Paraffin Problems in Gas Systems

Abstract

Abstract Gas wells, coal seam methane wells, glycol dehydrators, gas plants and gas storage wells can all experience paraffin problems that affect the cost to produce and handle gas. Problems with paraffin have been encountered from the formation through the gas plants to the storage wells. This paper will explain what paraffin is, why the problems occur and what type of treatments have effectively treated gas system problems. Case histories of various types of successful and unsuccessful treatments will be presented. Introduction Gas wells and coal seam methane wells may produce high gravity crude oil (> 40° API) or condensate (> 50° API) with the gas. Some wells may produce no liquids to surface but will still be producing oil or condensate into the well bore. The paraffin or n-alkane components account for a significant portion of a majority of these oils and condensates. These paraffins have a straight chain linear structure composed entirely of carbon and hydrogen. The melting points vary from –295°F for methane gas (CH4) to >240°F for Hectane (C100H202) and above.1 See Table 1. It is not known what the longest naturally occurring n-alkane in crude oil or condensate is, the longest observed by this author was a C103H208. The paraffins >C20H42 are the ones that can cause deposition or congealing in gas systems.2 These paraffins >C20H42 can deposit anywhere from the fractures in the formation rock to the gas storage wells.3 The deposits can vary in consistency from rock hard for the longest chain length paraffin to very soft, mayonnaise like congealing oil deposits caused by shorter chain paraffin. Crude oils and condensates have congealing points from < –90°F to >130°F. Paraffin can cause a great many types of problems including deposition from in the formation to the gas plant, congealing oil, interface problems, tank bottoms, stabilized emulsions, high line pressures, plugged flow lines, paraffin coated solids, under deposit corrosion, plugging of injection wells and filter plugging.4 One or all of these problems can occur in a gas production system. Many different types of treating programs have been used to control all of the various types of problems in different systems. Down hole problems have been treated by cutting or wire lining, heated tubing, coated tubing, fiberglass tubing, hot water circulation, hot oiling down the tubing, bacteria, magnets, enzymes, steam injection, solvent or condensate treatments, continuous or batch down hole chemical injection and squeeze treatments of crystal modifiers (PPD's). Chemistry of Crude Oil and Condensate The paraffin series of compounds or n-alkanes contain only hydrogen and carbon. The number of carbon atoms can range from 1 to >100. The ratio of carbon to hydrogen atoms can be shown by the formula CnH2n+2. This means that for every carbon atom we will have twice as many hydrogen atoms plus two.1 See Table 1. Reservoir fluids in gas wells are composed primarily of methane, ethane and propane which have very low boiling points. Depending upon a well's temperature and pressure characteristics liquid oil or condensate may be produced. The oil or condensate entering the well bore will contain the longer chain paraffins that give the liquid its cloud point. The cloud point is the temperature at which the longest chain length paraffin present in an oil or condensate becomes insoluble in that liquid. The cloud point indicates the temperature at which paraffin deposition will start. If the formation or equipment surface reaches the cloud point temperature of the liquid, paraffin deposition will start even though the bulk oil is still above the cloud point. As the surface temperatures of the equipment drops below the cloud point shorter chain paraffins will start to precipitate and deposit. The type of paraffin depositing will change as the condensate progresses downstream through the system. The melting point of the deposits will change as the type of paraffin changes. If the system cools sufficiently paraffins of < C36H74 will start to precipitate and will cause the congealing of the condensate itself. Many times congealing condensate will be misidentified as paraffin deposition. The only difference between deposited paraffin and deposited congealed condensate will be the melting point of deposit itself. A rule of thumb of the author is that if the deposit melts at < 120°F it is probably a congealing condensate problem.