Evaluating corrosive potential and impressing effective cathodic current levels based on surface methodologies exclusively are insufficient to effectively protect most wellbore casings. The timehonored solutions of inhibited drilling muds, isolating primary cement jobs, squeeze cementing, and material considerations do not provide a cost-effective means to address downhole corrosion in the Texas panhandle region. Accurate emplacement of cathodic protection systems is the key. The CPET* Corrosion and Protection Evaluation Tool effectively measures the corrosive profile and corrosive rates in the wellbore environment. This allows the operator to optimize his corrosion prevention expenditures.
*Mark of Schlumberger
External corrosion of well casings has always been a difficult and sometimes elusive problem to address in oilfield operations. While cathodic protection is second nature to the pipeline and facilities portion of the industry, corrosion processes and cathodic protection can be complex and are not well understood by most production personnel.
The problems with effective cathodic protection of well casings result from the extreme variation in the environment encountered when steel pipe is placed vertically in the earth. The expense and difficulty in accurately evaluating the level of protection required often leads to inexpensive generalized rules of thumb. These surface methodologies have proven insufficient when changes in the resistivity of geologic layers or stray current interference have not been taken into account. All these factors combined have resulted in perceived cathodic protection "failures." In addition, because of the long-term manifestation of corrosion processes, problems are often ignored well past the optimum point for preventive measures.
This paper describes a history of corrosion problems in a large group of wells in the Texas panhandle and the techniques used to reduce the associated failures.