Paraffinic Hydrocarbon

Key Takeaways

• Normal paraffins (n-alkanes) in crude oil are the primary target of biodegradation by reservoir microorganisms, and the degree of n-alkane depletion relative to branched and cyclic hydrocarbons is the most widely used geochemical indicator of biodegradation severity in subsurface petroleum accumulations: reservoir biodegradation occurs when meteoric water carrying oxygen and nutrients enters the reservoir from the surface recharge zone, enabling aerobic bacteria to oxidize the petroleum at the oil-water contact; the biodegradation preference order (Peters-Moldowan scale) begins with the preferential removal of n-alkanes (C1-C15 first, then C15-C35 waxes), followed by removal of isoprenoids (pristane, phytane), then branched alkanes, then steranes, then hopanes, with the aromatic hydrocarbons and polycyclic naphthenes being most resistant to biodegradation; a paraffinic crude oil that has lost its n-alkanes to biodegradation shows a characteristic "hump" of unresolved complex mixture (UCM) on the gas chromatogram where the n-alkane peaks formerly appeared, and has higher viscosity, higher pour point, higher sulfur content, and higher asphaltene content relative to the undegraded oil; in production terms, a biodegraded paraffinic crude may become heavy oil (API gravity below 20 degrees) or extra-heavy oil (API gravity below 10 degrees) requiring thermal stimulation or dilution with lighter hydrocarbons (diluent) to achieve adequate mobility for production and pipeline transportation.
• Wax appearance temperature (WAT, also called cloud point) and pour point are the critical thermal properties of paraffinic crude oils that determine the operational temperature requirements for production flowlines, export pipelines, and process equipment: the WAT is the temperature at which the first paraffin wax crystals appear when the crude oil is cooled from its reservoir temperature, typically measured by cross-polarized light microscopy (detecting the birefringent wax crystals as they nucleate) or by viscometry (detecting the abrupt increase in viscosity at the crystal nucleation temperature); the pour point (the lowest temperature at which the oil continues to flow under standard test conditions per ASTM D97) is typically 3-10 degrees C below the WAT; paraffinic crude oils from West African fields (Bonny Light from Nigeria, Escravos from Nigeria, Kuito from Angola), Australian Carnarvon Basin fields, and Libyan fields commonly have WAT values of 30-50 degrees C and pour points of 20-40 degrees C, creating cold flow challenges in deepwater flowlines where the ambient seawater temperature of 4 degrees C at depths below 1,000 meters is far below the WAT; the consequences of allowing a paraffinic crude line to cool below the WAT include gelling (the crude oil transitioning from a pumpable liquid to a rigid semi-solid gel that cannot be restarted by normal pumping pressure), pipeline plugging (when the gel solidifies against the pipe wall and progressively reduces the flow area until the pipe is completely blocked), and in the most severe cases, the irreversible consolidation of the wax network into a hard deposit requiring mechanical or thermal remediation.
• Chemical wax inhibitors (also called wax crystal modifiers or pour point depressants) are polymeric additives that cocrystallize with the developing wax network as the crude oil cools below the WAT, modifying the wax crystal morphology from the interlocking plate-like crystals that form an immobile gel to smaller, rounder crystals that remain dispersed in the oil and do not build a continuous gel network: effective wax crystal modifiers include polyacrylate polymers, ethylene-vinyl acetate (EVA) copolymers, and maleic anhydride copolymers whose pendant alkyl groups match the chain length of the dominant paraffin species in the crude (the polymer must have alkyl side chains of the same length as the wax molecules to intercalate effectively into the growing wax crystal); the selection of the appropriate wax inhibitor requires matching the polymer chemistry to the paraffin carbon number distribution of the specific crude, which is determined by gas chromatography (GC) analysis of the crude oil wax fraction; the inhibitor dosage (typically 50-3,000 ppm by mass of oil) is optimized by laboratory flow loop testing that measures the cold-flow behavior of the treated crude at temperatures near and below the WAT under flow conditions representative of the pipeline; the temperature at which a wax inhibitor must be injected (the injection temperature must be above the WAT so the inhibitor can dissolve in the oil and be present as individual polymer molecules when wax crystal nucleation begins) determines the placement of the chemical injection point in the production system — typically at the wellhead or subsea tree, upstream of the cold section of the flowline where the temperature drops below WAT.
• Paraffin content analysis of crude oil by high-temperature gas chromatography (HTGC) provides the n-alkane distribution from C1 to C100+ that is used to predict cold-flow behavior, to design wax management programs, and to characterize the source of waxy crude for refinery scheduling: HTGC separates the paraffin homologs by their boiling points on a high-temperature GC column capable of operating above 400 degrees C (necessary to volatilize the C50+ paraffins that have boiling points above 600 degrees C at atmospheric pressure), and the detector response at each elution time is calibrated to the known n-alkane concentration to produce a quantitative carbon number distribution; the total wax content by mass (the gravimetrically measured fraction of the oil that precipitates when the oil is cooled to 0 degrees C or lower, typically by the UOP 46-85 or IP 329 test methods) correlates with the n-alkane content above C18 in the HTGC distribution; paraffinic crude oils are defined by the ASTM-standard base classification as having a paraffin base, mixed base, or naphthene base character, with paraffin base crudes having predominantly paraffinic residua after distillation of the light fractions (gasoline and kerosene), naphthene base crudes having predominantly naphthenic (cycloparaffin) residua, and mixed base crudes having intermediate character; the base classification affects refinery yield predictions, lube oil production potential (paraffinic crudes yield high-quality waxy lube base stocks after dewaxing), and asphalt quality (naphthenic crudes yield better quality asphalt than paraffinic crudes).
• Subsurface wax deposition in wellbore tubing is a production engineering challenge in waxy oil wells where the crude oil cools below the WAT as it travels up the tubing and the pressure decreases, causing wax to precipitate and adhere to the tubing wall and progressively restrict the flow area until the well requires intervention: the wax deposition profile in the tubing (the depth at which deposition begins and the rate of radial thickness increase over time) depends on the WAT and wax composition of the crude, the tubing temperature profile (which is determined by the wellbore heat transfer model accounting for the geothermal gradient, production rate, fluid properties, and tubing/casing dimensions), and the flow regime (wax deposition is more severe in laminar flow where the radial heat transfer is diffusion-limited and the wax crystals have time to adhere to the cold wall surface before being swept away by turbulent mixing); routine intervention to remove wellbore wax deposits includes wireline-conveyed mechanical cutters (wax cutters or scrapers) that are lowered through the tubing to scrape the wax from the wall, hot oiling (pumping heated crude oil down the annulus to melt the wax deposits from the outside of the tubing and flush them to the surface), and hot watering (similar to hot oiling but using heated water as the heat transfer fluid); chemical wax dissolvers (aromatic solvents including toluene, xylene, and proprietary formulations) are less effective for downhole wax removal than for surface equipment cleaning because the temperature in the wellbore may be too close to the WAT for the dissolvers to work efficiently.