Study On Corrosion Behavior And Law Of Salt-resistant Polymer Flooding Injection Pipes

The salt-resistant polymer flooding technology is a new polymer flooding technology to deal with the excessive mineralization in the produced water,reduce the polymer hydrolysis rate and maintain polymer viscosity in the formation.Salt-resistant polymer flooding technology can effectively improve oil recovery,but there is a certain risk of corrosion in the gathering pipeline due to the high mineralization of the formulated polymer produced water and the presence of formation microorganisms.This paper addresses the field data of salt-resistant polymer flooding gathering pipeline in Daqing Oilfield.Dynamic and static weight loss coupons,electrochemical tests,X-ray diffractometer and scanning electron microscope were used.The main findings are as follows:The corrosion process and corrosion mechanism of 20#carbon steel coating after damage in salt resistant polymer flooding system under sterile and SRB system are revealed.The corrosion process of 20#carbon steel with coating damage in sterile environment is essentially the autocatalytic process of oxygen concentration cell.The corrosion products are adsorbed on the surface of carbon steel to form a local concentration cell.The hydrolysis of Fe²⁺ions makes the local area alkalized,resulting in the alkaline stripping of the coating.In the later stage of the experiment,the coating bulge and coating protection failure.On the basis of autocatalytic reaction of SRB system,through anaerobic respiration of bacteria under corrosion products and biofilms,electrons are directly captured from Fe⁰for metabolism,resulting in pitting of carbon steel under biofilms.The generated Fe S has good electrical conductivity,reduces charge transfer resistance,and increases the corrosion rate of 20#carbon steel after coating damage.In the experimental temperature range of 25～45°C.The corrosion rate of 20#carbon steel in sterile and SRB system increases with the increase of temperature.The increase in temperature accelerates the movement of ions within the corrosive medium,reduces the conductive resistance of the solution,promotes cathodic charge exchange,and produces a great value of corrosion rate at the experimental temperature of 45°C.The increase in temperature accelerated the hydrolysis of salt-resistant polymers and reduced the protective properties of adsorption on 20#carbon steel,and also promoted the reproduction of SRB populations and accelerated the dissolution of the anode under the biofilm.In the speed range of 1～3r/min,with the increase of rotational speed,the corrosion rate of 20#carbon steel in sterile system increases,and the corrosion rate of 20#carbon steel in SRB system increases first,then decreases and then increases.The erosion effect of the system increases with the increase of rotational speed,the ability to peel corrosion products is strengthened,and the uncorroded carbon steel is continuously exposed to the corrosive environment,and the corrosion rate increases.The maximum corrosion rate of SRB system under 2r/min corrosion condition is due to the synergistic effect of SRB and erosion corrosion.With the increase of rotational speed,the adsorption capacity of SRB weakened,a large number of bacteria were stripped off the surface of carbon steel,resulting in a decrease in the corrosion rate of carbon steel.When the rotational speed was 3r/min,the erosion corrosion increased,a small amount of bacteria participated in the corrosion process,and the corrosion rate increased slightly.In the pressure range of 0～20 MPa,with the increase of pressure,the corrosion rate of20#carbon steel in sterile system increases,and the corrosion rate of 20#carbon steel in SRB system increases first and then decreases.With the increase of pressure,the activity of Cl^-increases,the concentration of corrosive ions per unit volume increases,and the pitting corrosion of carbon steel surface is promoted.The maximum corrosion rate is obtained at the experimental condition of 20MPa.The corrosion rate of SRB system gradually increased in the range of 0～15MPa,and the pressure reaches 20MPa,the bacterial reproduction is affected,and the corrosion rate of carbon steel decreases.The corrosion process and corrosion mechanism of 20#carbon steel under-deposit corrosion and its synergistic effect with SRB in salt-resistant polymer flooding system are revealed.At the beginning of the experiment,the scale layer has a certain protective effect on20#carbon steel.In the middle of the experiment,corrosive ions and SRB reached the surface of carbon steel through the porous scale layer,which increased the corrosion rate of carbon steel,and SRB promoted the under-deposit corrosion of carbon steel.In the later stage of the experiment,the corrosion products fall off and the under-deposited carbon steel is exposed,resulting in the potential difference between the bare steel and the under-deposited carbon steel,forming a corrosion galvanic.Under the driving force of the galvanic,the under-deposited carbon steel is used as the anode and the bare steel is used as the cathode.The two are coupled to corrode the under-deposited carbon steel,causing serious pitting corrosion.The corrosion rate of carbon steel in SRB system is higher than that in sterile system,which proves that SRB promotes the under-deposit corrosion of 20#carbon steel.