Prediction of Corrosion Inhibitor Performance Using Simulated CO2/H2S Environmental Autoclave and Flowloop Tests - I
The hot topic series describes a test program and the result targeted towards evaluating corrosion inhibitor performance in CO2/H2S environments. After preliminary screening of the inhibition characteristics and performance of 5 corrosion inhibitors (designated A,B,C,D & E) under simulated CO2/H2S environment in a high temperature, high pressure flowing autoclave, the best two corrosion inhibitors (corrosion inhibitors A & B) were selected for further flow loop testing under simulated North Sea pipeline service conditions.
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This hot topic series describes a test
program and the results targeted towards evaluating corrosion
inhibitor performance in CO2/H2S
environments. After preliminary screening of the inhibition
characteristics and performance of 5 corrosion inhibitors
(designated A, B, C, D, & E) under simulated CO2/H2S environment in a
high temperature, high pressure flowing autoclave, the best two
corrosion inhibitors (corrosion inhibitors A & B) were
selected for further flow loop testing under simulated North Sea
pipeline service conditions.
The worst case pipeline service conditions (90% synthetic field
brine/10% hydrocarbon (depolarized kerosene) environment with H2S (partial pressure 0.075 psia [0.5 KPa]) and CO2
(partial pressure 75 psia [0.5 MPa]) were simulated at 300 F.
(149 C.) in a laboratory flowloop and the performance of the
selected corrosion inhibitor formulations A & B on API 5L
X-65 carbon steel and 0.5 percent Cr X-65 steel were evaluated at
two concentrations (100 ppm and 250 ppm by volume).
The corrosion rates were monitored at stirred (reservoir),
laminar and turbulent flow regimes at wall shear stresses
corresponding to a range of velocities (4.2, 5.3, 6.3, 7.0, 7.9,
13.1, 15.4, 17.4 and 21.1 ft/sec) using linear polarization (LPR)
and weight loss techniques.
The test results indicated the following:
IntroductionThe results presented in this hot topic are the second phase of a
study performed to evaluate the feasibility of using inhibited
carbon steel rather than duplex steel as a major material of
construction in a proposed North Sea pipeline.
- Provided the proper concentration of
corrosion inhibitor is applied continuously, carbon steel
can be satisfactorily inhibited under laboratory test
conditions that simulate pipeline worst case corrosion
- Corrosion inhibitor "B" was not
as effective as corrosion inhibitor "A"
requiring about 2.5 times as much "B" as
"A" to accomplish the same level of corrosion
control under these simulated laboratory test conditions.
- The results of this corrosion inhibitor
evaluation program do not support the use of 0.5 Cr X-65
material over X-65.
- The flowloop test data obtained at higher
wall shear stresses indicated, within the limits of the
wall shear stress parameters, there was little or no
effect on the corrosion inhibitor film being laid down
continuously on the metal surface.
In the first phase of this study, the results of high
temperature-high pressure flowing autoclave tests established
that it was possible for commercially available corrosion
inhibitors to be thermally stable at 300 F. (149 C.) as well as
provide >90 percent corrosion inhibition to carbon steels
(X-65 and 0.5% Cr enhanced X-65) under laboratory test conditions
simulating conditions predicted for the proposed North Sea
pipeline (see Tables 1 and 2).
In this hot topic the results of the Phase 2 studies are
reported. In this study, the two best performing corrosion
inhibitors from Phase 1 were tested in a flow loop apparatus to
evaluate their performance under varying conditions of flow over
a longer period of time. The performance of these two corrosion
inhibitors was determined on both X-65 and 0.5% Cr enhanced X-65
specimens at inhibitor concentrations of 250 and 100 ppm. Data
was measured for static, laminar and impingement flow conditions.
Maximum flow conditions for the flowloop tests were established
at 10 ft/sec based on a flow regime calculated using the Briggs
and Brill (B-B)1, as well
as the Taitel-Dukler (T-D)
models. The Briggs and Brill model predicted the occurrence of
intermittent slugs with a liquid film velocity of 6.2 ft/sec. On
the other hand, the Taitler-Dukler model, which is considered by
the authors to be more representative, indicated annular flow and
no slug flow in the pipeline. A velocity of 10 ft/sec liquid
velocity was selected as a very conservative flow rate parameter
for the flow loop tests. The wall shear stress (t) due to a liquid
flow velocity, v, in the field pipeline was calculated as
t= frn2/2where r= density of
the liquid and f = friction factor
The liquid flow rate in the laboratory flow loop corresponding to
the above shear stress value was calculated using the above
equation and Moody diagrams which provide relationships between
pipe diameter (D) and roughness factor (e/D) and f, e/D,
and Reynolds number (Re = D/??; ??= viscosity of the liquid.)
The effect of flow rate on the inhibitor film was studied using
the flow loop apparatus at up to nine (9) flow rates
corresponding to 40%, 60%, 80%, 100%, 120%, 330%, 456%, 660% and
865% of the maximum (100%) anticipated North Sea pipeline wall
shear stress value using a velocity of 10 ft/sec (Table 3). Other flowloop test parameters
included were: 300 F. (149 C.); pp CO2 of 75 psia; pp
H2S of 0.075 psia; 90% Synthetic North Sea Pipeline
brine (Table 4); 10% depolarized kerosene.
These test parameters represented worst case senario for the
proposed North Sea pipeline.
Prior to initiation of the Phase 2 study, flow loop apparatus
performance was tested using the BP Protocol Calibration Test. The flow loop test
apparatus encountered no problems passing the require calibration
The chemical supplier of the two corrosion inhibitors to be
evaluated in the flow loop test, provided preliminary flowloop
test data developed in their laboratory.
The test parameters used were: 180 F. (82 C.); pp CO2
of 70 psia; X-65 carbon steel specimens; 70% synthetic North Sea
pipeline brine; 30% depolarized kerosene; flowrate - 2.2 ft/sec.
These flowloop test conditions were less rigorous than the Phase
2 flowloop test conditions. However, the test results were in
line with the performance indicated in the Phase 1 tests.
Both the BP Protocol Calibration Test results and the chemical
supplier's flowloop test results tend to add support and
credibility to the results reported in study.
Input Data - Field
Note: Tau [wall shear stress} value calculations based on a
velocity of 10 ft/sec
Calculated Data - Flowloop Apparatus
|Constituent||Desired Mixture - mg/L||Actual Mixture - mg/L|
|Sodium - Na||20,000||20,835.30|
|Calcium - Ca||2,000||2,000.00|
|Magnesium - Mg||1,500||1500.4|
|Barium - Ba||50||50|
|Chloride - Cl||40,000||39,994.90|
|Sulfate - SO4||20||20|
|Bicarbonate - HCO3||100||100.3|