The Co <sub> In The Oil And Gas Development Process 2 </ Sub> Corrosion And Protection Technology

The corrosion of steel in petroleum has been threaten the petroleum natural gas industrial safety and the stability produce. In the last several decades scholars at home and abroad carried on a great deal of research of the CO₂ corrosion mechanism, breaking forms and the protection technology , but there still are a lot of problems did not be resolved. On the base of this work, the existence form of CO₂, the interaction of CO₂ and oil and produced water were studied in the tube pressure funnel of oil and gas store and well. Applying the super and critical fluid theories, it was pointed out in deep oil store and the oil well pressure funnels that CO₂ exists in the super and critical fluid when the pressure is higher than CO₂ critical pressure and the temperature is higher than critical temperature of 31.6 ℃. And it become a kind of stability colloid with the interaction of crude oil, natural gas and produced water. In oil well pressure funnels the pressure and temperature was lower with the distance reduce between oil and ground. Therefore, in oil and gas rising process, CO₂ is being in the condition of its pressure, capacity, density and solubility in produced water changing. When the pressure is lower than 7.39 MPa in the well tube, the super and critical fluid is changed into compressed air, which makes the capacity expand rapidly and solubility in water rapidly reduce. Then it enters the carriage pipe on ground to gather to carry. The theory and practice of the research proved that CO₂ corrosion research has much limitation which was under the leading of CO₂ partial pressure adopted for long time at home and abroad. It can to be applicable to the crude oil and natural gas transportation and the low pressure part(< 739MPa) of the mouth of oil well. (certainly at this time CO₂ was compressed more and greatly, but we still like to approximately think the compressed air unwillingly can make use of P_CO2 theories) But it can't been used in oil and gas pipe with several thousand meters deep where CO₂ is in the super and critical fluid state, pressure and temperature are changing. So this text think, it is applicable in theory that CO₂ mole percent under the total pressure function of the different position of the well tube substitutes CO₂ partial pressure. And it can evaluate the CO₂ corrosion and corrosive behavior in the high pressure and temperature of oil well tube with several thousand meters. Under this kind of theory, the CO₂ corrosion electrochemical process was analyzed adopting corrosion thermodynamics, the dynamics theories. Aimed at the development of some oil well ofdomestic, corrosive velocity was measured and corrosive produce was analyzed in different well depth and temperature of J55, N80 and PI 10 by using of SEM, EDS, XRD, XPS, EIS. And it was contrasted with the corrosion in C02 partial pressure. The evaluation of protection technique in common use was carried on. And the Ni- Fe- P compound layer was developed which has excellent function of anti- C02 corrosion.This research divides five parts.The first part is researching to manufacture the high temperature and pressure kettle (40MPa 250°C) which is applicable of C02 high temperature and pressure and Ag/ AgCl reference electrode in dynamic electro-chemistry test.The second part studies for the theories. Adopting the super and critical fluid theories and corrosive electrochemistry theories, the theory of Pco2 and C02 mole percent in lately total pressure was calculated with corrosive electro-chemistry thermodynamic and dynamic. The corrosive electric potential of catelectrode in saturated aqueous solution with 35MPa, 120°C is lower 17.2mv than Pco2 =1.5MPa, 120°C. And the apply the calculation theories about hybrid water pH value which dissolved a great deal of inorganic salt, The pH value of saturated C02 aqua in the simulating produced water on different temperature and pressure. Which provides the theories for judging the corrosive trend in oil and gas produced water of oil steel.The third part is about: The corrosion velocity of X56 steel in oil transportation, the analysis of microstructure of corrosive film, the polarization curve and Tafel slope coefficient of kathode and positive electrode of X56 steel under the conditions of special temperature and pressure, electrochemical impedance spectroscopy, were tested used modern physics and the chemistry means ( SEM, EDS, XRD, XPS, EIS).The result certificate:(1) In the flowing state, the C02 corrosion is local corrosion of X56 steel and the appearance takes on platform corrosion and cavity corrosion.Under the condition of Pco =0.5²Mpa^ 80° C , the biggest corrosion velocity can reach 19mm/a of X56 steel, belonging to serious corrosion in the standard of NACE.