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Optimising the cathodic protection to oil and gas well casings

Strong, Grahame (1995) Optimising the cathodic protection to oil and gas well casings. PhD thesis, Murdoch University.

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Abstract

Cathodic Protection (CP) of oil wells is associated with a number of interesting and commercially very important problems. Perhaps, the most difficult of these concerns the determination of the optimum applied current required to give best protection. Under-protection may lead to unnecessary and costly repairs while excessive protection increases power consumption, shortens the anode life and, in certain circumstances, induces corrosion on nearby unprotected structures. Since the external surface of the casing is inaccessible, indirect methods must be used to assess CP levels and to establish current settings.

Accordingly, a critical examination of existing methods is made to ascertain their usefulness and some new techniques are developed to model the well casing's response to CP more accurately. Field data, collected from the Barrow Island Oil Field, are used in this examination. The results obtained suggest that techniques based on E-log I, empirical modelling of the potential drop within the casing, and downhole logging are all unsatisfactory: they are either inconclusive, based on unsound principles or compromised by experimental error. Based on practical and theoretical principles, the most applicable and accurate method of assessing CP levels in the field is to determine the current density at the casing surface using advanced downhole logging techniques.

Since the acquisition of surface current density profiles requires expensive downhole logging programmes, an effort is made to develop a better method of using an attenuation equation that utilises the limited amount of field data that can be collected in this manner. The results show that, although this new method can simulate potential gradients within the casing better than Schremp's model neither method accurately simulated the profile of a slightly under-protected well. In particular, models based on simple power transmission lines are unable to incorporate satisfactorily corrosion currents originating from the casing.

A comprehensive numerical model is also developed to take into account the electrochemical behaviour of the steel and the heterogeneous nature of the earth. This model is based on the finite difference method of solving the Laplace equation. By taking advantage of the symmetry of the potential field around the well casing it is possible to model the three dimensional system using two independent variables. This allows the model to be incorporated into a minimisation program which then applies various protection criteria to calculate the minimum CP current required to fully protect the well. This approach provides an insight into the factors that influence the minimum current requirement - information which suggests directions for future field work and laboratory research into cathodic protection of well casings.

It is concluded that, at present, it is not feasible to optimise CP current requirements in a simplistic manner. The electrochemical behaviour of the steel and the geometry of the CP system are both important factors influencing the minimum CP current requirement. Consideration should thus be given to modifying the design of wells to incorporate external reference cells so that protection levels can be measured directly. With existing wells, the significant cost associated with accurately modelling the casing's response to CP, is justified only if it can be offset against the cost savings associated in optimising the CP current.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Physical Sciences, Engineering and Technology
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): May, Peter and Pang, J.T.T.
URI: http://researchrepository.murdoch.edu.au/id/eprint/52391
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