28% Chrome, 32% Nickel: A Case History on the Downhole Cutting of Exotic Completions

Abstract

Abstract Cutting production tubing using coiled tubing-conveyed cutting tools is a relatively common practice in today's industry. However, the move towards higher and higher chrome and nickel content in the completion metallurgy has meant that the existing tools and techniques require further revaluation and refinement. Cutting material with 28% chrome is much more difficult than cutting material with 13% chrome. A major international operator in Malaysia recently had a requirement to remove two production strings of 28-chrome tubing from two offshore wells. The two wells were to be worked over, requiring that the tubing be retrieved. High-chrome tubulars present problems for not only mechanical cutting techniques, but also chemical and explosive techniques. This paper lists the considerations that led to the adoption of the mechanical cut method. Because of the technical difficulties of the operations, significant equipment testing was carried out prior to the work. This paper further details the results of that testing and describes the execution of the work offshore. Finally, the findings and learnings derived from the interventions will be discussed. Background The two wells described in this paper are situated offshore Malaysia, towards the northeast part of the island of Kalimantan, formerly Borneo. They are situated in a water depth of approximately 300ft, completed with 7 5/8″, 33.7lb/ft Sanicro 28 tubing (SM2731–110). A 7–5/8″ × 9–5/8″ Model "SB-3" Hydraulic Set Production Packer anchors the production tubing to the casing. Both wells were to be deepened, requiring that the existing completion string be removed. The challenge was to retrieve the tubing, first having to disconnect the tubing from the packer. The same completion tubing was to be salvaged and run back into one of the wells as the new completion string, the lead-time on new 28Cr tubing being unacceptably long. The tubing metallurgy was 28% chrome, 32% nickel and 3% molybdenum. It had a minimum yield strength of 110,000psi and a surface hardness of 37 Rockwell C. This metallurgy represents a challenge to any cutting method. Much history of cutting regular carbon steels, and 13% chrome steels exists, but the specifics of this particular metallurgy made the cut a far more challenging prospect. Because of the level of uncertainty with this cut, several rounds of testing were instigated. Cutting Methods Considered Chemical Cutters Chemical cutters were considered for this application. The major problem preventing chemical cutters being used as the primary choice was that a restriction above the cut point meant that too large a standoff would be present at the point of the cut. The restriction in the wellbore was 5.953″, inside the safety valve. The internal diameter at the point of the cut was 6.765″, a diametrical clearance of 0.812″, assuming a perfectly sized cutting tool was available, which it wasn't. The standard size of tool used for this size of casing is 6.375″. It should also be noted that chemical cutters do not work so well in high-chrome tubulars. Additional chemical is typically required to make a cut. The corrosion resistant properties of the steel hinder the chemical process. Explosive Cutters Explosive cutters were also considered for this application. One main reason led to this method not being considered further. That reason was that it was felt that there was a chance that an explosive cutter could damage the 9 5/8″ casing behind the tubing. Since the well was to be deepened, it was imperative that these was no damage to the 9 5/8″ casing.