The corrosion of well casings in the Ventura Avenue field has become an increasingly important problem during recent years. In the seven-year period since the first leak occurred a total of 40 well casings have failed because of corrosion. A statistical prediction of the future failures now indicates that about 75 percent of the 250 wells drilled between 1938 and 1950 may be expected to fail unless some reduction in failure rate is effected. The inaccessible location of the leaks made it necessary to conduct most of the investigation on a basis of theory, statistics, surface measurements and remote controlled subsurface measurements. In some cases it was necessary to develop new tools in order to make the subsurface measurements. Among the possible causes of corrosion which were considered and, in general, refuted during this phase of the study were particular corrosive strata, defective steel, electric currents in casing or flow lines and interzonal migration of salt water. Concurrent attempts were made to develop means of preventing the failures. The more important methods considered were cathodic protection, insulating flanges on wellhead connections, protective coatings, extra casing strings and full-string cementing. Neither the study of possible causes nor the study of preventives yielded positive results. However, there were some indications that salt water could be migrating behind the well casings. These indications caused the adoption and use on 19 wells of the full-string cementing practice.
The turning point of the investigation occurred when, the corroded casing was recovered from one well. This made it possible to minutely analyze the material relevant to the problem including the corroded steel, the corrosion products, the formation adjacent to the leak and the residual drilling fluid cake. The results of the analysis plus further statistical study has led to the conclusion that bacterial activity is responsible for the corrosion which has been experienced. Research has also developed methods for inhibiting or destroying the bacteria. Alkaline environments were known to be inimical to bacterial activity and laboratory and field testing showed that the use of high pH drilling fluids would create such an environment adjacent to the well casings. The testing also showed that, in addition to probably preventing corrosion, the high pH drilling fluids are thoroughly satisfactory from an operational viewpoint. Therefore, this type of fluid is now used for drilling to the casing point in all Shell wells in the Ventura Avenue field. The foregoing partial solution to the problem is applicable only to wells yet to be drilled. To date no proven economical method for preventing bacterial corrosion in the existing wells has been found but study of the problem is still in progress.
