CO 2 CORROSION ISOPLOTS
APPENDIX E : WHAT OTHER FACTORS NEED TO BE CONSIDERED?
Apart from the factors relating to corrosion discussed above, there are a number of other factors that must be considered before finally selecting the material for downhole application.
Briefly, the main ones are as follows:
1. Mechanical Properties
Clearly it is necessary to ensure that the material selected for the downhole tubulars has sufficient mechanical strength. There are a number of issues that need to be considered in selecting the correct strength grade, i.e.:
(i) Maximum strength grade the can be used
There are a number of limits on the maximum strength of tubular that can be selected.
Principally:
(a) Low alloy steels.
The 155ksi grade low alloy steels (e.g. XT155) are unsuitable for sour service and should not be considered.
(b)13%Cr steels.
Generally the highest acceptable strength level for 13%Cr steel is the C95 grade. At strength levels higher than this it is difficult to ensure adequate toughness. Even at 95ksi the required toughness can be difficult to achieve and it is important to ensure the correct requirements (specification) are used.
(c) Alloyed Martensitic 13%Cr steels (e.g. ‘Super’ 13Cr) Presently available in two strength levels 95ksi and 110ksi.
(d) Duplex Stainless Steels.
Generally the highest acceptable strength level for the duplex stainless steels is 125ksi. There is a possible reduction in corrosion/stress corrosion resistance above this level. Duplex stainless steels with higher yield strength levels can be considered on the basis of specific environmental testing for the intended application.
(ii) Directionality of mechanical properties
It is important to note that the cold worked duplex stainless steels can suffer from a degree of directionality in their mechanical properties, as a result of isotropy induced by cold working processes. This can result in a lower strength in the transverse direction than in the
longitudinal direction. The magnitude of this effect will be dependant on the manufacturing process, but can be as high as 15% for high strength materials manufactured using the Pilger process. Therefore, if specifying cold worked duplex stainless steel this issue should be discussed with the supplier. However, for initial design purposes it is recommended that it is assumed that the strength in the transverse direction is 5% lower than in the longitudinal direction.
(iii) Effect of elevated temperature on mechanical properties
The strength of many materials decreases with increasing temperature. It is important to take such decreases into account in completion design. For the more commonly used downhole tubular materials the magnitude of this affect is as follows:
(a) Carbon steel, 13%Cr steel and Super 13%Cr alloys
Over the range of temperatures likely to be experienced downhole the reduction in strength is likely to be relatively small.
The present Casing Design Manual8 recommends that for low alloy steels up to and including Q125 a yield strength temperature de-rating factor of 0.03% per °F is applied, with de-rating commencing at 68°F.
Given the similarity in behaviour in this respect for low alloy steels and 13%Cr steel/Super 13Cr alloys, it would be reasonable to assume a similar temperature de-rating factor for both 13%Cr steel and Super 13Cr alloys.
(b) Duplex stainless steel
The reduction in strength with increasing temperature is greater for duplex stainless steels than for low alloy steels. This resulted in a design guideline for Miller that assumed a 10% reduction in yield strength at 120oC. The magnitude of this affect is likely to be different for different grades of duplex stainless steel and for tubulars made by different manufacturing routes. Therefore, this aspect should be discussed with the suppliers. However, for initial design purposes it is recommended that it is assumed that the strength is reduced at a rate of 8% per 100oC.
Where design yield strength has a critical influence on weight or grade selection, and conservative assumptions are economically undesirable, it will be necessary to ensure that the actual yield strength at the required elevated temperature is adequate for the design by undertaking mechanical tests. Tests should be undertaken both before (as a pre-qualification) and during (as a quality control check) manufacture of the downhole tubulars.
2. Flow-Induced Damage: Erosion and Erosion-Corrosion Resistance
Erosion can defined as the mechanical loss of material by the impact of liquid droplets, gas bubbles and/or solid particles (e.g. sand). The term erosion-corrosion is used to define the conjoint action of erosion and corrosion.
Guidelines have been developed for the avoidance of erosion/erosion-corrosion problems9. Amongst other things, these can be used to determine velocity limits for the avoidance of erosion/erosion-corrosion damage. Different velocity limits apply in different situations,
depending on the fluids/flow regime (gas, liquid or multiphase gas and liquid), the environment (corrosive or non-corrosive), the downhole tubular/equipment materials and whether or not solids are present.
8 Casing Design Manual, Issue 2, BPA-D-003, September 1999
9 ‘Erosion Guidelines Revision 2.1 (1999), J W Martin, Sunbury Report No. S/UTG/102/99, October 1999)
Summary guidance for evaluating erosional velocity limits is given in Section 2.1. below.
