Anti-scaling device minimizes contamination of wellhead flow lines

Table 1 shows typical precipitates encountered in oil and gas production, including sulfates such as calcium sulfate (anhydrite, gypsum), barium sulfate (barytes), strontium sulfate (celestite), and calcium carbonate. Other less common precipitates such as iron oxides, iron sulfides, and iron carbonate have also been reported.^19-21

Calcite deposition is generally a self-deposition process. The main factor in its formation is the loss of CO₂ from water to the hydrocarbon phase as the pressure is reduced. This removes the carbonic acid from the water phase, which kept the basic calcite in a dissolved state. The solubility of calcite also decreases with decreasing temperature (at constant CO₂ partial pressure).^22-24

Iron sulphide deposits are almost ubiquitous when hydrogen sulphide is produced and are often the result of pipe corrosion in the presence of H₂S²⁵

Scale formation occurs in water injection systems mainly when two incompatible waters are involved. Typical examples are seawater with a high concentration of sulphate ions and formation waters with a high concentration of calcium, barium and strontium ions. Mixing these waters can therefore cause precipitation of calcium sulphate, barium sulphate (BaSO4) or strontium sulphate. BaSO4 scale forms as an odourless white crystalline precipitate that is insoluble in water.^{26 27} Scale prevention using chemical inhibitors, continuously injected or squeezed into the formation near the wellbore, is generally considered the most cost-effective solution to this problem, but BaSO4’s physical hardness and chemical and thermal stability make it one of the most difficult types of scale to inhibit.²⁸

Prevention of stone formation

To investigate and evaluate the performance, suitability and ability of the SPD to address scale formation and production shortfalls, two units were installed at the Hassi Messaoud field on a 6-inch MD287 wellhead flowline and initially on an 8-inch section of a 6 x 8-inch MD525 wellhead flowline (the SPD was subsequently moved to the 6-inch section of the flowline).

Hassi Messaoud is the largest oil field in Algeria, about 650 km southeast of Algiers and covering about 2,000 km2, divided into two fields, northern and southern. It includes over 1,000 wells and was developed and is currently operated by the Algerian state-owned company Sonatrach. Total proven reserves are about 870 × 106 tonnes (6.4 billion barrels) and produces about 56,000 m3/day (350,000 bo/d). Most of the associated gas is reinjected into the reservoir to improve oil recovery, and this has recently been supplemented by gas from other fields in the region.