INTRODUCTION

ABSTRACT

Results are presented from an experimental investigation concerning corrosion of steels in high density brine packer fluids at high temperature. Variables examined include, temperature, acid gas (i.e. H2S and CO2) contamination and chemical inhibition. These results were compared with similar tests conducted on a conventional oil mud, The data indicate that the oil mud tended to have low corrosivity, but thickened and produced physical properties that would be not desirable for long term service at high temperature.

High density brine fluids were found to have varying corrosivity. Corrosivity increased with temperature and acid gas contamination, Inhibitor efficiencies at temperatures above 400 F (204 C) were relatively low (<65 percent) in most cases and highly variable depending on the specific formulation, In some cases, some commercial inhibitor formulations were found to actually accelerate corrosion, Recommendations for future work include examination of specific effects of steel type, development of inhibitors with higher efficiency and fluids with lower inherent corrosivity at high temperature.

Presented herein are the results of an intensive research program designed to evaluate the corrosion resistance of casing grade steels in high density packer fluids, These fluids are utilized in the annular space between the production tubing and casing to provide hydrostatic pressure to counterbalance the reservoir pressure, In deep hot wells, which have bottom hole temperatures in the range of 375-450ºF( 190 to 230ºC), there are significant concerns regarding thermal stability and deterioration of conventional fluids such as organic based (oil) muds which impair their ability to provide long term performance, Therefore, alternative fluids based on high density brines have been increasingly utilized, However, the ionic nature of these fluids combined with relatively low pH and interaction with acid gases (i,e, H2S and CO2) causes concerns regarding corrosion particularly as the temperature increases,

This study was conducted under multi-client sponsorship and involved exposure of steel coupons to selected packer fluid environments. The purpose of the program was to examined the corrosivity of these environments on steels commonly used as casing and tubing. Furthermore, due to the possibly aggressive nature of these fluids at elevated temperature, the efficacy of corrosion inhibitors were evaluated, This study included both commercially available inhibitor formulations in addition to non-commercial chemical species which were identified to have potential as corrosion inhibitor additives for use in high density brines

BACKGROUND

A range of high density brines of interest are shown in Table 1,,

These include both single and mixed brine system based halide salts (i.e. chloride and/or bromide) with a range of specific gravities typically from 1,1 to over 2,3, h the published literature, only limited data are available on the corrosivity of these systems on steel, (?-G)Despite this limitation several major factors have been identified related to corrosion in high density brine packer fluids. Corrosivity generally increases with ( 1) increasing specific gravity of the brine, (2) increasing temperature of exposure and (3) increasing aeration, These effects are highlighted in Figures 1, 2 and 4. Furthermore, it is expected that the severity of these environments can increase substantially when they are contaminated with acid gases, This latter situation is a major concern since contamination of the packer fluid by acid gases can occur as a result of packer or tubing connection leaks in wells with even moderate levels of H2S and/or CO2 gas part

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