WELL TUBULAR CORROSION IN THE COOPER/EROMANGA BASIN

Abstract

In September 1987 the Della-1 gas well blew out at approximately 19.5m (64ft) abovesea level (42.7m (140 ft) KB) due to corrosion of the production casing and tubing.

The production casing failure and other similar corrosion occurrences were considered to be due to sulphate-reducing bacteria which have been identified in a large number of wells in the Cooper Basin. It was considered possible that iron sulphide was being deposited on the casings in the surface-to-production casing annulus at the air/water interface promoting the formation of anodic sites and therefore corrosion.

Further investigations of the evidence indicates that sulphate-reducing bacteria are not the major contributors to the corrosion as was initially believed. Field studies, laboratory analysis and ongoing well programs show that the process of differential aeration is the prime cause of the casing corrosion. Corrosion has been found to occur predominantly at a depth of between 18.3m (60 ft) and 36.6m (120 ft) above sea level and occurs over a band of 6.1 m (20 ft) to 9.1m (SO ft) in each well in conjunction with the external water table.

As a result of this corrosion failure SANTOS has initiated a regular program of well maintenance, annulus inhibitor top-ups and pressure testing. A total of 315 wells have been tested to date, production casing corrosion problems have been identified in 35 wells, 31 wells have been repaired and four wells abandoned.