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.
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:
- 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.
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.
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
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.
- 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
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)