The Performance of Downhole Materials in Sour Environment of West Kuwait Oil Fields
Authors: Al-Hashem A. , Carew J. , Salman M. 
Published: December 2007

An investigation was carried out to establish the suitability and performance of candidate corrosion inhibitor packages for downhole production applications. The study covered both H2S and CO2 corrosion performance of carbon steel and low alloy steel tubulars in simulated West Kuwait oil field conditions. The performance of the corrosion inhibitors was established through the use of a number of inhibitor selection methods by systematic determination of efficiency, persistency and chemical compatibility of both corrosion and combined corrosion/scale inhibitor packages. The results have led to the determination of effective means of corrosion mitigation to enable wider uses of carbon and low alloy steels for downhole applications.

Introduction

Corrosion has wide ranging implications on the integrity of Materials used in the Petroleum Production industry. It manifests itself in several forms amongst which CO2 corrosion (sweet corrosion) and H2S corrosion (sour corrosion) are by far the most prevalent form of attack encountered in oil and gas production. The implication of these types of attack can be viewed in terms of their effects on both capital and operational expenditures and health, safety and the environment (HSE). Increasing water production gives rise to these types of attack and potential corrosion of carbon and low alloy steels in the presence of acidic gases (1–3). One means of corrosion control is through the use of effective inhibitor packages. Subject to continuous and sufficient availability, corrosion inhibitors can offer adequate corrosion mitigation against the risk of CO2 corrosion for the safe and trouble free application of carbon and low alloy steel production tubulars.

This study dealt with the evaluation of corrosion performance of candidate tubular steels in the absence and presence of a selected number of inhibitor packages recommended by chemical suppliers. The environmental conditions represented those of west Kuwait downhole production conditions and tubulars were according to API 5 CT grades L80 and C90 carbon and low alloy steel.

Experimental Methods

Test Materials

Two grades of carbon steel tubular were used throughout the test program. These alloys are API 5CT grade L80 and C90 type I. Both steels were Cr-Mo-B-Ti modified offering improved SSC resistance. The elemental analysis and mechanical properties of these two grades of steel are shown in Table 1and 2. Coupons were mechined from grades L80 and C90 tubulars and were sandblasted prior to testing. They were then degreased using acetone, dried and stored in discators prior to testing. After each test, the coupons were cleaned and degreased by rinsing in 15% hydrochloric acid followed by rinsing in sodium carbonate and distilled water, then rinsed with acetone and dried.

Inhibitor Packages

Three inhibitors package were obtained from three chemical companies. The recommended chemicals were based on water chemistry and production conditions and are described in Table 3.

Wheel Test

Wheel testing was carried out in accordance with industry practice to determine corrosion rate of exposed samples in semi-static conditions in the presence of candidate inhibitor packages over a 96-hour period. The test was intended to rank inhibitors chemicals. The samples were examined subsequently by means of weight loss and visual observations. Data were generated in the absence of corrosion inhibitors to determine the actual corrosion rate both in the absence and presence of crude oil. Among the chemical packages, two best performing chemicals were selected for further electrochemical evaluations.

Test Materials

Two grades of carbon steel tubular were used throughout the test program. These alloys are API 5CT grade L80 and C90 type I. Both steels were Cr-Mo-B-Ti modified offering improved SSC resistance. The elemental analysis and mechanical properties of these two grades of steel are shown in Table 1and 2. Coupons were mechined from grades L80 and C90 tubulars and were sandblasted prior to testing. They were then degreased using acetone, dried and stored in discators prior to testing. After each test, the coupons were cleaned and degreased by rinsing in 15% hydrochloric acid followed by rinsing in sodium carbonate and distilled water, then rinsed with acetone and dried.