Krishnan_subbed.qxp 27/3/09 04:02 Page 70
Alloy K-500 – Past Failures in Downhole Completion Tools
Table 1: Environmental Conditions Under Which Alloy K-500 Downhole Tools Failures Were Observed
Environment BHP (MPa) BHT (°C) CO
2
(kPa) H
2
S (kPa) HCO
-
3
(mg/l) Cl
-
(mg/l) pH
1 24.1 95* 1,207 1,690 DNA DNA 3.7
2 DNA 86 345 1,214 1,480 10,900 5.9
3 16.55–27.41 149–177 2,251–6,709 0.07–2.34 141–1,513 8–20,574 3.36–5.37
*Temperature reported is the estimated temperature at the location of the failed component rather than downhole conditions.
BHP = bottom-hole pressure; BHT = bottom-hole temperature; DNA = does not apply.
Figure 1: Microstructure of a Failed Alloy K-500 Subsurface Safety Valve
considerable build-up of corrosion scales (see Figure 3), but the safety
Showing Intergranular Cracking (Environment 1)
valve landing nipples did not show much evidence of the same (see
Figure 4). Testing of the scales discovered on the components revealed
mostly nickel sulphide and copper sulphide.
Residual hydrogen in a material is a good indication of hydrogen pick-
up during corrosion, and indicates susceptibility to embrittlement.
Therefore, interstitial hydrogen analyses were performed on most of
the failed parts. Typically, the hydrogen content of mill-produced
Alloy K-500 would be below 5 parts per million (ppm); the analysis of
the failed components revealed a hydrogen content of 7–200ppm
close to the fracture surfaces and 14–58ppm away from the fracture
surfaces. Baking a sample from one of the Alloy K-500 failed
components near the fracture surface at 650°C led to the reduction
of the hydrogen content from 112 to <1ppm.
It was discovered that failed Alloy K-500 components were often
directly connected to components made from low-alloy steel or
13
Cr
stainless steel. Often, direct contact between two different grades of
alloy metals can cause the lower grade to corrode through a process
Figure 2: Scanning Electron Microscopy of a Failed Alloy K-500
Circulation and Production Sleeve Component Showing Intergranular
known as galvanic coupling. However, in some cases involving this
Cracking (Environment 2) direct contact, the components made from Alloy K-500 revealed build-
up of corrosion scales even though the adjacent low-alloy steel
experienced no corrosion at all. Corrosion build-up was also discovered
on Alloy K-500 components that were separated from alloys with less
resistance to corrosion by elastomeric elements. As a result, these
failures were mainly attributed to sulphide stress cracking.
In contrast, the failures of the safety valve landing nipple
components in environment 3 were isolated to the threaded
connections and did not show many corrosion scales. These failures
indicate possible galvanic coupling with the adjacent
13
Cr stainless
steel tubing and, thus, hydrogen charging of Alloy K-500 safety
valve landing nipples leading to HE or galvanically induced hydrogen
stress cracking (GHSC).
Embrittlement can also be caused by exposure to mercury, and this
could lead to intergranular cracking and fracture similar to that
such as circulation and production sleeves (in environments 2 and 3). observed in the failures. Mercury production was reported in the
The H
2
S partial pressure varied widely from as low as 0.07kPa to as wells in environment 3, but the failed components showed no
high as 1,690kPa. Other parameters, including temperature, partial evidence of trace residues of mercury. Nonetheless, testing on Alloy
pressure of CO
2
and pH values, varied widely as well. K-500 was carried out with exposure to liquid mercury at room
temperature. All specimens failed in less than one week; however,
As demonstrated in Figures 1 and 2, the principal mode of fracture the hydrogen contents were measured after testing and found to be
propagation observed was intergranular cracking in all of the failures; <1ppm, which contrasts to the 7–112ppm of hydrogen found in
this is a good indication that failure was due to some type of HE various failures in this environment.
mechanism. As will be discussed later, the specific embrittlement
mechanism is usually indicated by whether the cracking is accompanied The Alloy K-500 used to manufacture the various components was
by corrosion. A considerable build-up of corrosion scales was found on obtained from two different sources, casting doubt on the theory of
components in environments 1 and 2. In environment 3, the circulation poor-quality supply. While one source used only a primary melt
and production sleeve components that failed also showed practice of electric furnace melting followed by vacuum oxygen
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EXPLORATION & PRODUCTION – VOLUME 7 ISSUE 1
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