Asphaltene Control Design Philosophies
In subsea wells, direct intervention with coiled tubing is very expensive and is not a viable means of control. Therefore, the strategy that has been proposed for control of subsea wells utilizes a combination of techniques to minimize deposition. This strategy is as follows:
Inject an asphaltene dispersant continuously into the wellbore (injection must be at the packer to be effective). Install equipment to facilitate periodic injection of an aromatic solvent into the wellbore for a solvent soak. Be financially and logistically prepared to intervene with coiled tubing in the wellbore to remove deposits. Control deposition in the flowline with periodic pigging with solvents. This strategy requires the installation of additional umbilical lines for delivery of asphaltene dispersant and large volumes of solvent, as well as a downhole line for injection of dispersant immediately above the packer. These hardware requirements add considerable project cost. Currently, there are no models for asphaltene deposition as a function of system pressure or other parameters.
Flow assurance modeling
Flow assurance modeling is helpful in understanding the pressure profile in the subsea system, especially where the bubble point is reached. Because the bubble point is typically the pressure at which asphaltenes are least stable, deposition problems would be expected to be the worst at this location.
If steady-state flow cannot be reached in a reasonable time due to the high viscosity of the fluids in the pipeline or difficulty in passing the gel segments through the outlet choke, consider displacing the pipeline contents with diluent hydrocarbon such as diesel or perhaps water. Another similar approach is to chemically treat the produced fluids being introduced at the pipeline inlet. Once the pipeline is filled with diesel, water, or treated fluids, flow can be accelerated rapidly due to the low viscosity and no concern about gel segments. If the gel does not break some options to consider are:
- Use a coiled-tubing system to push a tool into the pipeline and flush out the gel. Commercial coiled tubing equipment is available with an extended reach up to several miles.
- Generate pressure pulses in the gel. Because the gels will compress somewhat, a pressure pulse will move the exposed end of the gel and break a portion of it.With successive pressure pulses, a small segment of the gel can be broken with each pulse up to a limit.
- An easy first approach is to apply pressure to the gel and wait. There have been field reports of the gel breaking after several hours of exposure to pressure.
To plan, design, and operate a subsea pipeline to transport high paraffinic crudes, the following activities are recommended:
Measure key properties of the crude including cloud point, pour point, gel strength, effectiveness of pour point depressant chemicals, and viscosity as a function of temperature, shear rate, and dissolved gas. Assess the thermal conditions to be experienced by the crude for steadystate flow and transient flow during start-up, shutdown, and low production rates. Based on crude properties and thermal data, develop operational plans for shutdown, start-up, and low-flow situations. A key decision for startup is whether to break the gel with pressure or to employ more costly means to prevent gel formation. If gel breaking or warm-up is required for restart, the necessary flow control equipment and possibly a static mixer at the pipeline outlet will be required. Asphaltene design and control guidelines are still in the early stages of development. There is no predictive model for asphaltene deposition, as there is for wax and hydrates. Asphaltene design and control are usually considered with wax design and the following steps and analyses are carried out:
- Define samples to be taken and analyses to be performed.
- Perform wax modeling to provide:
WATof live fluids Location of deposition; Rates of deposition; Amount of wax deposited; Pipeline design parameters.
- Assess asphaltene stability under producing conditions.
- Provide chemical and thermal options.
- Determine pigging frequency.
- Design cost-effective solutions for prevention and remediation of wax and asphaltenes.
 ASTM D97-09, Standard Test Method for Pour Point of Petroleum Products, American Society for Testing and Materials, West Conshohocken, PA, 2009.
 ASTM D5853-09, Standard Test Method for Pour Point of Crude Oils, American Society for Testing and Materials, West Conshohocken, PA, 2009.
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