ABSTRACT

Results from an experimental investigation carried out in high density brine packer fluids are presented. Different variables were examined, namely temperature, time of exposure, chemical inhibition and brine composition and density. Tests to compare the performance of different classes of stainless steels were carried out by autoclave exposure under different deareated brines solutions: NaC1/NaBr, CaCIJCaBrz and Ca.BrJZnBrz, CaClz, CaCIJCaBrJZnBrz. General corrosion, localized corrosion (pitting and crevice), galvanic corrosion and resistance to environmental cracking were evaluated both in absence of corrosion inhibitor and with the addition of two different commercial inhibitors. Duplex steels were very resistant to the localized corrosion, although pitting and crevice were present in some combination of brine and temperature. The martensitic steel was very sensitive to the general and the localized corrosion. Brine CaClJCaJ3rz p = 1.75 g/cm3, showed at 200 C, a pitting on 13%Cr lower than expected. This is probably due to the general corrosion which show a mechanism competitive with localized corrosion. As a result, pitting corrosion is a matter of big concern when applications in heavy brine are considered. In fact only the most alloyed materials at temperature below 200C can be considered as immune. As a consequence, the effect of commercially available corrosion inhibitors is sometime lower than expected and there is a need of further improvements of corrosion inhibitors for the application to the CRA?S (Corrosion Resistant Alloys) in brine environments at 200°C.

INTRODUCTION

The use of solid-flee, high density brines, as packer fluid is a common practice in the oil field industry. Packer fluids are weighted fluids placed in the casing?tubing annulus of oil and gas wells. They provide a counter-pressure on the packer and assist in sealing the annulus against the inlet of the production fluid.

High density brines are especially used in deep hot wells where problems related to the thermal stability and deterioration of high density oil-base muds cannot be prevented. High density water-base or oil- base muds are not stable suspensions when left static in a well for a long time. On the contrary when muds are left as a packer fluid, the solids tend to settle out while in the static state, These solids on top of the downhole tool will cause a difficult and expensive retrieval of the tool.

Therefore, alternative fluids based on high density brines have been increasingly utilized. The salts primarily used to make these brines are KC1, NaCl, CaCIZ, CaBrz and ZnBrz. The need to comply with hydrostatic pressure requirements leads sometime to discard single salt brines having inadequate specific gravity, so mixture of two or more salt brines have to be taken into consideration. Mixtures of salts are more corrosive than a single salt brines [1,2]; the brines which contain the ZnBrz have the highest corrosion rates since the pH of these solutions is acidic.

However the high ion concentration of these fluids makes inhibition difficult due to limited dispersibility and volubility of the most conventional inhibitors in these environments. Commonly used corrosion inhibitors are films forming amines or other low-molecular-weight organic products, characterized by a sulfur containing group. Moreover, corrosion inhibitors have been originally designed for use with carbon steels rather than CRA materials and their efficiency has to be proven especially at temperature higher than 200C [3].

Some works show a relatively low corrosion rate if brines and a proper corrosion inhibitor are added [4,5]. A common observation in these studies has been that the corrosion rate

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