Laboratory Testing and Prediction of Asphaltene Deposition in Production Wells
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During primary oil production, when the thermodynamic conditions within the well tubing lie inside the asphaltene-deposition envelope (ADE) of the produced fluid, flocculated asphaltene particles could start being deposited on the tubing wall, causing a restriction in the tubing inner diameter that results in loss of production. This paper presents a methodology that begins by determining the ADE in the laboratory. Moreover, asphaltene-deposition rates for the tubing conditions can be measured using high-pressure/high-temperature coaxial-cylinder technology.
In-Situ Asphaltene Testing
At some thermodynamic states, asphaltenes exhibit a behavior called flocculation—that is, asphaltene particles or micelles aggregate or flocculate into larger particles or flocs. The locus of all thermodynamic points in a phase diagram at which flocculation occurs is called the ADE. Accurate measurement of asphaltene solubility at in-situ conditions inside the ADE is extremely difficult. Many asphaltenic reservoir fluids exhibit irreversible asphaltene flocculation. This means that, once the upper ADE boundary is crossed, some of the asphaltenes will not deflocculate and go back into stable suspension by simply reversing the thermodynamic path.
The spectrophotometric near-infrared (NIR) onset method uses the observation that there is a sharp increase in light absorption or attenuation in the NIR region of the electromagnetic spectrum at the onset of asphaltene flocculation.
Transmittance of light through a sample of asphaltenic oil has also been used to detect the onset of asphaltene flocculation. There is a sharp decrease in the transmittance of NIR light through a sample of asphaltenic oil at the onset of asphaltene flocculation.
Along with the spectrophotometer, an NIR imaging system is also used in the attempt to decipher the asphaltene behavior of a fluid.
One can change the thermodynamic conditions in the vessel containing the sample and obtain the oil’s ADE. State-of-the-art high-pressure/high-temperature imaging technology is routinely used in the laboratory to detect asphaltene particles very clearly upon isothermal pressure depletion. The asphaltic heavy-phase-formation phenomenon is indicated in the images to be a consequence of many asphaltene particles forming, aggregating, and settling in the NIR cell under the prevailing thermodynamic conditions. This indicates that the asphaltene-phase behavior responsible for asphaltene-induced formation damage and asphaltene-deposition problems takes two forms—asphaltene flocculation into larger particles or heavy-asphaltic-phase separation.
If a heavy liquid phase has been indentified as a source of asphaltene precipitation for a given sample, an important test to perform is a high-pressure/high-temperature viscosity/density test. To perform this test, the fluid is brought to a thermodynamic condition below its upper ADE, typically just above bubblepoint. The fluid is allowed then to equilibrate at those conditions and, after a day, is displaced through the viscosity and density instrument. As the fluid is displaced from the cylinder containing the oil toward the end of the displacement (i.e., approaching the bottom of the NIR cell or cylinder), a sedimented heavy phase is encountered. This is signified by the sudden increase in viscosity.
Similar to that process is the sedimentation test. In this test, a known amount of reservoir fluid is charged in a high-pressure vessel. The system is brought to a thermodynamic-equilibrium condition below the oil’s upper ADE (i.e., just above bubblepoint). After flocculation, the asphaltene particles grow in size enough that they begin to settle under the influence of gravity, depending on the flow dynamics of the container. Asphaltene particles will form a layer at the bottom of the cell that looks like a separate, distinct thermodynamic phase. The relative amount of this phase is a function of the asphaltene-particle-size distribution and the nature of the suspended phase (asphaltene phase) and suspension medium (liquid phase) at the prevailing thermodynamic conditions. This phase would have very low mobility if formed inside the formation, pipelines, or production equipment, but it is especially devastating at the bottom of wells and inside storage tanks.
The sample is then left to sediment for 7–10 days. The contents are then displaced while they are scanned with NIR light. A small sample of the top layer is collected while displacing for a gravimetric analysis. Once the interface of the asphaltene layer is reached, its volume is recorded. Using these volumetric measurements, along with the gravimetric analysis of the sample from the supernate, the extra layer is quantified.
