Treatment of Metal Surfaces to Alter Their Thermal Radiation Characteristics
Publication Date: 2004-Aug-03
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Gasses entering a well bore will contain evaporated water. This evaporated water does not consist of tiny particles of liquid held in suspension in the gas but is itself a gas as invisible as the gas with which it mixes. The volumeof water that canbe held evaporated in a gas is determined by the gas’s chemical composition, temperature and pressure. When, at a given pressure and temperature, a gas is holding all the evaporated water it can, the gas is said to be saturated or slits dew point or-st 400%-relative-humidity. As the gas flows up the well bore the temperature of the adjacent formation progressively declines.-as.will the gas’ pressure. As the gas flows up the well bore it will lose heat to the progressively cooler formation so that the temperature of the gas will also decline. At some point in the well bore gas pressure and temperature may reach a point that the volume of water evaporated in the gas exceeds what the gas can hold and liquid water begins to condense out of the gas. If the gas’ flowing velocity is high enough aerodynamic drag forces on the waterdroptets wiiIcarryihe~m~ut-of the welL If the gas velocity is not high enough the water will begin to collect as fog, “rain” and slugs within the well bore. Produced gas must now fight its way through this water gauntlet which acts to increase the back pressure on the producing formation, reducing the inflow of gas. At some point gas production may fall to a level where it is not economical to produce the well or production- may-stopzalltogether. The produced gas in the production tubing can be heated to keep it above its dew pointand this is the-subject of patents held by Centrilift. Because the adjacent formation is a massive heat sink it is economically desirable to thermally insulate the production tubing andthe heating source from the formation. Having the annulus, between the production tubing and the casing cemented into the formation, filled with a low pressure gas (vacuum) will substantially reduce convective heat losses to the formation -and ia also the-subject of-patents bald b~yCe-ntrilift.Thermal radiation, however, is not reduced by the vacuum and now become the primary mechanism of heat loss totheformation. Thermal radiation losses can be greatly reduced by highly polished, reflective surfaces. For example if the casing inside diameter surface were highly polished or coated with a highly reflective coating of silver or aluminum, radiation losses would be substantially reduced. This is not practical, economical or sustainable in a gas well environment. The quantity of thermal energy (heat) radiated from a surface is a function of the surface area, the forth power of the surfaces’ absolute temperature and a surface property called emissivitiy. An ideal “black” surface has an emissivity of 1 and will emit all of the thermal energy theoretically possible as described by Kirchoff’s Law. At temperatures of interest here most surfaces have an emissivity between .85 and .95. In other words these surfaces will emit thermal radiation at a rate of 85 to 95% of an ideal black surface. A galvanized (zinc coated) metal surface, however, has a measured emissivity of 0.23 when new and 0.28 when “dirty”. If the outside surface of the production tubing was galvanized, radiation heat losses will be substantially reduced and, in conjunction with a vacuum in the annulus, the amount of energy needed to heat a gas well to keep the gas above dew point will be substantially reduced. Galvanizing facilities are widely available and the process is a standard, very widely used industrial commodity. Galvanized surfaces are robust and will hold up well in ga~ well environments.