Study On The Scaling Mechanism In The Produced Water Of CO₂ Displacement

Nowadays, oil displacement by gas injection is the main direction to increase the recovery ratio of the tertiary recovery in the oilfield. So far, about 3% of the world crude oil production was obtained by the gas injection to increase its recovery ratio. The CO₂ displacement will bring about severe corrosion and scaling problem in the metal equipments, and lead to huge economic losses. Therefore, the study on the CaCO3 scaling mechanism in the produced water of CO₂ displacement and its prediction model has important theoretical and practical meanings, which can provide theoretical and technical supports for the technology of CO₂ displacement.This paper mainly focus on three points of view (chemical thermodynamicsH, Hcrystallization kineticsH, hydrodynamics) to study the Ksp constant of CaCO3 and the scaling tendency, which are mainly influenced by the above factors. And by the technique of AFM, the scaling tendency of CaCO3 is also studied in the condition of hydrodynamics. The result shows that the content of the scaling ions, temperature, pressure, pH and salt effect largely affect the scaling process, what's more, the temperature and pressure are the main factors that influence the scaling process. Moreover, when the flow velocity is below 0.9m/s, the fluid state in the tube transits from liminal flow to turbulent flow, the growth rate of the crystal nucleus is similar. Since the erosion of the fluid, the probability of the nucleation over the material surface is reduced when the flow velocity is further increased. However, in the range of the experimental flow rate, 1.5m/s is in the transition state, the growth of the crystal and the erosion of the fluid come to a dynamic equilibrium. The compacted scale layer is prone to form at 75℃, 1.97m/s or the bottom of the well, the time needed to form a stable scale layer is about 900s. A predition model for CaCO3 scaling is also constructed to help the scale inhibiting work.In this article, Q235 steel and N80 steel are chosen to study. In the condition of simulated oilfield water and continuous air blow, the EIS of the Q235 steel after the scaling process under different time and temperature is measured. Associated with SEM and EDAX, the conclusion is that the scaling products increased with the increase of temperature of scaling time, however, the corrosion products also increased, which is the result of the porosity of the scaling products. What's more, at 50-60℃, it may be different, the scaling film is very compact and can protect the steel in some way.In the condition of the original oilfield water and continuous N2 blow, it can be concluded that the scaling films on the Q235 steel are all very compact and the corrosion is more serious with the increase of the temperature. At 50℃, the scaling film is the most compact and the corrosion is well distributed, the scaling film can protect the steel somehow.Under the condition of high temperature and pressure, the corrosion on the N80 steel is very serious, the porosity of the scaling film just offers large infiltration lanes to the corrosion process and speeds up the corrosion of the steel. In the condition of the original oilfield water and continuous N2 blow, the inhibitor HGS-9BS can inhibit the corrosion well on the N80 steel. At the same time, the inhibitor can also change the crystal formation of CaCO3, which can make the crystal dropped off the steel surface more easily and play a part in the removal of the scale.