Another crucial step is to quantify the amount of solid depositing on the pipe wall at certain thermodynamic conditions. For this, a special high-pressure sheared-concentric-cylinder asphaltene-deposition apparatus is used. A vessel capable of 30,000 psia and 350°F holds hot live oil at specified (tubing) conditions, and it shears it against a cooler surface that emulates the tubing wall. This test is carried out for an hour, and the deposit is then quantified.
The most dominant factors of asphaltene deposition in primary production are flocculation of asphaltene micelles into larger particles near the wall because of a loss of pressure and temperature and adhesion of flocculated asphaltene particles at the wall that results in a compositional gradient between the bulk of the fluid and the wall. These result in diffusion of more asphaltene micelles, flocculation into larger particles, and adhesion at the wall. The streaming potential between wall and fluid is thought to be partly responsible for the force of adhesion.
As the fluid is being cooled or its pressure is depleted, its temperature and pressure arrive at the onset of asphaltene flocculation at some point in the pipe and asphaltene particles begin to form. At this point, the temperature difference between the fluid and the wall is at its highest. As a result, the attraction of the asphaltene crystals toward the wall is at its highest. As the asphaltene-micelle concentration of the fluid near the wall is depleted, more asphaltene micelles diffuse through the boundary layer to replenish the boundary-layer asphaltene-micelle concentration. The concentration of asphaltene micelles in the bulk fluid becomes uniform primarily through convective mass transfer. However, within the boundary, where the flow is laminar, an asphaltene-micelle-concentration gradient is set up, which causes mass transfer to take place by means of diffusion. Any disruptions of the diffusion process can result in diminishing of the asphaltene-deposition rate.
It is evident that the diffusion of asphaltene micelles through the laminar boundary layer is largely responsible for asphaltene deposition in liquid-full conduits. Hence, quantifying the diffusion mechanism is the key to predicting asphaltene deposition in liquid-full systems. This work uses the rotating-concentric-cylinder approach to determine experimentally the asphaltene-deposition rate through the diffusion mechanism without shear and with shear. The overall approach consists of measuring the amount of asphaltene deposited on a cooler concentric cylinder and using it along with other oil-related and pipe data to estimate the initial asphaltene-deposition rate on the system carrying the oil. It is called “initial” because it refers to the asphaltene-deposition rate that is expected to occur in the pipe during the first 24 hours.
The following approach is used to simulate the asphaltene-deposition rate in a given facility producing oil at a specified flow rate:
- Use the asphaltene-deposition-apparatus data and a Levenspiel plot to convert batch data to plug flow to estimate the initial asphaltene-deposition rate on the pipe.
- Tune the constants of the asphaltene-deposition simulator to enable it to predict accurately the 24-hour asphaltene-deposition rate.
- Use the tuned asphaltene-deposition simulator to calculate the asphaltene-deposition rate in the system.
After the simulator is tuned to calculate accurately the initial 24-hour asphaltene-deposition rate, the model was run to predict the asphaltene deposition in the system. It is recommended that all remedial operations, such as well intervention, pigging, and chemical treatment, be geared toward maintaining the asphaltene-deposit thickness in the pipe to an absolute maximum of 1% of the pipe inner diameter. The 1%-of-pipe-radius deposit-thickness level of 0.051 in., typically signaling the need for intervention, was predicted to be reached in a little more than 12 days. Chemical injection should lower the frequency of intervention requirements by diminishing the asphaltene-deposition rate.
Live-Oil Chemical Testing
Having determined that the oil has an ADE and that, within the field-expected thermodynamic conditions, there will be asphaltene deposition, it is critical to explore the feasibility of possible chemical treatment. For that, a test similar to the sedimentation test is conducted. A sample is loaded in the high-pressure/high-temperature vessel and brought within its upper ADE. It is then treated with a known amount of preselected chemical asphaltene inhibitor. At this stage, typically, the goal is to determine feasibility of chemical treatment. The exact amount of chemical treatment required can be determined by performing this test several times.
Once the chemical treatment of the oil in the vessel is complete, the sample is mixed in situ and then left for a few days to sediment just like in the sedimentation test. If the chemical is successful at inhibiting the asphaltenes, no asphaltic layer should be observed on the bottom of the vessel when displacing the fluid, as happens with the untreated sample. This process is illustrated in Figs. 1 and 2.
Laboratory Testing and Prediction of Asphaltene Deposition in Production Wells
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