Chemical Scavengers for Sulfides in Water-Base Drilling Fluids
Authors: Garrett R.L. , Clark R.K. , Carney L.L. , Grantham C.K. 
Published: December 2007

This state-of-the-art paper discusses the reaction chemistry of commercial scavengers in water-base drilling fluids. Parameters that affect the reliability, of these materials for removing sulfides are emphasized. Practical problems of scavenger use, such as effects on mud performance, Practical problems of scavenger use, such as effects on mud performance, how to add the scavenger to mud, and rig-site monitoring methods, are considered.

Introduction

Excessive risks may be incurred from unquestioned reliance on sulfide scavengers during H2S drilling. Such risks can result from a lack of reliable information about how the scavenger should function and what factors (chemical and physical) control its behavior in a mud. A need for a summary about this often controversial subject prompted us to write this paper. We conclude that at prompted us to write this paper. We conclude that at present, no available commercial product approximates present, no available commercial product approximates die "ideal scavenger" defined here. Some products offer advantages, while some have major disadvantages because of characteristics of their chemical type and the mud's influential environment. Differences in chemical behavior in a mud are very important for safe H2S drilling, especially if a scavenger is being relied on greatly for H2S control and safety.We based the general scavenger chemistry presented here on the sulfide chemistry in the literature. Field experience with scavengers influenced aspects of this paper, but our specific laboratory studies and those of paper, but our specific laboratory studies and those of others are the major foundation for comments on factors influencing scavenger behavior in mud systems.

Summary of Scavenger Application

This summary of the practical aspects of using or not using a chemical as a sulfide scavenger in muds is based on discussions given later of background chemistry and test results. Only two types of scavengers are being marketed to the drilling industry currently:zinc-base chemicals andiron-oxide, Fe O.

Copper compounds, although used in the past, are not considered satisfactory today because of drillpipe corrosion caused by reaction of copper ions with iron.

No Scavenger

A scavenger may not always be needed to control minor sulfides that result from drilling or similar small amounts of H2S entering a mud, although companies' policies may vary on this practice. A mud's natural scavenging capacity plus neutralization by mud alkalinity often are enough to control a minor amount of sulfides on a day-today basis. A change in operations, such as increasing mud weight, may reduce a small H2S influx to a negligible amount; then a scavenger is not needed. Where H2S influx could occur, it is important to measure the mud's soluble sulfide content frequently and quantitatively and to be prepared to add a scavenger if persistent filtrate sulfides appear. persistent filtrate sulfides appear. Zinc-Base Scavengers

Two general types of zinc-base scavengers are now available;very slightly water-soluble inorganic compounds, the most common being basic zinc carbonate; andhighly water-soluble organic zinc chelate compounds.

For mud applications over a broad pH range, zinc-base additives provide effective scavenging by a rapid and irreversible reaction with sulfides to form solid ZnS.

JPT

P. 787