High-pressure/high-temperature (HP/HT) wells, with the presence of small amounts of hydrogen sulfide (H2S), generally in the parts per million (ppm) range, has caused considerable difficulty in completing such wells in compliance with NACE MR0175/ISO 15156. The central and most recognizable limitation this standard imposes is the 0.05 psia H2S partial pressure threshold for sour service. The drawback is that the standard prohibits C110 steel from being run to surface, restricting its use to depths where the temperatures are ≥ 150°F (65°C). Moreover, till the industry carries out a formal scientific study that elucidates the correct means to address the potential for sulfide stress cracking (SSC) based on H2S content, fugacity, and pressure, the insufficiency of NACE MR0175/ISO 15156 to correctly predict the SSC of high strength components will result in the increasing irrelevance of the standard.

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Is NACE MR0175/ISO 15156 Becoming Irrelevant?

Bruce Craig, FNACE

MetCorr

In recent years the petroleum industry has had to drill and complete wells in ever deeper

reservoirs in order to meet the growing worldwide demand for oil and gas. This means that wells,

now referred to as high pressure high temperature (HPHT), are encountering extremely high

pressures, typically greater than 15,000 psi (69 MPa) with temperatures beyond 350oF ( 177oC).

Many of these wells have been drilled and completed in the Gulf of Mexico and onshore Texas

and Louisiana. The presence of small amounts of H2S, typically in the ppm range, has caused

considerable difficulty in completing such wells, especially when trying to comply with the

requirements of NACE MR0175/ISO 15156. The primary and most recognizable limitation this

standard imposes is the 0.05 psia (0.3kPa) threshold for sour service. Thus for a well with 15,000

psig bottomhole pressure there need only be 3.5 ppm H2S for NACE MR0175/ISO 15156 to be

invoked, yet it is almost impossible to accurately measure this amount of H2S. Because of this

difficulty in accurately measuring H2S it has been argued by some companies that high pressure

wells should be considered sour from the start. While this is certainly conservative and a safe

way to address the problem, other companies chose to ignore the possibility of any H2S at all.

Considering the former approach, a few examples will help show how even a conscientious

operator will eventually face a dilemma if they chose to abide by NACE MR0175/ISO 15156.

The first example (these are all real cases not simply theoretical exercises) is a steel

casing tieback string that must tie the production liner back to the surface. This string is the first

production casing outside of the tubing that may be exposed to H2S if there is a leak in a

connection and historically is always designed to be compliant with NACE MR0175/ISO 15156.

Figure 1 shows such a design. Due to the high pressures and high tensile loads the tieback is

typically a heavy wall high strength steel casing. In HPHT wells the production tieback is often

casing with outside diameters that are 8-5/8", 9 - 7/8" and 10 - 3/4" and wall thicknesses greater

than 0.750 inch (19 mm) even sometimes exceeding 1 inch (25.4 mm). This is because there are

2

the combined loads of axial and collapse or burst that require high strength for this string. It has

been found during design of HPHT wells that API 5CT Grade Q125 casing may be required for

strength, however, since Q125 has no resistance to SSC, a heavier wall C110 casing, which is

currently a non-API sour service grade, can often handle the mechanical design. But NACE

MR0175/ISO 15156 does not allow C110 to be run to surface, instead it restricts this steel to

depths where the temperatures are ≥ 150oF (6 5o C) . This means that to comply with NACE

MR0175/ISO15156, Grade T95 would be the only choice that would meet the standard all the

way to the surface (for all temperatures). This is extremely restrictive and some would argue

punitive. Furthermore, it creates serious technical issues. For example, if an 8-5/8" OD, 1.00"

wall thickness C110 would suffice for the tieback casing but must be compliant with NACE

MR0175/ISO 15156 (cannot be run to surface), T95 would need to be selected. But this would

require increasing the wall thickness of the T95 to 1.187" (30.1 mm) in order to maintain the

appropriate mechanical loads which translates to a reduction in the inside diameter. This

decrease in ID generally causes dimensional issues and constraints that cannot be easily

overcome, such as sufficient room for the subsurface safety valve as shown in Figure 1.

Additionally, increasing the wall thickness leads to hardenability problems during manufacturing

and at some point the wall thickness required simply cannot be manufactured. So the well

designer and the oil company have two choices - don't complete the well or run several thousand

feet of corrosion resistant alloy (CRA) that meets NACE/ISO 15156 for a tieback. This decision

is increasingly being faced by numerous oil companies and generally they chose neither of the

two choices above but instead run C110 to surface. Other less conscientious companies don't

consider any of these issues, instead just running API 5CT Q125 casing that has no resistance to

sulfide stress cracking (SSC). Therefore, it can be seen that trying to complete HPHT wells can

lead to a dilemma that generally results in less than strict adherence to NACE MR0175/ISO

15156. While it is recognized that NACE MR0175/ISO 15156 has a provision to tests alloys for

a specific well applications and use the results to justify the use of those alloys that are not

compliant, the reality is that other than for a few major oil companies this is never done. Rather

companies just run the alloys based on manufacturers test data and sales brochures. A good

example of this is the arrival in the marketplace of C125 steel casing for marginally sour wells.

