Hydrogen sulfide (H2S) is hazardous to human health, living organisms and more generally to the environment. Historically, this is the reason wells found with sour gas were often carefully plugged and abandoned.

Steel tubular such as drillpipe may be exposed to H2S during drilling operations in the event of loss of well control or if the drillpipe is used with underbalanced drilling techniques. If unfavorable combinations of different factors coincide, this contact can lead to crack initiation that can propagate and lead to catastrophic failure even with stresses largely below the yield limit of the steel.

With the increasing demand of gas worldwide, some highly sour oil and gas reservoirs have been explored in Russia, the Middle East, China and North America. The development of such fields represents significant technical challenges regarding drillpipe integrity and operations safety. The combination of tubular failures due to sulfide stress cracking (SSC) and rising HSE concerns when dealing with sour gas led industry to develop new drillpipe grades with enhanced resistance to SSC.

Selecting H2S-resistant drillpipe

Material selection for drillpipe in a sour environment is not straightforward since there is no dedicated international standard. Both API and ISO do not include any requirements for sour service drillpipe.

The NACE Material Recommendation MR0175 was written in 1975, although it left drilling products out of the scope because those tools are generally used in a controlled environment (drilling fluids). It nevertheless clearly defines four application domains that provide a range of susceptibility to H2S related to well conditions. NACE MR 0175 is considered a reliable selection guide for casing and tubing material. Most of the current or future highly sour field conditions fall outside of this diagram.

NACE also defines normalized test methods gathered in the NACE Testing Methods TM-0177. These were created in 1977 and reviewed in 2005. Four testing methods are specified by NACE for oil and gas tubulars: A, B, C and D. The testing methods are not equivalent, and each can play a specific role.

Method A evaluates the suitability for service by testing the material’s resistance to axial stresses (pure tension), which can be close to the maximum operational stresses that will actually be applied to the drillpipe.

At a regional level, however, the “Industry Recommended Practices” (IRP) Volume 1 was published in 2004 in Canada. The standard was created with people’s safety in mind and provides material property specifications and guidelines for manufacturers, including quality control, testing and inspection of drilling products intended to be used in critical sour wells.

Decade of H2S-resistant drillpipe use

Sour service drillpipe as defined in the IRP has been used for a decade in Canada as well as in other regions and continents along with a variety of other proprietary sour service grades of drillpipe and bottomhole assemblies.

For instance, the development of several gas fields in the Sichuan Basin in China has involved numerous drilling and safety challenges due to the depth of the reservoir and to the high content of sour gas (around 14% H2S). The occurrence of several drilling incidents due to H2S during exploration and development led the operating company to select highly engineered drilling products to avoid such problems in the future.

Based on the experience and expertise of well-known manufacturers, fit-for-purpose grades of drillpipe have been selected to resist these harsh sour conditions and encourage safe drilling conditions. Such proprietary grades largely exceed the resistance of API grades to SSC and are being manufactured using the NACE TM0177-Method A and as per specifications largely inspired by the IRP 1.8.

Starting from December 2006, several critical wells have been drilled using sour service grades instead of standard products. Since late 2006, it has been proven that the use of such sour service grades minimizes the risk of failure, even in the harshest well sections, and no incident has been reported so far.

A more recent example is the exploration of the Kurdistan region in northern Iraq, where the extraction of oil and gas reserves began in 2007. Significant amounts of H2S gas are present, with reservoir depths easily reaching more than 6,100 m (20,000 ft). Throughout 2010, the use of standard grades of drillpipe led to several drillstring failures due to SSC as a result of well control loss.

Today most operating companies drilling in this part of the world carefully select H2S-resistant drillpipe, and the tubular inventory in this region largely meets IRP requirements. One of the first operators to have switched its inventory to sour service grades has drilled several exploration wells with two rigs equipped with 5,000 m (16,400 ft) of IRP 1.8-compliant drillpipe. The strings have been used in severe environments containing 18% to 20% H2S and high-pressure conditions. No SSC failure has been reported during the year of operation.

Highly sour fields: New frontier

With the increasing demand of domestic gas in different parts of the world, some highly sour oil and gas reservoirs are being explored with H2S content beyond what could have been imagined a decade ago. To explore, appraise and develop these new fields, which often combine sour gas and deep/complex well profiles, significant safety challenges need to be overcome, including maintaining drillpipe integrity.

From a normative standpoint, regional initiatives are still pretty active with the upcoming revision of IRP Volume 1 but also the emerging use of IRP Volume 6 in the industry. This volume was published in 2004 and addresses critical sour underbalanced drilling, although it has barely been used until very recently.

Also, the Chinese government has put together its own standard to specify more stringent requirements toward sour service drillpipe used in sour gas wells. The standard has been issued by the National Energy Administration of China and implemented in 2012. The specification is largely influenced by the IRP 1.8 and 6.3 sections and includes high-strength steels and SSC requirements in the assembly zone of the drillpipe.

From a product standpoint, the new challenges associated with the particularly sour fields require new highly engineered drillstring solutions to increase the safety margin related to SSC failure risks, especially in the upset and welded zones. Sour service drillpipe has long been used with tool joints and tubes fulfilling separate criteria for sour service as defined by the IRP.

Both the upset area and the friction weld present some challenges for preserving SSC resistance due to metallurgical factors such as heterogeneous microstructure, different chemical compositions between the tool joint and the pipe body and high hardness values close to the weld line.

Latest innovations

Manufacturers and end users are perpetually looking for new grades able to extend the current drilling envelope and push the frontiers of sour gas development in a safe manner. One of the latest products developed to address the highly sour field market exhibits SSC resistance in the weld and pipe body upset areas, which exceeds the requirements of IRP 1.8.

To achieve such results, several modifications of the drillpipe tool joint chemistry and the post-weld heat treatment have been implemented. As a result, the hardness gap between the pipe body and the tool joint has decreased drastically. The weld line is no longer distinguishable on micro-hardness mappings, indicating a more homogeneous microstructure. The SSC resistance of both the welds and upset areas has been assessed by the NACE TM177 Method A and Solution A. This new grade of drillpipe has been fully qualified to provide higher safety margins compared to existing grades on the market.