Completion Fluids
Casing that is not cemented should be protected by oxygen-free. high-pH and thixotropic completion fluid. Residual dissolved oxygen initiates corrosion pitting and promotes subsequent bacterial growth. Oxygen contained in most completion fluids is best controlled by chemical conversion to a harmless reaction product. Common scavengers used to remove oxygen are zinc-phosphate and zinc-chromate. These inhibitors are used at concentrations of 500-800 mg/1. Low pH values, on the other hand increase hydrogen availability in fluids which initiates hydrogen-induced stress cracking. Completion fluids should be thixotropic in order to suspend solids and maintain the required hydrostatic head of the fluid column. This reduces the stresses on casing due to collapse and buckling loads.
As discussed earlier, both hydrogen and sulphide components of hydrogen sulphide are instrumental in bringing about sudden failures in casings. Hydrogen sulphide may enter the completion fluid from formations that contain H2S. or originate from bacterial action on sulphur compounds commonly present in completion fluids, from thermal degradation of sulphur-containing fluid additives, from chemical reactions with tool joint thread lubricants that contain sulphur, and from thermal degradation of organic additives.
Scavengers and film-forming organic inhibitors are utilized in the treatment of water-based completion fluids. Common inhibitors used to remove H2S from completion fluid are iron sponge, zinc oxide and zinc carbonate and sodium or
potassium chromate. Iron sponge is a highly porous synthetic oxide of iron and
reacts with H2S to form iron sulphite, whereas zinc oxide and zinc carbonate remove H2S by forming precipitates of sulphide, whereas chromates remove H2S by oxidation process.
Film-forming organic inhibitors have been found very effective in protecting casing from contaminants. They are typically oily liquid or wax-like solids with large chains or rings with positively-charged amine nitrogen group on one end. Their structure can be represented as follows:
R. NH2 R2.NH R3N [R4X]+
Primary Secondary Tertiary Quaternary
where:
|
Fig. 6.3: Diagram illustrating the theory of cathodic protection. I" = current required to produce complete cathodic protection. Current must exceed equilibrium corrosion current, I’, to provide any protection. Corrosion will cease when the flow of cathodic current (/") increases cathodic polarization to the open circuit potential (Ea) of the anode as shown at point.4. |
R represents the hydrocarbon chain or ring portions of the molecule.
In water, the amine groups take on an additional hydrogen that gives them a net posit ive-charge. Thus, the polar amine groups are adsorbed to the casing and the hydrocarbon portion forms an oily, water-repellent surface film. The amine inhibitors actually work best where H2S is present and 02 is absent, because they can react with H2S to form a complex compound which helps to build a protective film. (For details see Jones. 1988.)