Many companies will run this grade of casing in wells back to surface in violation of

MR0175/ISO 15156 without regard to testing or any other qualification simply because the

demand to complete HPHT wells requires its use.

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Another example is the large useage of modified 13Cr tubing alloys (also referred to as

Super and Hyper). These alloys are not compliant with NACE MR0175/ISO 15156 when they

are used with a minimum yield strength of 110,000 psi, which is a mainstay of the industry in the

US and widely used for wells containing H2S at partial pressures in excess of 0.05 psia. Again,

with few exceptions, no laboratory testing is done qualify these alloys for a specific application.

The intent of this paper is not to condone the widening industry neglect of NACE

MR0175/ISO 15156 but to point out that if this standard is to continue to have relevance in the

petroleum industry it must begin immediately to put this standard on a scientific footing rather

than simply rely on old empirical rules. It is well known that the 0.05 psia limit was an empirical

choice based on experience and has worked admirably for many years, especially when wells

were lower pressure and partial pressure was a legitimate estimation of the potential for SSC.

However, it is now recognized that at high pressures the partial pressure is not correct and

fugacity along with reduced solubility of H2S due to methane influences must be used to evaluate

the potential for SSC1. Until the industry carries out a formal scientific study that elucidates the

correct means to address the potential for SSC based on H2S content, fugacity and pressure, the

inability of NACE MR0175/ISO 15156 to correctly predict the SSC of high strength components

will result in the increasing irrelevance of NACE MR0175/ISO 15156. As operators continue to

ignore this standard and have success they will increasingly neglect even the consideration of

MR0175/ISO 15156.

In addition to well construction there are other areas where NACE MR0175/ISO 15156

is intentionally ignored or within certain industry standards, an apparent attempt to side step the

NACE MR0175/ISO 15156 requirements has been institutionalized. For example, some

manufacturers of API 6A wellhead and Christmas tree equipment offer customers equipment

manufactured in accordance with NACE MR0175-2002 in order to allow the use of such

components as 17-4 PH stainless steel which has been essentially eliminated in the NACE

MR0175/ISO 15156 standard. Many operators purchase such equipment which in effect is an

outdated standard. Yet, the current edition of API 6A/ISO 10423 (19th edition, July 2004)

specifically states that MR0175: 1999 is to be used, even though ISO15156 was in effect when

6A was released.

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In the case of API 610 for pumps the 10th Edition, dated October 2004 (well within the

time frame of NACE MR0175/ISO 15156) which is the most recent edition, the standard refers

to MR0175 and not NACE MR0175/ISO 15156. It states that any undated references to

standards applies to the latest edition, which many manufactures interpret as MR0175-2003,

again so the more stringent NACE MR0175/ISO 15156 requirements don't have to be applied. In

both the 6A and 610 cases the application of earlier editions of MR0175 instead of NACE

MR0175/ISO 15156 is not just occurring in a few unknown manufacturers or their customers but

from many of the major manufacturers and operators.

It has been 5 years since the issuance of NACE MR0175/ISO 15156 and still many

sectors of the oil industry continues to ignore this standard in favor of earlier editions of

MR0175or in some instances completely ignores the use of any sour service standard. This is a

dangerous situation but if NACE MR0175/ISO 15156 continues to set stringent requirements

without a firm scientific foundation and ignores the successful use of newer alloys (ie,

Super/Hyper 13Cr-110) then it will become irrelevant.

References

1. P. R. Rhodes, L.A. Skogsberg, R. Schofield, R. D. Kane and E.A. Trillo, Corrosion 2007,

Paper No. 07110, NACE, 2007.

Acknowledgment

The author gratefully acknowledges the assistance of Bob Moe at Viking Engineering in

preparing the schematic in Figure 1.

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Figure 1 – An example of an HPHT well completion.

ResearchGate has not been able to resolve any citations for this publication.

  • P R Rhodes
  • L A Skogsberg
  • R Schofield
  • R D Kane
  • E A Trillo

P. R. Rhodes, L.A. Skogsberg, R. Schofield, R. D. Kane and E.A. Trillo, Corrosion 2007, Paper No. 07110, NACE, 2007.