(2) the C02 corrosion produced film of X56 steel divides three layers. The appearance and structures of each layer are different, and each layer is all not quite compressed. The outer layer is FeCO3 and (Fe^Ca... )COi .The middle layer and inner layer are FeCOi.In the fihns there exist material channel because of films non-compressed and holes and corrosive mediator which arose serious local corrosion.(3)By investigating the corrosive velocity and electro-chemistry behavior of X56 steel in oil transportation, the C02 corrosive mechanism of X56 steel in the condition of 80°C n PCOi =0.5²Mpa. The cathode is subjected to the hydrogen precipitation control, the anode is subjected to the formation speed of Fe2+ control. C02 promoted anode respond and accelerated the anode to dissolve. With the C02 partial pressure and temperature increasing, the corrosion velocity of X56 steel increased.The fourth part is about: the corrosion velocity, structure of corrosive produced fihns and the film electro-chemistry test were analyzed of oil steels such as J55, N80 and PI 10 in oil and gas well pressure funnel with different pressure and temperature. Respectively carrying on the corrosion velocity measurement of the three oil steels in self-made high temperature and pressure kettle under the condition of 6MPa,60°C > 10MPa,80°C -. 18MPa,100°C, 24MPa, 120°C, 36MPa,160°C and 1.4m/s flow speed. The structure of the corrosion fihns of the three steels was analyzed by modern physics and chemistry means (SEM, EDS, XRD, XPS). The influence of the pressure, the temperature to the structure of C02 corrosion produced fihns of oil steel was studied. And the corrosive electro-chemistry test of the films was taken, including polarization curve and electrochemical impedance spectroscopy (EIS). The result certificates:(1) C02 the corrosion velocity of three kinds of oils steel is with the PI 10 steels are most seriously, N80 take second place, the J55 is lowest.(2)The most serious C02 corrosion district is the segment of pipe used in oil well that the temperature is placed in 80 100 °C, at this time C02 corrosion take on the serious local corrosion on the even corrosion base. The corrosion produced films divide three layers, mainly being constituted by the FeC03 crystal. The film layer has characteristic of loose film, many holes layover the body incompletely, the base metals existing much corrosive pitting.(3 ) CO2 corrosion produced film was formatted under the condition that the pressure is greater than 18 MPa, temperature is higher the 120°C. Its structure is divided into two layers, constituting of FdCaiOs and CaCC>3. The film is much compressed and lay over thebase completely and protect the base metals well. The corrosion produced film especially formed in 36MPa> 160°C can cut off the corrosive reaction, which was confirmed by electro-chemistry polarization curve and AC impedance spectrum. From here this article puts forward the technique conceive that oil pipe steel is protected by corrosive film formed spontaneously on the oil pipe steel in the bottom of oil well.The fifth part is about the evaluation and research of C02 corrosion and protection technology.(1) The efficiency of the five kinds of inhibiters extensively applied at home and abroad currently was appraised adopting the study way of C02 mole percent under total pressure. The result proved that under the condition of 80 °C and 1.5 MPa five kinds of inhibiters showed fine efficiency (above 85%). Under the condition of the high pressure and temperature, the inhibited efficiency of all inhibiters is lower 60%. And we carried on the academic study to the forming reason of this kind of phenomenon.(2) Studied to develop the Ni-Fe-P compound plating technique, this kind of films have the non-crystal organization, fine tightness and fine bonding with the body metals. Within the scope of 0.5³6 MPas,30¹60 °C , Of this coating layer C02 corrosive test and electro-chemistry test were carried on to prove that this kind of layer has good function for resisting to C02 corrosion.