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Heavy crude oil, characterized by its API gravity less than 20 degrees and high viscosity, poses challenges in transportation due to its unique composition. This paper explores various aspects of heavy crude oil transport, including the selection of positive displacement pumps, pressure drop calculations, and methods to overcome transport problems. Key correlations for pressure drop analysis are discussed, and solutions to address the challenges of heavy crude oil transport are presented.
Heavy Crude Oil, Positive Displacement Pumps, Pressure Drop Correlations, Transport Challenges, Solutions
Heavy crude oil, with an API gravity of less than 20 degrees, exhibits higher specific gravity and viscosity compared to lighter crude oils. This high viscosity is primarily due to the presence of higher molecular weight compounds and a significant amount of asphaltenes. Heavy crude oil typically has a viscosity ranging from 10^3 to 10^6 cP and an API gravity below 20API. To efficiently transport heavy crude oil, positive displacement (PD) rotary pumps are commonly used. Four common types of PD pumps include internal gear, external gear, timed lobe, and vane pumps.
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Before selecting a pump, factors such as required flow rate, differential pressure, temperature, particle size in the liquid, abrasive characteristics, and corrosiveness of the liquid must be considered. Proper suction conditions are essential for PD pumps to function effectively, as incomplete filling of pump cavities can reduce pump flow.
Internal gear pumps are well-suited for high-viscosity liquids but are susceptible to damage when pumping large solids. Their ability to operate at low speeds makes them ideal for high-viscosity applications with minimal inlet pressure requirements.
Internal gear pumps have successfully handled liquids with viscosities exceeding 1,320,000 cSt and low-viscosity liquids like propane and ammonia.
External gear pumps are typically employed in high-pressure applications such as hydraulics. They operate similarly to internal gear pumps, with two gears meshing to produce flow. Proper adjustment of pump speed is crucial when handling thick liquids, as gear teeth must adequately fill with viscous fluids.
External gear pumps may face challenges under critical suction conditions, and torque requirements increase with rising viscosity, potentially requiring stronger vane materials.
Lobe pumps use lobes that do not make contact, driven by external timing gears. While suitable for low-viscosity liquids, their performance diminishes with high-viscosity fluids, often requiring significant reductions in operating speed.
Vane pumps offer better dry priming capability than other positive displacement pumps. They utilize a rotor with radial slots positioned off-center in a housing bore, with vanes that slide in and out as the rotor turns. Vane pumps typically operate at 1,000 or 1,750 rpm, excelling with low-viscosity liquids that fill pump cavities easily.
High-viscosity applications may demand significantly reduced speeds and stronger vane materials for optimal performance.
Pressure drop correlations play a crucial role in assessing hydrostatic and frictional fluid losses during heavy crude oil transport in wellbores and pipelines. Several empirically derived correlations account for various flow conditions:
Single Phase - Wellbores and Pipelines:
Multi-phase - Pipeline:
Multi-phase - Wellbore:
The commonly used correlations include the Fanning Correlation, Panhandle Correlation, Modified Panhandle Correlation, Weymouth Correlation, and Fanning Gas Correlation.
The Fanning correlation, divided into Fanning Liquid and Fanning Gas, provides a means to calculate pressure loss due to friction effects for both single-phase gas and liquid flows. It relies on the Fanning friction factor, density, average velocity, pipe length, and diameter.
The Fanning Correlation can be expressed as:
Δf =
(gc⋅D⋅f⋅ρ⋅v2⋅L) / 2
Where:
The Fanning Correlation applies to both single-phase gas (Fanning Gas) and single-phase liquid (Fanning Liquid) flows.
The Panhandle correlation, originally developed for single-phase gas flow through horizontal pipes, has been adapted to account for vertical, inclined, and horizontal flow. It considers both the horizontal and vertical components of pressure drop.
The Panhandle Correlation can be expressed as:
Δf =
(gc⋅D⋅κ⋅(ρG⋅V2)⋅L) / 2 + (ΔHH / 144)
Where:
The Panhandle equation incorporates a flow efficiency factor (( E )), generally ranging from 0.8 to 0.95, to account for real-life conditions.
The Modified Panhandle correlation is an adaptation of the original Panhandle equation, designed for single-phase gas flow. It accounts for horizontal, inclined, and vertical flow, considering both frictional and hydrostatic pressure losses.
The pressure drop due to friction in the Modified Panhandle Correlation is given by:
Δf =
(gc⋅D⋅κ⋅(ρG⋅V2)⋅L) / 2
The vertical component of pressure drop is calculated using the hydrostatic head equation:
ΔHH =
(144(P1 - P2) - (ρG⋅g⋅Δz)) / 144
Where:
The Modified Panhandle equation incorporates a flow efficiency factor (( E )), typically ranging from 0.88 to 0.94, to represent real-world conditions.
The Weymouth correlation, designed for single-phase gas flow in pipelines, calculates pressure drop due to friction and accounts for horizontal, inclined, and vertical pipes. It relies on the Weymouth friction factor and incorporates a flow efficiency factor (( E )) for real-life scenarios.
The pressure drop due to friction in the Weymouth Correlation is given by:
Δf =
(gc⋅D⋅κ⋅(ρG⋅V2)⋅L) / 2
The hydrostatic pressure difference is calculated as follows:
ΔHH =
(144(P1 - P2) - (ρG⋅g⋅Δz)) / 144
Where:
The Weymouth equation utilizes a flow efficiency factor (( E )), typically set at 115% for practical use.
The Fanning Gas Correlation, also known as Multi-step Cullender and Smith, calculates the hydrostatic pressure difference (( Delta PHH )) and friction pressure loss (( Delta Pf )) for single-phase gas flow. This correlation employs standard equations to determine pressure losses.
