ABSTRACT

This paper presents the outcome of a comprehensive program of work to establish the suitability and performance of candidate inhibitor packages for downhole production applications. It covers both CO2 and H2S corrosion performance of carbon and low alloy steel tubulars in simulated field conditions. Inhibitor performance was established through using a number of inhibitor selection methods by systematic determination of efficiency, persistency and chemical compatibility of both conosion and combined corrosion/scale inhibitor packages. The results have led to 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 con'osion (sweet corrosion) and HzS 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 conosion of carbon and low alloy steels in the presence of acidic 1,2,3 gases . One means of corrosion control is through the use of effective inhibitor packages. Subject to continuous and sufficient availability, corrosion inhibitors can offer adequate CO1TOsion 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 LS0 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 1 and 2 respectively.

Coupons were machined fl'om grades L80 and C90 tubulars and were sandblasted prior to testing. They were then degreased using acetone, dried and stored in desiccators 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 were obtained from three chemical companies. The recommended chemicals were based on water chemistry and production conditions and are described in Table 3. The specific chemical composition of the three-inhibitor packages is proprietary. However, inhibitor-1 is made up of quaternary ammonium chlorides and methanol; inhibitor -2 consisted of alklypyridinium salts, oxyalkylated alkylphenols and amine salts in water and isopropanol;, and inhibitor-3 is a combination of organic amines and organic phosphates. Inhibitors -3 provides scale inhibition along with conosion protection.

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 w

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