Methodology For Materials Selection Basis of Design, and Equipment Testing Criteria

Abstract

This paper will share some of the collaborative efforts completed within DeepStar® Global Deepwater Technology Development Program as part of Phase XII projects in X500 Drilling, Completions & Intervention committee leading to gap identification, industry standards, and guideline deployment. Selecting safe economical materials for production from the increasingly more important HPHT horizons is a serious challenge because of the required combination of ‘safe and economical’. This challenge is being discussed in the literature as well as the standard writing community at great length. The effort aims at economical materials remaining within safe limits without being overly conservative. It is well known that qualification for sour service has occurred experimentally within a matrix of pH and H2S partial pressure ever since NACE MR0175 was first established some 40 years ago. It has now been recognized within the past 5+ years that it is the H2S concentration in solution which is responsible for the corrosion damage. As the total pressure increases the solubility is no longer proportional to the partial pressure, rather a fugacity correction needs to be applied. As the total pressure still increases further into the realm of 15,000 to 20,000 psi a heretofore not fully appreciated effect occurs in that the high methane pressure reduces the solubility even more than would have been predicted by the fugacity correction alone. Its consequence regarding corrosion, however, is still not well established because testing at these high pressures is, and will be for some time, problematic. This makes it clear that acquiring representative and relevant environmental data for the MSBOD is only part of the task at hand. The realistic translation of field environment to the test environment is the other part. Hence, appropriate physicochemical models for extreme conditions must become an integral part of the development of the BOD. In this environment there are two aspects that need to be vigorously pursued. The qualification procedures for the proposed metallurgy need to be clearly delineated and their relevance demonstrated. Test protocols need to be strengthened and results need to be interpreted statistically for relevance. In this context methods for identifying (system) in situ conditions based on external analytical data will also be critically discussed. A methodology will be proposed whereby the H2S partial pressure on x axis of the familiar pH-pH2S diagram can be displaced by the fugacity based H2S concentration. This will result in a reduction of excess conservatism while maintaining an adequate safety margin. This work was initiated and funded by DeepStar, Global Deepwater Technology Development Program as part of Phase XII projects in the (X500) Drilling, Completions & Intervention Committee.

Improved High-Pressure, High-Temperature (HPHT) Materials Qualification Using Dissolved H2S Concentration as the Sour Service Scalable Metric

“…13 The intrinsic overconservatism of the above NACE/ISO policy becomes consequential when selecting materials for mildly sour HPHT wells with wellbore pressures >10,000 psig. [5][6]14 (The policies leading to over/under-conservative design will be addressed in the following paragraphs. )…”

“…Thus, qualifying materials based on equivalent P H2S may lead to over-conservatism particularly when qualifying materials for service conditions with total pressures >10,000 psig (69 MPa). [5][6][7]12 In addition to over-conservative design concerns, the Standard also allows for a simplification for systems whose total pressure exceeds the bubble point/saturation pressure (e.g., "gas-free wells"). The assumption is made that the severity of the environment at pressures above the bubble point can be characterized by the P H2S calculated at the bubble point, which may be nonconservative when considering the modern f H2S and C H2S approaches.…”

“…However, there is recent evidence that SSC susceptibility correlates better with H 2 S fugacity (f H2S ) and/or C H2S at elevated total pressures. [5][6][7] The recent publication of ISO 15156-1:2020 now allows users the option to define environmental sour severity either by f H2S , activity (a H2S ), or C H2S in addition to P H2S . These alternative sour severity metrics incorporate the thermodynamic nonideality effects on H 2 S partitioning between gas/liquid hydrocarbon and aqueous phases at elevated pressure, temperature, and composition.…”

“…13 The critical issue is therefore to determine which parameter, f H2S or C H2S , best characterizes higher-pressure sour systems for material qualification purposes. On the one hand, several authors [5][6][7] discuss differences in H 2 S solubility in highpressure vs. low-pressure systems and the relationship between concentration, fugacity, and activity. No evidence of the controlling SSC mechanism was provided.…”

Casing structural integrity and failure modes in a range of well types - A review

“…But, the situation downhole make tubulars (casing and liners) vulnerable to failure of different kinds; because of 'triobocorrosion' (Das and Samuel, 2018). Hausler et al (2017) pointed out that the performance of tubular hardware (tubing and casing) depends on tubes properties, existing and applied stresses and the environment in which the tube is operating. High level standard specifications for casing, tubing and line pipes applicable to oil and gas wells are documented by the API under API specification 5CT.…”