INTRODUCTION
The in-situ permanent expansion of casing and tubing is rapidly becoming implemented in oil and gas well completions. Over 25,000 feet of solid, not slotted, casing have been expanded downhole by one company since November 1999. Previous laboratory work has shown that tubular expansion affects the toughness of the final product; however, proper material selection of the product to be expanded can yield a final product with acceptable toughness.
Recently, laboratory expansion of selected grades of seam welded and seamless OCTG steels has been used to study the effects of expansion (cold work) and strain aging on the hardness of five grades of C-Mn steels as well as on the susceptibility of four different heats of seamless P-110 tubing (also C-Mn) to sulfide stress cracking.
Both cold work and strain aging were found to increase the hardness of these steels as well as dramatically reduce the susceptibility of the P-110 to sulfide stress cracking in an aqueous solution of 5wt% NaC1 at a pH of 4.2 with a cover gas containing a partial pressure of H2S of _<0.15 psi ( _<1.0 kPa). These results are significant because they also offer further insight into the effects of handling damage or cold straightening operations on the performance of all high strength OCTG steels in sour production environments. New approaches to the evaluation of steels for sour service will be required to accommodate this phenomenon. Further study may determine how to "design" steels to be resistant to this effect.
The effect of in-situ mechanical expansion of oil country tubular goods (OCTG) upon their mechanical properties has been studied; some of this work has been reported earlier I~,21. The process involves either pushing or pulling a mandrel and lubricated cone assembly through the inside of OCTG of smaller inside diameter than the cone and expanding it in-situ, e.g., downhole. This method of well completion technology and repair can decrease the cost of well completion, make completions in difficult formations possible, and can salvage damaged completions by in-situ well repairs (e.g., cladding across damaged casing intervals) I31. Over 25,000 feet of solid, not slotted, casing has been expanded downhole by a single company since November 1999. This aspect of well completion technology is expected to grow significantly in the first quarter of the 21 st century.
While the early laboratory work in this area has demonstrated that toughness and collapse strength are reduced because of the expansion process [1,2,41, it was also demonstrated that the final toughness of the product is acceptable if the proper grade of steel is specified for the process. The collapse strength is reduced because of the Bauschinger effect. Some of the collapse strength can be restored by a suitable thermal treatment (strain aging). This has been demonstrated in the laboratory for expanded tubulars I41; it has also been used commercially for increasing the collapse of UOE linepipe [51
The study presented in this paper extends previous work to include the effects on expansion (cold work) up to 22% and strain aging on the hardness of five grades of C-Mn steels and on the susceptibility of four different heats of P-110 (C-Mn) to sulfide stress cracking.
