Techniques to Control Corrosion in Drilling Operations

Contamination by carbon dioxide or hydrogen sulfide from the formation can be quite serious if large volumes of gas are allowed to enter the fluid column. This is best prevented by properly controlling the hydrostatic pressure. When drilling operations are at a pressure near that of the formation or otherwise under pressure, larger quantities of formation gas can enter the mud and more acid contamination wfll occur. 

Contamination can occur while drilling either gas- or water-bearing formations, so it is customary to provide an alkahne buffer to help neutralize them. In most cases the alkaline buffer is used to preserve driUing fluid properties, as well as to reduce corrosion problems. Alkaline materials have limitations and may be insuflicient to neutralize the acid gases if serious contamination is occurring. Under these conditions much of the gas may be vented to the atmosphere from the surface pits or disposed of even more efficiently by degassing equipment In addition, drilling fluid properties can be adjusted to facilitate the escape of the gas. 

Copper compounds also are used as sulfide ion precipitators. The copper compounds are efficient in precipitating sulfide, but can cause accelerated corrosion of steel. Basic copper carbonate is used to combat the sulfide ion problem. Copper carbonate has very limited solubility in water and, as with zinc compounds, the solids react with sulfide ions. 

The limited solubility of copper carbonate in drilling fluids becomes a corrosion problem as a result of an electrochemical reaction, whereby the copper ion is displaced from solution by iron going into solution, causing metalhc copper to be plated on the steel equipment. For this reason, copper compounds are not generally recommended in drilling fluids. 

Acid gas contamination has resulted from drilling fluid materials that have been altered by temperature, microbiological activity, or electrochemical effects. Contamination originating from thermal breakdown of drilling fluid additives is conditioned by time and temperature. 

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Requirements for Corrosion Controi in the Petroieum and Petrochemical industries 163 6.1.3 Techniques to Control Corrosion in Drilling Operations... 

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