ABSTRACT

Acid systems are used to improve productivity through either near-wellbore damage removal or through dissolving scale inside the wellbore during production. This paper describes the qualification methodology applied in the search for effective scale dissolvers/stimulation fluids with low corrosivity. The identification of suitable acid systems is becoming increasingly more challenging.

Furthermore, the operators try to lower the cost of the acid treatment itself. This may be achieved through optimization of the inhibitor package, which constitutes a significant part of the chemical cost.

A corrosion testing program was performed aimed at evaluating scale dissolver packages to determine which package(s) were least corrosive and acceptable for use in wells constructed with 3Cr80 alloyed steel and L80 13 Cr (API 5CT grade) (1) tubulars. Laboratory exposure corrosion tests were carried out at 60°C and 80°C. The scale dissolver packages consisted of 7.5%HCl, 15%HCl and 28%HCl including corrosion inhibitor packages. The main challenge was to optimize the acid formulation for 3Cr 80 alloyed steel. There is only limited data available for this material.

Corrosion resistance of the tested alloys was evaluated in terms of mass loss and localized corrosion. The results of this program successfully identified the optimized scale dissolver packages for both 3Cr80 alloyed steel and L80 13 Cr (API 5CT grade), respectively.

INTRODUCTION

Acid systems are commonly used to improve productivity through either near-wellbore damage removal or through dissolving scale inside the wellbore during production. A number of different acid stimulation packages are available for removal of debris to permit unrestricted hydrocarbon flow. Acidic fluids can include different types of acids, such as hydrochloric (HCl), acetic, formic, or combinations of such acids. Since these can be corrosive, corrosion inhibitor packages are required to prevent severe corrosion damage to the well construction materials. Furthermore, inhibition additives for acid systems usage in North Sea oil wells require adherence to regulations calling for continual improvement in environmental characteristics while maintaining performance.1 As environmental standards are continually tightened, especially in the North Sea area, options for different corrosion inhibitor chemistries that will meet the criteria are becoming more limited. Challenging conditions for acid inhibition are ever present with (1) high temperatures, and (2) the use of metallurgies, e.g. 3Cr80 alloy steel, for which only limited data on scale treatments is available.2,3 3Cr80 has been developed as an economical material alternative for moderately corrosive services, and has been in service for more than 16 years as tubing on the Norwegian Continental Shelf (NCS).3

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