Observation of Asphaltene Destabilization at Elevated Temperature and Pressure
- J.X. Wang (Petroleum Recovery Research Center, New Mexico Inst. of Mining and Technology) | K.R. Brower (Petroleum Recovery Research Center, New Mexico Inst. of Mining and Technology) | J.S. Buckley (Petroleum Recovery Research Center, New Mexico Inst. of Mining and Technology)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 2000
- Document Type
- Journal Paper
- 420 - 425
- 2000. Society of Petroleum Engineers
- 1.8 Formation Damage, 4.3.3 Aspaltenes, 4.1.2 Separation and Treating, 4.2 Pipelines, Flowlines and Risers, 4.1.5 Processing Equipment, 5.2.1 Phase Behavior and PVT Measurements
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An optical observation cell was added to a mercury-free pressure/volume/temperature apparatus to extend studies of the onset of asphaltene precipitation to elevated temperatures and pressures. Observations for asphaltic, paraffinic, and light-to-medium gravity crude oils are reported as a function of temperature, pressure, and mixing time. Precipitants ranged from heptane to propane.
Studies at ambient conditions have shown that the refractive index at the onset of precipitation is an important characteristic of oil/precipitant mixtures. Calculated values of refractive index as a function of temperature, pressure, and composition are being correlated to observations of asphaltene separation at elevated temperature and pressure, as a predictive test.
Changes in temperature (T), pressure (p), and composition during oil production can destabilize the colloidal dispersion of asphaltenes in oil. The name "asphaltene" refers to the heaviest, most polar crude oil components that are insoluble in n-heptane and soluble in toluene.1 More generally, asphaltenes can be considered to be the material in an oil that can aggregate in response to changes in conditions.
Aggregated asphaltenes can cause a variety of problems in injection and production wells, surface facilities, pipelines, and in refinery operations.2 Within the formation, aggregation of asphaltenes might result in changes in permeability, relative permeabilities, or both. As oil is recovered from ever more challenging horizons, it is increasingly important to be able to anticipate and prevent asphaltene aggregation.
Although there has been considerable effort focused on predicting the appearance and extent of asphaltene precipitation, existing models require extensive oil characterization and even then they may not be predictive. There is a need for a method of oil characterization with respect to asphaltene precipitation that is simple and accurate.
The attractive interactions experienced by colloidal-sized asphaltene aggregates near the onset of precipitation are probably dominated by nonpolar van der Waals forces.3 These attractive interactions increase as the difference in refractive index (RI) between the colloidal asphaltenes and the rest of the oil phase (the maltenes) increases. RI of the oil represents the volume-weighted average of the contributions of all its components. If other factors (i.e., T, p, and composition) are constant, RI of the maltenes can decrease with decreasing pressure, increasing temperature, and addition of low-RI components or removal of the high-RI fraction.
Generally, anything that decreases maltene RI also decreases asphaltene stability. An exception is the effect of increasing temperature which causes thermal disaggregation4-6 even though RI decreases. The onset of precipitation in response to addition of precipitant has been found to occur at a value of RI (denoted PRI the RI at the onset of precipitation), which is constant for a given crude oil and precipitant.3,7
Some asphaltene problems are clearly related to varying amounts of solid material plugging the formation or causing fouling during operations. The amount of asphaltene present in an oil is readily quantified by simple solvent precipitation tests,1 but whether that asphaltene causes problems depends on whether or not it reaches instability during its removal from the reservoir and subsequent transport to the refinery. Detecting the onset of asphaltene precipitation presents greater technical challenges than quantifying the amount initially present in the oil, especially at reservoir conditions. Methods have been developed based on filtration,8, IR,9 viscosity,10 and conductivity,11 to name only a few. Optical observation, although sometimes hindered by fluid opacity, is the most direct and least-equivocal method available for detecting the appearance of asphaltene aggregates.
Seven crude oils, representing a wide range of oil properties, were selected for this study. The volatile fractions of all these oils were removed before storage and they were used as received. Table 1 shows the physical properties of these crude oils, including their appearance under microscopic examination before addition of any solvents or precipitants. In Table 2 are listed the hydrocarbons used as precipitating agents and solvents in this study.
A Ruska 2370 mercury-free pressure/volume/temperature (PVT) system was used for this study with adaptations for input of additional fluids and sampling through an optical cell. A schematic diagram of the main elements of the system is shown in Fig. 1.
Oil/hydrocarbon mixtures were inspected visually at constant conditions of T and p using an optical cell. The optical cell has two glass windows constructed of 3/8-in. plate glass, cut into 1-in.-diam disks, and mounted in a stainless-steel holder at a fixed distance from one another. The cell has inlet and outlet ports to allow fluid in the cell to be replaced as required. The separation distance could be varied by means of shims and was 0.010 in. in the tests reported here. The cell was pressure tested up to 10,000 psi and was used to observe mixtures at pressures up to 5,000 psi.
Transmitted light was provided by an optic-fiber bundle positioned directly behind the cell. The cell was mounted vertically, immediately inside the oven door. Observations were made through the glass door using a 45X microscope (Edmund Scientific) with a 6.3-mm field of view and 65-mm working distance.
Maintaining Single-Phase Conditions.
Tests were performed with crude oil-propane mixtures to determine the bubblepoint pressure for several oils and varying temperatures. Typical results are listed in Table 3. Pressures above the bubblepoint were maintained in all tests with propane to insure single-phase conditions.
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