A simplified workflow for completion tubing metallurgy selection

We’re all familiar with the sort of H₂S and CO₂ partial pressure material selection graph shown in Figure 1. Here, the appropriate metallurgy for a well based on the partial pressure of the CO₂ and H₂S as well as operational temperature is divided into zones. Increasing CO₂ pushes us up the Y-axis moving from standard API specifications into 13Cr and the 9 Cr. Meanwhile, increasing H₂S along the X-axis moves us into sour service tubulars and (if there is sufficient CO₂ present) into the world of Duplex and Super Duplex materials (red zones). It is important to note that the scales are logarithmic. For example, partial pressures of CO₂ which require 13Cr material (assuming no H₂S) range from about 3 psi to 50 psi, whereas 9Cr material ranges from 50 psi to 1400 psi.

Figure 1 – Metallurgy Selection Map for CO₂ and H₂S Partial Pressures

Often these quick calculations are good enough for the selection of the metallurgy for most completions. For environments which are not overly challenging, 13Cr is adequate. However, there are a couple of additional factors which need to be considered. Firstly these graphs tend to be overly cautious, being supplied by the tubing manufacturers. Thus they can result in the over-specification of materials. Secondly, while they do give a ‘nod’ to the impact of temperature, they do not include the impact of pH on the corrosion of the material.

The following paragraphs define a simple workflow that can be adopted to improve the robustness of a material selection process as part of a conceptual completion design or a basis of design.

Partial Pressure Determination

The partial pressure for the analysis should be calculated at the bubble point of the hydrocarbon. However, most reported CO₂ and H₂S level will be at separator conditions or at standard temperature and pressure. So the first step is to determine the volume of CO₂ (%) and H₂S (ppm) at the bubble point. There is not a linear correlation between say standard pressure and temperature and bubble point but rather a logarithmic reduction of concentration with increasing pressure of the form:

Where:

H2Svol is the concentration of H2S in ppm
P is pressure in psi
a and b are constants dependant on the concentration of the H2S at standard pressure and temperature.

The equation above shows reasonable accuracy across a range of pressures. An approximation of a and b can be determined from the equations:

a = -0.267*H₂S + 11.955

b = 2.369 * H₂S -76.762

Where H₂S in this case is the concentration at standard temperature and pressure. A similar process can be used for determining the volume of CO₂ at pressure based on an initial percentage at standard conditions.

So, as an example, if we have an H₂S content of say, 850 ppmv at standard conditions and we wish to calculate the H₂S concentration at the bubble point (say it’s Pb = 1200 psi) then:

a = -214.995

b = 1936.9

H₂Svol = -214.995 * ln(1200) +1936.9 = 486 ppmv at Pb

Calculating the partial pressure for this volume is simply the bubble point pressure multiplied by the H₂S volume at the bubble point divided by 1,000,000. In this example we get a partial pressure of 0.582 psi.

Why the Bubble Point?

Why do we calculate the partial pressure at the bubble point, especially when the concentrations of H₂S and CO₂ are much higher at standard conditions? This is because it is the maximum partial pressure that we are interested in and despite the reduction in H₂S and CO₂ concentrations with increasing pressure, the bubble point will always results in the highest partial pressure as demonstrated in the graph in Figure 2.

Figure 2 – H2S Partial Pressure and Concentration at Different Pressures

We will assume that in the absence of H₂S, the levels of CO₂ present would require the use of 13Cr tubing with a partial pressure of 10 psi. This combination of partial pressures (H₂S = 0.582 psi and CO₂ = 10 psi) puts us right in the middle of the 22Cr region of our partial pressure chart as shown in Figure 3.

Figure 3 – H2S and CO₂ for 22Cr Tubulars

We could stop here and order up our 22Cr tubing and hope that the lead times and cost didn’t break the budget, but there are additional checks that can be quickly carried out to ensure that we are making the right decision.

BS EN ISO 15156

Reference to BS EN ISO 15156 which covers material selection for environments with H₂S states (Part 3, Table A.19 in the 2009 standard) that 13Cr material may be suitable if the the partial pressure of H₂S is 1.5 psi of less. (H₂S PP = 0.582 psi in our example). However this is dependant on the pH of the in-situ fluids being greater than or equal to 3.5, so it is important to understand what the pH of the in-situ fluids is.

Approximate Determination of pH

An estimate of the pH of the fluid can be determined from the graph shown below which is based on BS EN ISO 15156 Part 2 Appendix D. Let’s say that our reservoir is 70 deg C. We take the two partial pressure values for H₂S and CO₂ and add them (10 + 0.582 = 10.582 psi). Interpolating between the 20 deg C and 80 deg C lines given in the publication, we can calculate that the pH as a function of temperature and pressure. Figure 4 below shows the linear correlation between the cum of CO₂ and H₂S partial pressures and pH. The equation in the top left hand corner of the graph is an example of the pH calculation at 70 deg C

Figure 4 – pH as a Function of H₂S and CO₂ partial pressures

H₂S and pH Domain Maps

Having established the pH, we can use a number of domain maps which indicate the safe and unsafe zones for the use of 13Cr tubulars in sour environments. The figure below shows a selection which include those of a steel supplier (blue) and an operator (red). You can see that both of these are more conservative than that of API 15C.

Figure 5 – pH and H₂S Domain Maps from Various Sources

Workflow Recap

Obtain CO₂ and H₂S concentrations at standard conditions
Calculate the concentration at the bubble point pressure (Pb)
Calculate the partial pressures at the bubble point pressure
Check outcome on standard partial pressure map.
Calculate the pH based on reservoir temperature and total partial pressure of CO₂ and H₂S
Check suitability based on domain maps

Summary

CO₂ and H₂S partial pressure material maps are a useful first step in material selection but they tend to be overly pessimistic compared to national and international standards.

This is a simple workflow and it should be noted that in cases of uncertainty refer to a qualified metallurgist.

Finally a cautionary note. Different standards and sources use different units (as you can see in the selection of figures above.

References and Links

BS EN ISO 15156 Petroleum and Natural Gas Industries – Materials for use in H2S-Containing Environments in Oil and Gas Production – Part 3: Cracking-Resistant CRA’s (Corrosion Resistance Alloys) and Other Alloys September 2015 (British Standards Institute Website)

Heidersbach R. Metallurgy and Corrosion Control in Oil and Gas Production 2nd Edition 2018 (Amazon UK Link)