RIASSUNTO
Abstract
Sixteen chrome coiled tubing (16Cr CT) was introduced in the spring of 2003 and over 200 strings have been put into field use as velocity strings. Following preliminary testing, two 16Cr CT reels were deployed at Prudhoe Bay, Alaska to evaluate feasibility as an intervention workstring. The two reels performed a variety of standard CT applications on a daily basis. Observations and data were gathered to determine operating guidelines, applicability, and limitations. The field trial indicated that 16Cr CT can be deployed in the field with only minor operational modifications.
16Cr has superior abrasion resistance in 13Cr production tubulars and little CT surface (external) wear was seen during the field trial. The second reel developed a pinhole failure earlier than expected; however, analysis of the adjacent material indicates that 16Cr has increased low cycle fatigue life when compared to standard carbon steel CT. Additional testing is ongoing, and it is felt that the conditions resulting in the failure can be mitigated to avoid future premature failure.
This paper documents the lab and field trial results. Standard operating procedures for 16Cr CT are described that provide easily implemented guidelines. 16Cr has applicability as an intervention workstring, particularly in corrosive environments and in areas where abrasive 13Cr production tubulars must be endured.
Introduction
Prudhoe Bay performs over 400 coiled tubing (CT) well interventions per year with three CT units on 24-hour operations. The standard CT used is 1-3/4, 80,000-psi (80 ksi) yield strength carbon steel. CT reels are typically retired due to wear and 80% coil life after ~700,000 running feet in carbon steel completions, or approximately every 1-1/2 months. Running footage is defined as the distance run into a well, not the round trip footage. Along with coil life, running footage is used as an effective way to estimate the number of jobs that can be completed before a string should be taken out of service.
Development and testing of 16Cr CT. Corrosion resistant alloy 16Cr CT was developed to offer a cost effective CT solution for applications in wet CO2 environments where carbon steel CT was not suitable. In particular, operators were embracing the advantage that CT had to offer as ""velocity?? or ""siphon?? strings to reestablish the productivity of aging gas wells. The relatively thin walled carbon steel CT easily corrodes in wells that produced CO2. The situation becomes more complicated when the CO2 is in the presence of produced water, which creates an accelerated electrochemical corrosion circuit. The produced water may be direct water and/or condensed water, created as gas traveling to the surface passes through its critical condensation temperature. These conditions generate carbonic acid which can create extremely corrosive environments.
Significant testing has been completed by the manufacturer to define the practical operational limits of 16Cr CT (Reference 1). Some of the testing which applies to Prudhoe Bay's operational environment is described below.
Wet CO2 corrosion. The main corrosion damage mechanism at Prudhoe Bay has been identified as CO2 corrosion, commonly referred to as sweet corrosion. It is caused by the relatively high temperatures (200 - 220 oF) and high CO2 content (12%) found in the reservoir gas and miscible injected gas.
16Cr CT susceptibility to wet CO2 environments was evaluated using 5% CO2 gas sparged through synthetic seawater at 200oF. More aggressive downhole environments were simulated using 500 psi CO2 in a 5% sodium chloride solution at 250oF. All tests showed no visible change to the 16Cr surface.
Sulfide stress cracking and chloride stress corrosion cracking. H2S ranges from 0 to 500 ppm at Prudhoe Bay. Production and well intervention fluids typically have 10,000 to 20,000 ppm chlorides. 16Cr CT was tested for resistance to sulfide stress cracking using C-ring and bent beam specimens in accordance with NACE TM-0177-96 and for resistance to chloride stress corrosion cracking in accordance with ASTM G123 modified. C-ring tests apply tensile stress to the CT specimen in the circumferential direction. This simulates loading dynamics that would be experienced from hoop stresses due to pressure. Bent beams apply tensile stresses to the test specimen in the axial direction, simulating loading in the load bearing direction.