High pressure/high temperature (HP/HT) wells require non-standard products. While the basic material specifications are well defined, the influence of higher temperatures on yield strength and the extreme pressures drive designs toward thicker pipe walls and higher strength casings. Another complicating factor often encountered is the presence of H2S and the risks associated with sulfide stress cracking (SSC).
At well temperatures over 300°F (148°C) material yield strengths can be reduced by 5 to
|Figure 1. Shown are 8 5/8-in. by 7-in. VAM HW-ST crossovers. (Figure courtesy of VAM)
10%. In other words, using a 95 KSI material such as API T95 could result in effective yield strength of 85 to 88 KSI in the high temperature portion of the well which reduces the maximum internal yield pressure (MIYP) rating of the casing.
For example, a 7-in., 35.0# (0.498-in. wall thickness) 95 KSI grade casing would have a MIYP of 11,830 psi at ambient temperature but at 350°F (176.5°C) the MIYP could be reduced to approximately 10,500 psi.
Overcoming yield-strength reductions
This lower pressure rating can have a significant influence on the well design and should not be overlooked. One common way to overcome the reduction in yield strength is to increase the wall thickness to the next heavier weight such as using 38.0# for the given example.
Moving to the 38.0# 95 KSI casing and allowing for the same temperature de-rating, would result in a MIYP of 11,550 psi, which is much closer to the 11,830 psi of the 35.0# casing at ambient temperature. The 38.0# casing provides higher internal and external pressure ratings but the drift diameter is 5.795 in. compared to 5.879 in. for the 35.0#. This could impact downhole tools and subsequent tubulars to be run and therefore is not a trivial matter.
This next heavier weight philosophy works well when working in moderate pressures and temperatures but when the well pressures are above 15,000 psi there are fewer standard API weights to use.
So it can be seen that as the casing outside diameter increases, it is more difficult to achieve the higher pressures utilizing conventional API tubulars. If there are SSC concerns, then the yield strength is limited to 110 KSI, which further reduces the options. So in the case of extreme HP/HT in the 20,000 psi range specifically designed casings are required and proprietary high pressure gas-tight connections replace the API 8rd and buttress threads.
Gulf of Mexico HP/HT well
Consider a recent Gulf of Mexico well, with 20,000+ psi bottomhole pressure (BHP), and H2S levels sufficient to limit the material to T95 grade in the upper portion of the production casing string. The operator required an 85¼8-in. by 7-in. tapered casing string as a production liner and preferred to use threaded and coupled gas-tight premium connections. The final design consisted of:
• 600 ft (183 m); 8 5/8 in.; 1.125-in.; wt T95 pipe (MIYP 21,680 psi)
• 7,600 ft (2,318 m); 7 in.; 1.000-in.; wt T95 pipe (MIYP 23,750 psi)
• 10,000 ft (3,050 m); 7 in.; 1.000-in.; wt P110 pipe (MIYP 27,500 psi)
The bottom 10,000 ft of the 7-in. portion of the string would be in service at temperatures > 275°F (134°C) such that SSC was not a concern and P110 material could be used in this section of the well. The upper section of the 7-in. and all of the 8 5/8 in., however, were required to be SSC resistant, and the T95 was required.
While these products met the design parameters for the well, none were readily available from inventories or existing stocks. The T95 casing strings were special ordered products with lead times of 6 months. In addition to the pipe, coupling stock was also required. Coupling stock usually requires a wall thickness of approximately two times the wall thickness of the pipe. So this required a coupling stock of about 2-in. wall thickness for both sizes.
Since this was T95 material, restricted yield strength, controlled hardness and NACE testing were also required. These properties are more difficult to achieve as the wall thickness increases. The thicker the wall, the harder it is to effectively quench and temper the pipe to achieve the required properties throughout the wall thickness. In this case, the pipes and the coupling stock were manufactured by V&M Tubes in Germany and successfully passed the required testing including NACE.
As the material was being produced, the selection of an appropriate connection was being finalized. It was more expedient to have the pipe threaded and coupled at the mill. This was particularly important since the delivery was going to be delayed due to an unplanned outage in the steel melting facility. Projected rig availability and drilling plans added to the need to reduce production time wherever possible.
Since a threaded and coupled connection was required, the VAM HW-ST connection from the tubulars company was considered. This connection had been specifically designed and successfully used on heavy wall pipes for more than 10 years. Prior testing data was first reviewed to determine if additional qualification testing was required. There was existing testing in wall thicknesses from 0.800 in. to 1.050 in. in sour service grades C90, T95 and C110.
Testing and running
After reviewing the prior testing, it was decided that only limited testing would be needed on the 8 5/8-in. material to supplement existing testing. There was sufficient test data available on the 7-in. connection to accept it without additional testing. Material for 8 5/8-in. pipe and coupling stock was air freighted to Houston so that testing could be performed to qualify the connection in time for mill threading. Samples were manufactured and make and break tests were performed to validate running performance. After the final makeup, the samples were subjected to an ISO Series B test with internal gas at pressures up to 22,400 psi and tension loads up to 2,384 KIPS. The testing was successful and the VAM HW-ST connection was approved for mill threading.
Another important consideration for this application was the crossover from 8 5/8 in. to 7 in. Due to the location in the well, this crossover had to be machined out of sour service material. This was a special order piece of T95 material with a 2 1/2-in. wall thickness, which allowed machining an 8 5/8-in. pin on one end and a 7-in. pin on the other end. Two crossovers were made and then bucked onto 7-in. pup joints, and an 8 5/8-in. coupling was then bucked onto the “upper” end. This provided an assembly with a pin looking down and box looking up for ease of running on the rig.
The casing string was successfully run in November 2006 as a single 18,200-ft (5,551 m) string in 18 ppg oil-base mud due to the H2S environment. A total of 343 joints of 7-in. 65.50# P110 were run and then crossed over to 7-in. 65.50# T95 for 262 joints. At the top of the string, 21 joints of the 85¼8-in. 90.0# T95 were run. The total air weight for the combined string was slightly more than 1.2 million pounds.
The casing running was performed by a contracted casing crew. All connection makeups were computer controlled and torque turn monitored. Out of the over 625 makeups, there were no rejects or backouts. All of the connections made up correctly with the initial makeup. The successful installation of this critical production casing string was a result of upfront diligence and planning on the part of the operator. This provided sufficient time for the specialized materials to be manufactured in the required dimensions. This also gave the time required for the needed connections to be evaluated, physically tested, and finally qualified prior to need.