Asphaltene Precipitation and Alteration of Wetting: The Potential for Wettability Changes During Oil Production
- Rashid S.H. Al-Maamari (Sultan Qaboos U.) | Jill S. Buckley (New Mexico Petroleum Recovery Research Center, New Mexico Tech)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- August 2003
- Document Type
- Journal Paper
- 210 - 214
- 2003. Society of Petroleum Engineers
- 1.2.3 Rock properties, 4.1.2 Separation and Treating, 5.4.2 Gas Injection Methods, 1.8 Formation Damage, 4.3.3 Aspaltenes, 5.4 Enhanced Recovery, 5.2.1 Phase Behavior and PVT Measurements, 4.1.5 Processing Equipment
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Wettability controls the distribution and flow of immiscible fluids in an oil reservoir and thus plays a key role in any oil-recovery process. Once thought to be a fixed property of each individual reservoir, it is now recognized that wettability can vary on both microscopic and macroscopic scales. Polar oil components can adsorb onto the pore-bounding mineral surfaces by several different mechanisms. In this study, we explore a transition in the mechanism of wettability alteration that relates to the asphaltene fraction and its stability in the oil phase.
Asphaltene stability was varied by adding n-heptane to samples of five crude oils. Conditions tested ranged from those in which asphaltenes were stable to mixtures in which aggregates formed and separated from the oil. The onset condition - in which the first asphaltene particles become visible - provides a reference point with respect to asphaltene stability in each crude-oil sample.
Muscovite mica sheets, treated with brine and with the crude oil/heptane mixtures, were examined using contact angles between decane and water as a measure of altered wettability on the oiltreated surfaces. A significant increase in oil-wet conditions very near the onset of asphaltene precipitation was observed with four of the five oils, indicating the potential for wetting changes during the course of oil production if conditions of asphaltene instability are approached. Implications of changes in the mechanism of wetting alteration during the course of production from an oil reservoir are considered.
Asphaltenes and Reservoir Wettability.
The potential for asphaltenes to adsorb onto high-energy mineral surfaces and thus to affect reservoir wettability has long been recognized. A good summary of early work is provided by Anderson.1 More recently, there have been a number of studies of asphaltene adsorption and resulting changes in wetting on smooth surfaces.2-5 What has been missing in previous studies is a simple measure of the stability or incipient instability of the asphaltenes, which is needed to distinguish between competing interaction mechanisms among these large, aromatic, somewhat polar molecules and mineral surfaces in the presence of brine.
COBR Interaction Mechanisms.
The polar components in crude oils can adsorb, in the presence of an aqueous phase, by distinctly different mechanisms, depending on factors that include brine composition and the ability of the oil to keep its asphaltenes dispersed. 6 Additional interactions that may occur between oil components and dry surfaces are not relevant in the context of an oil reservoir in which both aqueous and oleic phases are present.7 Crude oil/brine/rock (COBR) interactions can include the following:
Ionic interactions that involve ionization of acids and bases at the oil/water and solid/water interfaces (acid/base, ion-binding, and other specific interactions are included in this category).
Surface precipitation interactions that depend mainly on crude-oil-solvent properties with respect to their asphaltenes.
While these interaction mechanisms have been demonstrated for specific cases under conditions that clearly should be dominated by one class of mechanisms or the other, little is yet known about the intermediate conditions and the transition from one set of mechanisms to another.
We use the refractive index (RI), a readily measured property, to quantify solvent properties of oil/heptane mixtures as they relate to asphaltene stability.8-10 RI at the onset of precipitation is denoted as PRI. Above that value of RI, asphaltenes remain dispersed; below it, they flocculate. The value of PRI depends on both oil and added precipitating agent (n-heptane in this study). When the asphaltenes are stable (i.e., RImix>>PRI), we expect that ionic interactions will dominate. As the onset of precipitation is approached, a transition to surface precipitation as the primary interaction mechanism should take place.
Alteration of Reservoir Wettability.
The effect of recovery processes on reservoir wettability has been a recurring theme in the improved oil recovery literature.11-13 Many interesting questions have been raised, but few definitive answers have been provided because it is difficult to isolate the influence of wetting on measurable parameters (e.g., fluid-flow rates, pressure drops) from other variables. Even in laboratory corefloods, the effects of wetting alteration are complicated and difficult to isolate. Yan et al.14 used asphaltenes dissolved in toluene to effect changes in the wetting of Berea sandstone cores and the measured rate of water imbibition to assess the extent of wetting change. In many other studies, however, wetting states are deduced from endpoint relative permeabilities,11,13,15 sometimes after the addition of a large excess of precipitant.16 It is difficult to say conclusively how many of the changes reported were caused by a reduction of total permeability, by cycle-dependent hysteresis in the relative permeability curves,17 or by the actual changes in surface properties of the pore-lining materials.
The trends of wettability alteration demonstrated in this study have implications for reservoir wettability and changes that might occur during production.
Freshly cleaved Muscovite mica was used as the solid surface. Surfaces were cleaned with a mixture of nine parts 30% H2O2 and one part of 20% NH4OH in an ultrasonic bath for 30 minutes and then left in the cleaning solution overnight. Finally, the mica samples were rinsed extensively with double-distilled, de-ionized water.
Table 1 summarizes the brines used. All solutions were prepared from double-distilled, de-ionized water, with sodium chloride added to adjust the salinity.
HPLC grade hydrocarbons (heptane, n-decane, and cyclohexane) were purified by passing through dual-packed columns of activated silica gel and alumina.
Five crude oils were tested (Table 2). Two of these oils (Lagrave and Tensleep) had some aggregates in the laboratory oil samples. To ensure that the asphaltenes were well dispersed, alpha-methylnaphthalene (a-MN) was added to these two samples to make up the initial test "oil."
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