Materials Performance

NOV 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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65 NACE INTERNATIONAL: VOL. 53, NO. 11 MATERIALS PERFORMANCE NOVEMBER 2014 attached at the bottom hole assembly ; the depth is ~300 m. The expected well bottom temperatures are 600, 900, or 1,200 °C. The surface facilities include an air com- pressor, dehydrator, dry air injection pipe- line, and well head for the air/steam injector well. This equipment does not require any special material of construction. Corrosion susceptibility of injector well conduits depends on the gaseous environ- ment and operating conditions. The igni- tion in the vicinity of the bottom hole assembly creates high temperatures from coal combustion. Depending on the pro- cess control, the temperature can reach ~600, 900, or 1,200 °C. Hence, the study used these high temperatures to identify suit- able materials of construction. The pres- surized injection air does not allow the hot gases from coal combustion to enter the injector well. Hence, the injector well's sec- ond casing is exposed to air and high tem- peratures. This environment facilitates high-temperature oxidation. Producer Well Completion The producer wells are constructed ver- tically to a certain depth and then the wells' bottom portion runs horizontally. The wells are completed without any tubing. The surface casing, first casing, and second casing are completed in such a way that cir- culating water in the annulus between the first and second casings serves as a heat exchanger to reduce the produced gas tem- perature to 200 °C when it exits the pro- ducer well. The surface facilities include a well head for the producer well, a cooling water pipeline from the water source to the pro- ducer well's well head, a steam pipeline outlet from the well head of the producer well to the point where steam is utilized, and a f low line from the well head to the syngas processor. These surface facilities do not require any special materials of con- struction. The production casing at the bottom hole is exposed to high temperatures and a complex gas mixture. The UCG process temperatures of 600, 900, and 1,200 °C are used for the current study to identify suit- able materials of construction. The typical contents of produced gases relevant to the corrosion studies are shown in Table 1. In high-temperature corrosion from mixed gas, there is interaction between oxygen activity and other principal corro- dant activity. 2-3 The relative content of the various gases predominantly influence the corrosion attack preferentially as oxida- tion, sulfidation, carburization, and nitrid- ation. 4 A detailed analysis of the oxygen potential , sulfur potential , and carbon activity were compared with the equilib- rium pressures and activity of each from the standard Ellingham Diagrams. 3,5-6 The results are shown in Table 2. The alloys rely upon the oxidation reaction to develop a protective oxide scale to resist corrosion attack. To be protective, this oxide layer must be chemically and thermodynami- cally stable. In the syngas composition from the UCG process, ammonia (NH 3 ) is not pres- ent; instead the syngas has inert molecular nitrogen, N 2 . Furthermore, the atmosphere of the UCG process is predominantly oxi- dizing in nature as per the current study based on the syngas composition from the UCG process, so nitridation attack will not be significant. The conclusion is that the environment promotes high-temperature oxidation for both injector and producer well conduits. Material of Construction for Conduits of Underground Coal Gasification Wells The conduits of UCG wells—both injec- tor and producer wells—are exposed to predominantly high-temperature oxida- TABLE 2. STANDARD EQUILIBRIUM PRESSURE VS. POTENTIAL DUE TO COMPOSITION FOR O 2 , SULFUR, AND CARBON Temperature (°C) O 2 Potential (A) (P-H 2 O/P-H 2 ) or (P-CO 2 /P-CO) Atmosphere Equilibrium O 2 Pressure (B) Atmosphere 600 8.9 E-36-2.08 E-35 1.25 E-36 900 2.4 E-24-5.39E-24 3.2 E-25 1,200 1.38 E-17-3.08 E-17 1.85 E-18 Temperature (°C) S Potential (A) P-S 2 Atmosphere Equilibrium S Pressure (B) Atmosphere 600 1.754 E-22 2.994 E-18 900 3.88 E-16 6.624 E-12 1,200 2.23 E-12 3.810 E-8 Temperature (°C) Carbon Activity (A) P-CH 4 /P 2 -H 2 Atmosphere Equilibrium C Activity (B) Atmosphere 600 1.68 5.225 900 1.22 3.792 1,200 1.0 3.13 (A) From gas composition (B) From Ellingham Diagrams TABLE 1. TYPICAL CONTENTS OF PRODUCED GASES RELEVANT TO CORROSION STUDIES Gas Pressure (A) (psi) P-CO 2 7.9 P-H 2 S 0.05 P-CO 3.0 P-H 2 O 16.3 P-CH 4 0.8 P-N 2 23.0 (A) Expressed as partial pressure

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