Reduction of Casing Leaks By Using Cathodic Protection-East Texas Field
Authors: Pace Floyd Allen , Krupicki Stanley E. 
Published: December 2007

Our company has used cathodic protection successfully to reduce casing leaks as much as 75 % in the East Texas field, which produces oil from the Woodbine sand at approximately 3,600 ft (1181 m). Field operation information has been detailed for the casing cathodic protection system that began on an experimental basis in 1958 with 104 wells and was expanded to 1,034 wells by 1981.

Introduction

The company currently is operating 1,186 wells in the East Texas field. The East Texas field has been producing for 45 to 50 years, and some leases have a remaining estimated life of 50 years. Average well density is approximately 4 acres (1.6 × 10 to the 4th m2) per well (Table 1). Generally, cathodic protection can be justified in long-life fields and where casing leaks would cause a catastrophe, such as in sour gas fields. The East Texas field qualifies as long-lived. The majority of casing leaks in the field result from electrolytic corrosion from depths of 200 to 3,000 ft (65.6 to 984 m). The cathodic protection method of reducing casing leaks was installed first on 230 wells by other operators in 1956. We installed our first cathodic protection on 104 wells on an experimental basis in 1958. These wells were selected at random from various leases throughout the field and not necessarily from the high-leak-frequency areas. The need for cathodic protection was not critical in 1958 because we were experiencing only five leaks per year, but the predicted increase in casing leaks as the wells became older made cathodic protection more economical.

This paper covers the trends of casing leaks from 1958 to the present. It gives company history of cathodic protection, details increasing costs of casing repair and cathodic protection installation and maintenance, and chronicles results of cathodic protection effectiveness over 9 years.

Theory

Cathodic protection can be defined as reduction or prevention of corrosion of a metal surface by making it cathodic with impressed currents. For well casing, this can be accomplished with direct current (DC) electricity from an external source to oppose the discharge of current from anodic areas of casing immersed in soil or salt water. Corrosion caused by DC flow from anodic to cathodic areas on the same structure through an electrolyte is electrochemical in nature.Cathodic protection does not eliminate corrosion but transfers it from the structure under protection and concentrates the corrosion at another known location where the current-discharging anode(s) can be designed for maximum life and ease of replacement.Cathodic protection is of value only to the surface of the metal exposed to the same electrolyte as the anode. For example, only the external surface of the oil string of casing between the surface pipe and the top of the cement circulated around the oil string from the bottom is protected in the East Texas field wells discussed in this paper. Fortunately, this casing is exposed to native soil from about 100 to 3,200 ft (33 to 1050 m) below the surface, which is the area needing protection.

Problem Identification

After the field had been operated for 20 to 25 years, casing leaks began to increase with 80% of the leaks occurring in the upper 500 ft (164 m) of casing. The majority of the leaks were external, resulting from electrolyte corrosion. In 1957, a cumulative 85 casing leaks had occurred on our 1,500 wells.

JPT

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