These indicate the erosional velocity limits for which the rate of erosion will be 0.1mm/yr or less. Three main categories for solids content are used in these guidelines:
“Totally solids free” The flow streams are such that there is no risk of solids being transported in the fluids. It should be noted that even very low levels of solids can cause significant wastage (erosion or erosion/corrosion) rates. Hence it is very important to be sure there is no risk of solids entrainment before using these limits.
• “Nominally solids free” less than 1pptb (lbs per thousand barrels) for liquid systems, less than 0.1lb/mmscf for gas systems; no solids detectable.
• “Solids present” Solids detectable in the system. In this case the levels of solids will need to be known, or appropriate assumptions made regards their likely level.
2.1. Evaluation of Velocity Limits.
The following criteria indicate the maximum velocity limit to obtain an estimated rate of erosion of 0.1 mm/yr. or less.
Totally Solids Free Duties:
• Non Corrosive
⇒ Single Phase (liquid/gas) - no velocity limits for avoidance of erosion
⇒ Multiphase - limit velocity to 70m/s to avoid droplet/gas bubble impingement erosion.
• Corrosive
⇒ Single Phase Liquid - no velocity limits for avoidance of erosion
⇒ Un-inhibited wet gas/multiphase10 - limit velocity to 70m/s to avoid droplet/gas bubble impingement erosion.
⇒ Inhibited carbon steel wet gas/multiphase - use the API RP14e11 equation with C=200 or 20m/s (whichever is lower).
Nominally Solids Free Duties:
⇒ Single Phase Liquid - use the API RP14e7 equation with C=250 for carbon steel;
C=300 for 13Cr steel and C=450 for duplex stainless steel.
⇒ Multiphase - use the API RP14e7 equation with C=135 for carbon steel; C=300 for 13Cr steel and C=350 for duplex stainless steel.
10 For carbon steel it is assumed that the fluid has sufficiently low corrosivity to justify it’s use and is un-filmed (i.e. no carbonate film). For carbonate filmed carbon steel (see Section ????) use the
‘inhibited’ flow velocity limits.
11 API RP14e equation is: Maximum Allowable Velocity (ft/sec) = C/(mixture density in lb/ft3)0.5
⇒ Single Phase Gas - Evaluate using the Erosion Rate models in the Erosion Guidelines5. Assume a solids production rate of 0.1lb/mmscf if no specific data available.
Solids Containing Duties:
It is not possible to define a rational flow velocity for all possible operating conditions below which the rate of erosion will be below 0.1 mm/yr. Therefore, it will be
necessary to undertake an assessment of the likely erosion rate/acceptable flow velocities on a case by case basis using the relevant erosion rate models from the Erosion Guidelines5.
If the anticipated maximum flow rate is less than the critical velocity calculated using the information above, the effect of high fluid flow rates and/or sand erosion need not be
considered further. However, if the maximum flow rate is greater than the critical velocity then significant wastage may result. In this case further consideration of the likely wastage rates resulting from erosion/erosion-corrosion will be required. In the first instance reference should be made to the BP Amoco Erosion Guidelines5.
3. Galvanic Corrosion
This is the preferential corrosion that can occur to one of the metals when two different metals are electrically coupled in a corrosive environment. In such a couple one of the metals will act as an anode (i.e. it will corrode at an enhanced rate) and the other will act as a cathode (i.e.
there will be a certain degree of protection). The susceptibility of a material couple towards galvanic corrosion of the 'anodic' metal is influenced by a number of factors, such as the conductivity of the corrosive medium, the relative surface area of the two metal components and the magnitude of the potential difference between the two metals in the corrosive
environment.
In general, downhole tubular systems should be designed to eliminate galvanic cells where possible. For the purposes of downhole materials selection galvanic corrosion should be considered a potential problem if there is a corrosion-resistant alloy (CRA)/carbon steel interface. However, if such interfaces do not occur then the problem of galvanic corrosion can be discounted.
If galvanic corrosion between CRA jewellery and carbon steel tubulars is a concern,
consideration should be given to installing an internally coated CRA between the components.
In general, 9Cr-1Mo or 13%Cr steel against C-steel is not a concern, the major concern is for the higher metallurgies, e.g. nickel alloys, against carbon steel.
If there is a concern about any material combination that occurs in a downhole tubular design the relevant specialist/s should be consulted.
INDIVIDUAL ALLOY "GO/NO GO" CHARTS
-SULPHIDE STRESS CRACKING AND STRESS CORROSION CRACKING