Asphaltene precipitation

One of the major problems in all area of petroleum industry, including production, operation, storage and transportation, is formation of heavy organic compounds, e.g., asphaltene and wax, and their consequent deposition in equipment and pipelines. Change in crude oil composition, temperature and pressure leads to asphaltene precipitation. Thus, it is of vital importance to determine onset of asphaltene precipitation during oil production. In this regard, various techniques have been suggested ,  However, from practical point of view, one needs to decide which method serves to be proper one. To this end, present study gives a comprehensive critical review for different techniques serve to determine onset of asphaltene precipitation along with comparing their advantages and disadvantages. Since each method pre- sents its own definition for onset, such as onset of precipitation, clustering and deposition; it is necessary to distinguish their difference based on underlying physical mechanisms. It was concluded that mechanism of asphaltene precipitation for determining onset of asphaltene precipitation plays a substantial role.
Generally, asphaltene represents a fraction of crude oil compounds, which tends to form a solid-like phase once adding light hydrocarbons, normally n-heptane, into the crude. It is believed that asphaltene particles are partially dissolved in oil as colloidal or micellar form due to mean polarity of crude oil and presence of stabilizing compounds, e.g., resin  In recent years, precipitation, flocculation and deposition of these molecules have been identified and analyzed. As an important point, one should distinguish physical implication of these words; precipitation, flocculation and deposition.
Precipitation denotes formation of semi-solid phase by ag- gregation of solid particles. After beginning the precipitation process, particles with sizes of about 1 micrometers are formed through clustering process, which is called flocculation stage. In other words, formation of large aggregates from smaller ones is called flocculation. At the end, during deposition, asphaltene particles are formed on a surface, e.g., pipe wall or rock. Precipitation does not necessarily result in deposition, however, could be of effective contribution in deposition process.
Aphaltene precipitation during primary depletion
In normal pressure depletion, reservoirs that experience asphaltene precipitation usually have the following characteristics:
 Fluid in place is light to medium oil with small asphaltene content.
 Initial reservoir pressure is much larger than the saturation pressure. That is, the fluid is highly undersaturated.
 Maximum precipitation occurs around the saturation pressure.
Heavier crudes that contain a larger amount of asphaltene have very few asphaltene precipitation problems because they can dissolve more asphaltene. Leontaritis and Mansoori and Kokal and Sayegh compiled field cases with asphaltene precipitation problems during primary depletion. Extreme cases include the Venezuelan Boscan crude with 17 wt% asphaltene produced nearly without precipitation, whereas the Venezuelan Mata-Acema crude with 0.4 to 9.8 wt% asphaltene and the Algerian Hassi Messaoud crude with 0.062 wt% encountered serious precipitation problems during production.
Asphaltene precipitation during IOR gas injection
The injection of hydrocarbon gases or carbon dioxide (CO2) for IOR promotes asphaltene precipitation. Numerous field reports and laboratory studies on this phenomenon have been published.
Although it frequently manifests itself at the production wellbore at solvent breakthrough, precipitation can occur anywhere in the reservoir.
Asphaltene precipitation also may occur during solvent injection into heavy oil reservoirs. Butler and Mokrys proposed an in-situ solvent-extraction process for heavy oils and tar sands called VAPEX. This process uses two horizontal wells (one injector and one producer). The injection of solvent (e.g., propane) creates a solvent chamber in which oil is mobilized and drained toward the producer. In addition to the mobilization process, the solvent may induce asphaltene precipitation, which provides an in-situ upgrading of the oil.
Asphaltene precipitation and deposition
Asphaltene characteristics discusses the chemistry of asphaltenes and  the thermodynamic equilibrium of asphaltenes in petroleum fluids. Changes in pressure, temperature, and composition may alter the initial equilibrium state and cause asphaltene precipitation.
The region in which precipitation occurs is bounded by the asphaltene precipitation envelope (APE). [Also sometimes called the asphaltene deposition envelope (ADE).] Fig. 1 shows a typical pressure composition APE and a pressure temperature APE. For purposes of this page, precipitation refers to the formation of the asphaltene precipitate as a result of thermodynamics equilibrium and deposition refers to the settling of the precipitated asphaltene onto the rock surface in a porous medium. The onset conditions correspond to points on the APE. Within the APE, the amount of precipitated asphaltene increases as pressure decreases from the upper onset pressure to the saturation pressure of the oil. The precipitation reaches a maximum value at the saturation pressure and decreases as pressure decreases below the saturation pressure.
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Fig. 1 – Pressure-composition and pressure-temperature APEs (after Leontaritis).
Inside the reservoir, after precipitation has occurred, the asphaltene precipitate can remain in suspension and flow within the oil phase or can deposit onto the rock surface. The main deposition mechanisms are adsorption and mechanical entrapment. The deposited asphaltene may plug the formation and alter rock wettability from water-wet to oil-wet.
Experimental measurements of asphaltene precipitation
Proper planning procedures dealing with the onset of asphaltene precipitation requires knowing the precise conditions under which such precipitation will occur. There are several methods available that allow for such calculations and the ability to estimate when asphaltene precipitation will occur in order and thus how best to prevent and/or deal with it.
Measurements of asphaltene precipitation envelope (APE)
The APE defines the region in which asphaltene precipitation occurs. Accurate measurements of the APE and the amounts of precipitate within the APE are required for design purposes and for tuning existing models. The upper pressure on the APE is denoted by pAu and the lower pressure on the APE is denoted by pAℓ. Several techniques are available for determining the onset of precipitation with various degrees of accuracy.
