ABSTRACT:

This study applied a pH buffer as a corrosion inhibitor in brine packer fluids. The study systematically evaluated the effect of pH buffer on corrosion control of steel materials at elevated temperatures up to 425oF. The paper describes laboratory results of pH buffer in sodium chloride, potassium chloride, sodium bromide and sodium chloride/sodium bromide brines and field case histories for buffered brine packer fluids in high-temperature high-pressure wells. The pH buffer applied is cost effective, readily available and easily handled and monitored in the field. Results from buffer capacity, general corrosion and stress corrosion cracking tests are presented. Field case histories are presented to demonstrate the successful use of the pH buffer in brine packer fluids at high temperatures.

INTRODUCTION:

Sodium, potassium and calcium chloride and sodium, calcium and zinc bromide brines have been successfully used for well completions for more than 25 years. More recently, sodium, potassium and cesium formate brines are being explored for use as completion and packer fluids. These brines can be applied as single-, two- or three-salt mixtures based on density, crystallization temperature, and economic requirements. However, brine corrosivity is a major concern - especially when the brines are used as packer fluids (which remain in contact with production tubing and casing for an extended period of time). Generally, brine corrosivity increases with increases in temperature, brine density and zinc content. The corrosivity of brines also increases with a decrease in pH of brine solutions. In application, single-salt brines have density limitations, and mixtures of two or more salts are needed to provide adequate density for hydrostatic pressure requirements. In most cases, mixtures of salts are more corrosive than single-salt brines. Some single-salt brines have lower corrosion rates than others. Due to the corrosive nature of brines, a corrosion inhibitor becomes necessary to reduce corrosivity. Two types of inhibitors are conventionally used: film-forming amine and low molecular weight inorganic thiocyanate (SCN-) compounds. In application, thiocyanate corrosion inhibitors have unique characteristics compared with conventional amine-based inhibitors: excellent solubility in high-density brines and ability to control corrosion of brines on carbon steels and low alloy steels at high temperatures. Thiocyanate inhibitors were originally developed to control corrosion of carbon steels in ZnBr2 brines and also found application in non-zinc brines such as sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl2), sodium bromide (NaBr) and calcium bromide (CaBr2). Thiocyanate inhibitors for oilfield corrosion control include sodium thiocyanate (NaSCN), potassium thiocyanate (KSCN) and ammonium thiocyanate (NH4SCN). However, when applied at high temperatures, thiocyanate can decompose and yield hydrogen sulfide (H2S) or free sulfur. Burke et al.1 reported that NaSCN present in NaBr brines decomposes at temperatures above 302oF and forms hydrogen sulfide. Hydrogen sulfide certainly increases the potential of sulfide stress corrosion cracking (SSC) of steel materials. Isaacs et al.2 also reported that thiocyanate can form elemental sulfur and cause SCC of austenitic stainless steels. Ke et al.3 further reported that thermal decomposition of thiocyanate can induce sulfide stress cracking.

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