Abstract
Coiled tubing (CT) has been used extensively for underbalanced drilling in non-sour applications with great success; however, attempts to utilize it for sour service produced frequent failures due to tubing environmental cracking. Specific operational conditions of CT, involving a combination of cyclic plastic strain of CT material (associated with travel on/off a reel) and periodic exposure to sour environments, typically result in a significantly poorer CT performance in comparison with non-sour applications, or with sour service at stresses below yield.
An experimental program was implemented using Slow Strain Rate Test (SSRT) to evaluate suitability of CT candidates for sour underbalanced drilling. Out of a variety of standard tests for stress corrosion cracking (SCC) resistance, this test was chosen because it assesses dynamic cracking in the plastic region, accounting for synergy effects between plastic strain and exposure to sour environment. Effects of variable strain rate and loading path on maximum strain and CT life were examined for several CT material candidates. To facilitate assessment of actual CT products, full-body specimens were tested, offering a large test area with as-manufactured surface finish, inclusion of welds, and an option to apply a multi-axial stress state. Various modes of environmentally assisted cracking such as sulfide stress cracking (SSC), hydrogen induced cracking (HIC) and blistering, commonly observed in the field, were successfully simulated in the laboratory.
This paper presents a brief description of the test setup, and provides examples of test results obtained for several CT specimens of different material grades and pre-fatigued with a varying number of strain cycles. The study demonstrates applicability of the selected test methodology for initial selection of CT products for sour service.