Gravimetric technique
This technique is conducted in a conventional pressure/volume/temperature (PVT) cell. For a pressure below the pAu, precipitation occurs and larger particles segregate and settle at the bottom of the cell because of gravity. Asphaltene analysis (titration with n-pentane or n-heptane) of the oil shows a decrease in asphaltene content compared with the original oil. Pressure steps must be chosen carefully to capture the inflection point at pAu and pAℓ.
Acoustic-resonance technique
The acoustic-resonance technique has been used effectively to define pAu. The live oil is charged at a high pressure (e.g., 8,500 psia) into a resonator cell maintained at the reservoir temperature. The resonator pressure then is decreased at a very low rate (e.g., 50 psia/min) by changing the volume. The depressurization rate decreases with time to a typical rate of 5 psia/min toward the end of the experiment. Acoustic data exhibit sharp changes at pAu and at the oil saturation pressure, ps.
Light-scattering technique
Light-scattering techniques also have been successfully used to measure the APE. For dark-colored oil, a near-infrared laser light system (800×10-9 m to 2200×10-9 m wavelength) is required to detect asphaltene-precipitation conditions. The principle behind the measurements is based on the transmittance of a laser light through the test fluid in a high-pressure, high-temperature visual PVT cell undergoing pressure, temperature, and composition changes. A receiver captures the amount of light that passes through the oil sample. The power of transmitted light (PTL) is inversely proportional to the oil mass density, to the particle size of the precipitate, and to the number of particles per unit volume of fluid.The PTL curve exhibits sharp jumps at pAu, ps, and pAℓ.
Flirtation technique
In this method, the cell contents during a depressurization test are mixed in a magnetic mixer, and small amounts of the well-mixed reservoir fluid are removed through a hydrophobic filter at various pressures. The material retained on the filter is analyzed for SARA contents.
Electrical-conductance technique
This technique measures the change in the fluid conductivity with changes in concentration and mobility of charged components. Asphaltenes have large dipole moments, and, therefore, the conductivity curve exhibits a change in the slope when precipitation occurs.
Viscometric technique
The key point of this method is the detection of a marked change in the viscosity curve at the onset of precipitation because the viscosity of oil with suspended solids is higher than that of the oil itself.
Other techniques
Asphaltene precipitation has been detected through visual observations with a microscope. Measurements of interfacial tension between oil and water also can be used to detect the onset.
Comparison of different methods
Fig. 2 shows the results of Jamaluddin et al.‘s comprehensive comparison of measurements with the gravimetric, acoustic-resonance, light-scattering, and filtration techniques on the same oil. These methods, except for the acoustic-resonance technique, determine both the upper and lower APE pressure. The acoustic-resonance technique normally provides only the upper onset pressure. In addition to APE pressures, the gravimetric and filtration techniques also give the amount of precipitated asphaltene within the precipitation region. The gravimetric and filtration techniques are more time consuming than the acoustic-resonance and light-scattering techniques. Fotland et al. showed that the electrical-conductance technique can determine both precipitation onset and amounts of precipitate that are consistent with the gravimetric technique. The advantage of the viscometric technique is in its applicability to heavy crude oil, which may give some difficulties to light-scattering techniques, and in the low-cost equipment. In many cases, two measurement techniques are applied to the same oil to enhance data interpretation. MacMillan et al.[19] recommended the combination of light-scattering and electrical-conductance techniques, while Jamaluddin et al.[16] suggested the simultaneous application of light-scattering and filtration techniques.
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Fig. 2 – APE pressure measurements with different methods (after Jamaluddin et al.
Reversibility
The reversibility of asphaltene precipitation is a subject of some controversy.
Fotland and Wang et al. suggested that asphaltene precipitation is less likely to be reversible for crude oils subjected to conditions beyond those of the precipitation onset.
 Hirschberg et al. speculated that asphaltene precipitation is reversible but that the dissolution process is very slow.
 Hammami et al. reported experimental measurements that seem to support this conjecture. They observed that asphaltene is generally reversible but that the kinetics of the redissolution vary significantly depending on the physical state of the system.
Hammami et al. illustrates the laser-power signal (light-scattering technique) from a depressurizing and repressurizing experiment on a light oil that exhibits strong precipitation behavior. The laser-power signal increased linearly as the pressure decreased from 76 to 56 MPa. This increase results from the continuous decrease of oil density above the bubblepoint as the pressure is reduced. With further depletion between 56 and 52 MPa, a large drop (one order of magnitude) in the laser-power signal occurred. The onset of asphaltene precipitation was estimated to be 55.7 MPa and the laser-power signal dropped to a very low level at 45 MPa. The bubblepoint pressure for this oil is 33.5 MPa. On repressurization of this oil from 27 MPa (7 MPa below the bubblepoint), almost the entire laser-power signal was recovered, but the signal followed a slightly different curve. They further show that the repressurization laser-power curve lags the depressurization curve, which is an indication that the kinetics of redissolution is slower than the kinetics of precipitation. Their analysis also shows that the ultimate laser-power value reached from repressurization is higher than the predepletion value. Hammami et al. suggested that a large fraction of the precipitated asphaltene (the suspended solid) could easily go back into solution while a smaller fraction exhibits partial irreversibility or slow dissolution rate. The oil at the end of the repressurization process is partially deasphalted and is slightly lighter that the original oil.
Joshi et al. performed further experiments to study the reversibility process. Their results corroborate the observations of Hammami et al. for depressurization and repressurization experiments at field conditions; however, they observed that the precipitation caused by the addition of alkane at atmospheric conditions is partially irreversible. They explained that asphaltene precipitation with pressure depletion at field conditions (field asphaltenes) results from the destabilization but not the destruction of asphaltene micelles. On the other hand, asphaltene precipitation caused by the addition of an alkane solvent in the laboratory under atmospheric conditions (laboratory asphaltenes) strips the asphaltene micelles of their resin components, and the restoration of reformed micelles is a very difficult process.