Coiled tubing is an extremely useful tool in many well logging and workover applications in oil and gas production operations. Several important concerns regarding its use include the need for improved guidelines for the assessment of mechanical integrity, fatigue damage, and the effects of hydrogen sulfide in sour oil and gas production environments. This paper provides information regarding the use of coiled tubing in sour environments with particular emphasis on sulfide stress cracking, hydrogen induced cracking and stress-oriented hydrogen induced cracking and how they work synergistically with cyclic cold working of the steel tubing.
Coiled tubing (CT) is an extremely useful tool in many well logging and workover applications in oil and gas service. In these applications, the use of CT has allowed a means of providing well services that could not be otherwise performed using conventional techniques. Additionally, several oil companies are attempting to use CT in place of conventional threaded and coupled production tubing for permanent installation at substantial cost benefits. Current concerns in the oil and gas industry regarding CT include the need for reproved guidelines for the assessment of mechanical integrity, fatigue damage, and the effects of hydrogen sulfide. The available information concerning the use of coiled tubing in sour service 1slimited. The majority of the research work has focused on the effects of mechanical integrity and fatigue damage.
Effects of hydrogen sulfide on the serviceability of CT are particularly important as hydrogen sulfide can reduce the useful life by degradation in the useable strength of the tubing through a combination of hydrogen blistering, hydrogen induced cracking (HIC), stress oriented hydrogen induced cracking (SOHIC), sulfide stress cracking (SSC), and possibly weight loss corrosion. Additionally, these effects of hydrogen sulfide may work synergistically with the cyclic cold working of the steel which takes place during spooling and running the CT. Prior studies on carbon steels have shown that cold work may significantly reduce the SSC threshold stresses (see Figure 1). The limited amount of data in this area does not relate specifically to CT and its uses. However, this information does show cause for concern in this area.
Furthermore, the strength range for CT (60 to 100 ksi yield) covers a regime where both SSC and HIC may occur in steels. There are also specific concerns about the performance of the high frequency induction forge welded seam m well as the bias and butt welds which occur during CT manufacturing and in the field, respectively. Figure 2 shows a schematic representation of bias and butt welds used in CT. The specific concern the possibility of high weld and HAZ hardness ardor unfavorable microstructure resulting from these welds leading to an increase in SSC susceptibility. Figure 3 shows the relationship of HAZ hardness, H2S partial pressure and H2S concentration on the SSC resistance .
With only a limited amount of hydrogen sulfide testing performed on CT, there was a need to develop a better engineering database on the performance of CT. Therefore, CLI International Inc. conducted a multi-client program to expand the current database allowing a better understanding of the combined effects of strain cycling and resistance of CT to cracking in hydrogen sulfide environments. The program was supported by fourteen sponsors consisting of major oil and gas companies, service companies, CT manufacturers and materials suppliers. The program WM conducted over the period beginning November, 1992 and ending September, 1994. This paper summarizes some of the initial findings of this pr