Patent ID: 6031381
Filing Date: 2000-02-29
Classification: G01V

Abstract:
A method for electrically inspecting a specific portion of a cased well surrounded by a geological formation using at least three vertically spaced apart voltage measuring electrodes and with resistances being defined between the vertically spaced apart voltage measurement electrodes, comprising:providing first, second, and third vertically spaced apart voltage measurement electrode means that electrically engage the interior of the specific portion of the cased well with a first resistance being defined between first and second voltage measurement electrode means and a second resistance being defined between second and third voltage measurement electrode means;providing voltage detector means that provides the separate voltage outputs respectively from said first, second, and third voltage measurement electrode means;providing first A.C. current conduction means that generates and conducts a first A.C. current between a first current conducting electrode means in electrical contact with the interior of the cased well to a second current conducting electrode means located on the surface of the earth;providing second A.C. current conduction means that alternatively generates and conducts a second A.C. current between said first current conducting electrode means and a third current conducting electrode means in electrical contact with the interior of the cased well whereby said third current conducting electrode means is located on a first vertical side of said three voltage measurement electrode means and whereby said first current conducting electrode means is located on a second vertical side of said three voltage measurement means;obtaining measurements of the separate voltages from the three vertically spaced apart voltage measurement electrode means while first A.C. current is conducted through the formation that provides first measurement information relating to current flow into the formation;obtaining measurements of the voltage outputs from said first and second voltage measurement electrode means while said second A.C. current is conducted between said first and third current conducting means that provides first compensation information related to said first resistance;obtaining measurements of the voltage outputs from said second and third voltage measurement electrode means while said second A.C. current is conducted between said first and third current conducting electrode means that provides second compensation information relating to said second resistance; andusing at least one of said measurement information and said first and said second compensation information to determine the average wall thickness of the casing between the first and second voltage measurement electrode means.