ABSTRACT

This paper presents a current review and update of the role the presence of volatile fatty acids play in the production problems related to sea water injection. Such production problems as corrosion, formation souring, deep water subsea injectivity and decline in injectivity are discussed. Conceptual souring models, methods of sampling and analyzing for VFA and modern research methods developed towards VFA related sea water injection systems are reviewed. Areas for future research are indicated..

INTRODUCTION

With the renewed interest in the occurrence of formation souring associated with sea water injection, the role that volatile fatty acids (VFA) play in this production phenomena has become a matter of some importance and interest to practicing corrosion engineers. In this paper, souring is defined as sweet (hydrogen sulfide free) petroleum reserves that experience an increase in hydrogen sulfide production during the economic life of the reservoir. Two fairly recent examples of this phenomena are the Ninian Field located in the North Sea? and the Kuparuk River field on the North Slope?. In both cases, the souring was attributed to microbial sources and related directly to sea water injection.

Volatile fatty acids (VFA) are defined as water soluble short-chain carboxylic acids containing 1 - 5 carbon atoms per molecule. They have the general formula: R - COOH where R is a straight-chain or branched alkyl group containing 1 - 4 carbon atoms. This group of short-chain carboxylic acids are described below:

Name Formula Number of Carbon Atoms

Formic HCOOH 1

Acetic CH,COOH 2

Propionic CH,CH,COOH 3

isoButyric (CH,),CHCOOH 4

normal-Butyric CH,(CH,),COOH 4

iso-Valeric (CH,),CHCH,COOH 5

normal-Valerie CH,(CH,),COOH 5

The VFA?s are found in varying amounts in brines. However, the single-carbon carboxylic acid, formic ( HCOOH ), is not generally found in natural, unpolluted waters. The VFA?s occur predominantly as their anions in natural waters of P referred to in the literature3 as SCA H of 5 or greater. These short-chain aliphatic acid anions have been ?s. Early interest in these dissociated, ionic complexes (acetates, propionates, butyrates and valerates) was in the area of subsurface geology and geological exploration.

Over a period of approximately 25 years, this information has appeared in technical periodicals that are not usually on the corrosion engineer?s reading list. Among other things, this published work has shown that the VFA?s and their dissociated anions have geological significance in the following areas:

-Formation porosity enhancement as a result of subsurface mineral dissolution

-0 Precursors of natural gas.

-Possible petroleum proximity locators.

-Sub-surface ground water flow tracers.

A possible source of a major error in the determination of titratable alkalinity. It is not the purpose of this paper to discuss the geological importance of the presence of VFA?s in oilfield brines. However, since corrosion is a multi-disciplined science, it is important to be aware of work published in other fields that provides us with additional information that helps us to better understand VFA?s and SCA3?s role in production problems related to sea water injection. A few papers the corrosion engineer may find useful are listed among the references3-to 512

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