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LABORATORY STUDIES AND FIELD TESTS FOR THE DEFINITION OF NEW MATERIALS AND COMPONENTS FOR GEOTHERMAL USES.

Objective

CONTROL OF THE CORROSION IN HIGH ENTHALPY WELLS CAN LEAD TO A MORE ECONOMIC ELECTRICITY PRODUCTION.
The necessity of having adequate materials to handle hard drilling and well operation conditions has directed experimentation towards innovative steels for this type of activity.
A series of materials with high matrix fineness are in the experimental stage for drill pipes. The materials examined were AISI 4137 H ESR, API G 105 ESR and UNI 30 nickel chromium molybdenum 12 (all remelted under slag).
The electrochemical tests run at both 20 C and 80 C always indicate an active behaviour (generalized corrosion) for all the materials.
For the evaluation of the susceptibility to stress corrosion, tests were conducted by the exposure of U-bend test pieces in an autoclave at 200 C for 720 h. In no case was there formation of cracks.
The SSRT tests, conducted under the same conditions, evidence the presence of stress cracking induced by the hydrogen sulphide.

The materials examined for casings were API C90, UNI X2 chromium nickel molybdenum nitrogen 225 and UNI X1 nickel chromium molybdenum copper 31274.
Of each material, 3 tubes and 3 couplings were made. These will be lowered into a reinjection well for a field test.
The electrochemical tests on materials for tubing, performed in an autoclave at 80 C, indicate that the C90 steel is always active in such conditions.
The martensitic stainless steel 13-chromium displays the formation of passive metal, but the passive range is very limited and localized corrosion phenomena are evidenced.
The austenitic ferritic and superaustenitic stainless steels, on the other hand, display a wide range of passivity.
Subsynchronous resonance tests (SSRT) were conducted in air (as reference) and in the environment at the velocity of deformation. The C90 steel displays extensibilities in air and in the environment that are comparable with each other. This result means that the C90 steel is not susceptible to stress corrosion in the test environment, probably because it corrodes generally and not locally, as shown by the electrochemi cal study. The absence of stress corrosion cracking is documented by the macrographic aspect and by the appearance under the scanning electron microscope (SEM) of the fracture surface, which is completely ductile.
The 13 chromium steel, on the contrary, showed an extensibility in area and an extensibility in aggressive solution. This sharp drop in ductility indicates the presence of stress corrosion.
The duplex and superaustenitic steels display extensibilities in air and in the environment which are comparable to each other, and hence did not undergo stress cracking.

The development of geothermal energy has been hindered by the fact that the resistance of present materials to mechanical stresses and corrosion is not always sufficient. This project focused on the problem of choosing the material to be used in a geothermal environment. The components examined were drilling pipes and reinjection tubing. The following materials were examined for use as drilling pipes:
carbon steel type AISI 4737H (normal and electroslag remelting (ESR));
carbon steel type UNI 30NiCrMo12 (ESR);
carbon steel type API G705 (normal and ESR).
The following steels were examined for reinjection tubing:
carbon steel type C90;
13% chromium martensitic stainless steel;
22% chromium austenitic ferritic stainless steel;
28 chromium 32 nickel 3.5 molybdenum superaustenitic stainless steel.
These samples were tested for mechanical properties and corrosion resistance in the laboratory and samples of reinjection tubing were placed in a geothermal injection well.

For drilling pipes, it was found that the ESR process offers significant advantages over the basic metallurgical and mechanical properties. These advantages permit significant improvements in the resistance to the initiation of localized corrosion phenomenathat precede the propagation due to corrosion fatigue, in connection with or parallel to preexisting mechanical damage.

The overall corrrosion fatigue behaviour of the welded joint is satifactory. Inhibition with line offsets the disadvantage due to corrosion damage, bringing the crack propagation kinetics to what is observable in fatigue in air.

For reinjection tubing, the perfect stability of the duplex and superaustenitic stainless steels in the test environments was evidenced, while the carbon steel and martensitic stainless steel proved to be strongly susceptible to stress corrosion phenomena.
DEVELOPMENT OF MATERIALS FOR USE IN GEOTHERMAL EXPLOITATION, IN PARTICULAR FOR INJECTION WELLS. SEVERAL STEELS WILL BE TESTED FOR CASING MATERIAL :
- LOW CARBON STEELS TYPE AISI 4137 ESR AND UNI 30 NICRMO12 ESR AS WELL AS
- CARBON STEEL API G105 SR,

TEMPERATURE UP TO 200C WILL BE USED AND THE INFLUENCE OF H2S AND CO2 BE FOLLOWED.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Ente Nazionale per l'Energia Elettrica SpA (ENEL)
Address
Via Andrea Pisano 120
56100 Pisa
Italy