Abstract

Cathodic protection (CP) is often used to protect steel well casings from external corrosion. However, it is difficult to measure the CP potential distribution along the well casing, which brings about difficulties on CP design and effectiveness evaluation. In this paper, the polarization characteristics of casing steel in different soil layers were obtained by polarization testing at different soil depths in the field. Based on measured polarization characteristics, geometry of well casing and soil resistivities of different soil layers, a numerical simulation model for CP of well casing was established. CP current requirement and CP potential distribution of well casings were estimated by calculating and field testing. Additionally, the effects of soil resistivity and casing polarization characteristics on CP potential distribution of well casing were studied by numerical simulation.

Introduction

Deep well casing is an important part of oilfield production. In the long service life of well casing, corrosion can result in wall thinning and even perforation of the casing due to contacting with soil, water and other naturally occurring substances within the formation. The most economic and effective method to decrease corrosion of well casing is cathodic protection (CP). However, the vertical depth of casing is several kilometers, and CP current requirements of casing in different layers are quite different [1-2]. At the same time, the conductivity of different formations will affect the distribution of CP current. It is difficult to measure CP potential distribution along well casing, which brings about difficulties on CP design and effectiveness evaluation.

With the development of computer, many studies have shown that numerical simulation can determine the distribution of CP current and potential [3-8]. But things such as soil structural model and appropriate boundary conditions have to be addressed as the technology is applied for well casing. In this paper, E-log I method was used to determine the current requirement of single well casing. The polarization characteristics of well casing steel at different depths were measured by integrated coupon reference assemblies put at different depths. The numerical simulation technology was adopted to analyze the influence of soil resistivity on the CP potential distribution along casing.

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