Paraffin Deposition in Petroleum Production

Abstract

Introduction Precipitation of solids, generically called Precipitation of solids, generically called "paraffins," is a well-recognized production problem. The severity and nature of the production problem. The severity and nature of the problem can be predicted by use of accurate problem can be predicted by use of accurate composition of the C7+ fraction and principles of solution thermodynamics. principles of solution thermodynamics. Paraffin problems commonly result from cooling and subsequent precipitation of highmolecular-weight hydrocarbons during Iransit from the reservoir to the surface, Paraffin deposits form on the wall of downhole tubulars and in low-velocity zones near entrances and exits of chokes, collars, or similar restrictions in the flow path. Costly preventive chemical treatments or remedial preventive chemical treatments or remedial workover procedures are required to cut away the paraffin deposit with mechanical knives or scrapers, As an alternative, hot oil can be used to melt these deposits. Even more significant, but generally unaddressed, is the problem of backpressure on the reservoir caused by paraffin deposits, in the production tubing, which decreases the fluid withdrawal rate and defers production. Heavy hydrocarbons can also production. Heavy hydrocarbons can also precipitate within the reservoir, reducing the precipitate within the reservoir, reducing the permeability at distances that are permeability at distances that are untreatable from the wellbore, thereby posing a large production restriction. production restriction. Precipitation of solid paraffin is actually an example of fluid/solid phase equilibrium. It is explained in terms of established principles of thermodynamics of solutions-i.e., principles of thermodynamics of solutions-i.e., the solution of higher-molecular-weight hydrocarbons in lower-molecular-weight hydrocarbons that act as a solvent. Simply put, high-molecular-weight solids precipitate whenever anything occurs that precipitate whenever anything occurs that decreases the carrying capacity of the fluid solvent. Three classes of problems associated with paraffin deposition in petroleum production are supercritical fluid phenomena in production are supercritical fluid phenomena in the reservoir, temperature changes in the production tubing; and pressure and production tubing; and pressure and temperature changes in surface facilities, gathering systems, and trunklines. Supercritical Solvent Effects In the Reservoir In deep, hot reservoirs, fluid have equilibrated over geologic time. The reservoir temperature typically exceeds the critical temperature of several of the light ends, such as methane and ethane, and associated nonhydrocarbon gases, such as nitrogen and CO2. These "supercritical" components are known to act as effective solvents for heavy ends. The most notable application of supercritical-solvent extraction technology is the decaffeination of coffee with CO2. Similar applications demonstrate the general affinity of supercritical fluids for macromolecular solutes. When a well begins producing hydrocarbons and associated fluids to the surface, a pressure gradient is established. resulting in pressure gradient is established. resulting in flow of reservoir fluid through the porous medium. The carrying capacity of the supercritical-solvent fraction decreases dramatically as the pressure decreases in the direction of the wellbore. Precipitation of dissolved heavy ends, in the reservoir rock, is predictable. Accumulation of precipitated heavy ends in the reservoir is precipitated heavy ends in the reservoir is a plausible contribution to relative permeability decrease with cumulative production. permeability decrease with cumulative production. Temperature Changes in Production Tubing Production Tubing Solids deposition in production tubing, the most common example of the paraffin problem observed in production, is primarily a problem observed in production, is primarily a result of decreasing temperature, which causes a certain portion of the highmolecular-weight fraction to precipitate. Mechanisms other than decreasing temperature, such as pressure change, can reduce the carrying capacity of the solvent fraction of the reservoir fluid during transit from bottomhole to the surface. Pressure change resulting in liberation of volatile light ends as gas may prove to be a very significant mechanism in terms of its impact on overall recovery of oil and gas. Pressure and Temperature Pressure and Temperature Changes in Surface Facilities and Pipelines Precipitation of paraffin in surface Precipitation of paraffin in surface equipment results from lower fluid temperature and from a decrease in fluid pressure. In separators, precipitation results from flashing and the resulting decrease in the amount of light ends that are pan of the solvent fraction. Paraffin deposition in surface equipment is a relatively minor technical problem because it is easily accessible and most of the heavier fraction has already precipitated in the wellbore and the reservoir. Nonetheless, paraffin deposition in trunklines and transmission lines exposed to severely cold climatic conditions remains an expensive aspect of getting oil to the marketplace, especially when pipelines are on the ocean floor, such as at the Beatrice field in the North Sea and the Hibernia field offshore Newfoundland, or in Arctic regions, such as the Alaskan North Slope, the Daqing and Shen-Yang fields in northern China, and fields in Siberia. Reservoir Engineering of Paraffin Fluids Today's reservoir fluid must be thought of as a complex mixture of light ends (methane, ethane, propane, butanes, pentanes, hexanes, etc.) and heavy ends (a near continuum of molecular weight fractions characterized as distributions of paraffins, naphthenes, aromatics, resins, bitumens, asphaltenes, etc.). It is not sufficient to characterize reservoir fluids simply in terms of weight percent C7+, ASTM D-86 Pour Point, and API gravity. Point, and API gravity. Reliable sampling and compositional analysis of the reservoir fluid, initially and during production, are essential. Accurate knowledge of fluid composition enables the engineer to forecast and define the severity of paraffin deposition problems associated with production. JPT P. 1024