (Bangestan Formation)

Our new PVT cells have been equipped with Solid Detection Systems (SDS). Using this system, onset of asphaltene flocculation at high pressure-high temperature and different miscible gas injection conditions, can be measured.
Gas injection processes may be unavoidable programs for enhance oil recovery in so many oil fields all over the world.  Among these processes miscible gas injection is more favorable and a better option than the immiscible ones [Sirvastava, R. K., 1995]. Decreasing in interfacial tension between different phases and consequently less drag forces for fluid flow in porous media is a favorable miscible injection outcome. As it is known some unfavorable phase separations may occur during miscible gas injection. One of these phase separations is asphaltene deposition from liquid oil phase into a new asphaltene phase. Asphaltene precipitation, flocculation, aggregation and finally deposition in petroleum reservoir are consequent steps that in them new phases are formed. Therefore it seems miscible gas injection effect on asphaltene deposition is a necessary subject that should be considered in EOR programs.
 
The mechanism of pressure effect on amount of asphaltene deposition and/or dissolution is not completely known. Pressure variation may have two distinct effects on asphaltene deposition. This means increasing in pressure can results in increasing or decreasing of asphaltene solubility in oil. Once petroleum mixture is at a pressure below its bubble point pressure, pressure increase leads to increase in asphaltene deposition [Nancy, E. Burke et al., 1990]. Here we may say that increase in pressure, causes some light hydrocarbon fractions transfer (pass) from gas phase to liquid phase, which may cause precipitation of heavy fractions.