The friction pressure loss is determined using the Fanning friction factor, while the hydrostatic pressure difference considers the compressibility and density variations of gas during pressure changes. The Fanning Gas Correlation applies to pipes of all inclinations and is implemented as a multi-segment procedure.
The Fanning equation calculates friction pressure loss for both single-phase gas and liquid flows. It relies on the Fanning friction factor, density, velocity, length, inside diameter, and gravitational acceleration.
The Fanning Friction factor equation is the same for single-phase gas and liquid flows:
Δf =
(gc⋅D⋅f⋅(ρG⋅V2)⋅L) / 2
Where:
The calculation of hydrostatic head differs for gas and liquid due to gas compressibility and varying density with pressure and temperature. The hydrostatic pressure difference is determined by sequentially calculating pressure changes in small steps to account for density variations with pressure.
The hydrostatic pressure difference equation is:
ΔHH = ∑i=1n [(144(Pi-1 - Pi) - (ρG⋅g⋅Δz)) / 144]
Where:
The transportation of heavy crude oil presents challenges due to its low mobility and restricted flow characteristics. Several factors, including wax and asphaltene deposition on the pipeline's internal wall surfaces, affect the flow of heavy oil. Table 1 provides an example of the composition of medium, heavy, and extra-heavy Mexican crude oil, showcasing variations in API gravity and substance content.
| Parameter | Medium | Heavy | Extra-Heavy | |
|---|---|---|---|---|
| API gravity | 21.27 | 11.90 | 9.17 | |
| Molecular weight (g/mol) | 314.8 | 486 | 507.8 | |
| Sulfur content (%) | 3.40 | 5.0 | 4.80 | |
| Water content (%) | 1.80 | 0.05 | 0.05 | |
| SARA analysis | ||||
| Saturates | 26.53 | 7.94 | 15.00 | |
| Aromatics | 14.74 | 5.28 | 19.11 | |
| Resins | 47.60 | 70.93 | 46.78 | |
| Asphaltenes (from n- C7) | 11.13 | 15.85 | 19.11 | |
This table highlights the differences in API gravity and the presence of various substances in medium, heavy, and extra-heavy crude oil. These distinctions categorize them as medium, heavy, or extra-heavy crude oils.
The challenge lies in effectively transporting heavy crude oil through pipelines. Several methods can accomplish this:
One approach to ease the transportation of heavy crude oil is viscosity reduction. Heating the oil can reduce its viscosity, making it more flowable. However, this method requires energy and careful temperature control to avoid excessive heating that may cause undesirable chemical changes in the oil.
To minimize drag and improve the flow of heavy crude oil, drag-reducing agents (DRAs) can be added. DRAs are chemicals that modify the fluid's rheological properties, reducing its resistance to flow. This approach can enhance the overall efficiency of oil transport through pipelines.
In-situ oil upgrading technologies aim to improve the properties of heavy crude oil directly at the well site. These methods can include thermal processes, such as steam-assisted gravity drainage (SAGD), or chemical treatments to reduce viscosity and enhance flowability. In-situ upgrading minimizes the challenges associated with transporting highly viscous crude oil.
Despite the various methods mentioned above, the transport of heavy crude oil still faces several challenges:
Wax and asphaltene deposition on the internal surfaces of pipelines can lead to flow restrictions and increased frictional losses. These deposits can solidify and accumulate over time, reducing the pipeline's capacity and necessitating costly maintenance and cleaning operations.
Maintaining the right temperature for heavy crude oil during transport is crucial to prevent excessive viscosity and wax precipitation. In colder climates, insulation and heating systems may be necessary to ensure the oil remains sufficiently fluid throughout the journey.
Heavy crude oil often contains corrosive components that can lead to pipeline corrosion. Proper material selection, coatings, and corrosion monitoring are essential to mitigate this issue and ensure the integrity of the pipeline over time.
When heavy crude oil undergoes rapid pressure changes in the pump cavities, cavitation can occur, causing damage to the pump and reducing its efficiency. Adequate suction conditions and pump selection are vital to prevent cavitation issues.
Addressing the challenges encountered during the transport of heavy crude oil requires strategic remedial measures and solutions:
To mitigate wax and asphaltene deposition, regular pipeline cleaning and the use of chemical inhibitors can be effective. Chemical treatments can prevent the buildup of solid deposits and maintain the pipeline's flow capacity.
Implementing efficient temperature control systems, such as electric heat tracing or hot oil circulation, helps maintain the oil's temperature within the desired range. This prevents wax formation and ensures smooth oil flow.
Corrosion prevention measures include the selection of corrosion-resistant materials for pipeline construction, internal coatings, and corrosion inhibitors. Routine inspection and monitoring programs help identify and address corrosion issues promptly.
Proper pump selection, including consideration of suction conditions, is crucial to prevent cavitation. Regular pump maintenance and monitoring of suction pressure conditions help ensure the efficient operation of PD pumps.
The transportation of heavy crude oil presents unique challenges due to its high viscosity and specific gravity. To overcome these challenges, various methods, such as viscosity reduction, drag reduction, and in-situ oil upgrading, can be employed. However, problems like wax deposition, temperature control, corrosion, and pump cavitation must be carefully managed through remedial measures and solutions to ensure the efficient and reliable transport of heavy crude oil through pipelines.
Heavy Crude Oil Transport and Pressure Drop Analysis. (2024, Jan 03). Retrieved from https://studymoose.com/document/heavy-crude-oil-transport-and-pressure-drop-analysis
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