| With the rapid development of the national economy,China is facing the problem of the high dependence on imported oil and gas resources.However,the unconventional hydrocarbon resources in China are abundant,and the exploitation of unconventional hydrocarbon resources can effectively alleviate the shortage of oil and gas resources.Ball-activated multistage fracturing technology is one of the most efficient methods for the exploitation of unconventional hydrocarbon reservoirs,and the application of dissolvable magnesium alloy ball seats can significantly improve the production efficiency of unconventional oil and gas.However,in the extremely harsh downhole environments,the dissolvable magnesium alloy ball seats are not only corroded by groundwaters or fracturing fluids,but also eroded by sand-bearing fracturing fluids,leading to the abnormal operation of ball-activated multistage fracturing system.In order to improve the corrosion resistance and erosion wear resistance of dissolvable magnesium alloy ball seats,the Fe-based amorphous coatings had been prepared on dissolvable magnesium alloy substrates via high velocity oxygen-fuel(HVOF)spraying technology.In this work,the effects of spraying parameters,such as oxygen flow and kerosene flow,on the microstructure and properties of Fe-based amorphous coatings were studied,the immiscible interfaces of Fe-based amorphous coating/dissolvable magnesium alloy were improved by introducing Ni60 interlayers,and the bonding mechanisms for these interfaces were discussed.The results show that Fe-based amorphous coatings are mainly composed of amorphous splats and amorphous oxides,but there are a few crystallized splats,pores and unmelted particles in the coatings.Oxygen flow and kerosene flow affect the properties of Fe-based amorphous coatings through porosity and amorphous content.The hardness,bonding strength,corrosion resistance and wear resistance of Fe-based amorphous coatings decrease with the increase of porosity,and the corrosion resistance decreases with the decrease of amorphous content.When the oxygen flow is 53.8 m3/h and the kerosene flow is 26.5 L/h,the Fe-based amorphous coatings possess the best performance.By introducing the Ni60 interlayers with the thickness of 150 μm,the compactness,amorphous content,corrosion and wear resistance of Fe-based amorphous coatings are reduced slightly,but the bonding strength of the Fe-based amorphous coatings are improved significantly(56.9 MPa).In the process of HVOF spraying,the heat released by molten droplets and generated by droplet impact can partially melt the substrates,and tiny molten pools can be formed at the droplet/substrate interfaces.Atomic diffusion can occur in these molten pools,resulting in the localized metallurgical bonding at the droplet/substrate interfaces.Therefore,the improvement of bonding strength is mainly due to the replacement of the Fe-based amorphous coating/magnesium alloy interfaces by the Ni60 interlayer/magnesium alloy interfaces and the Fe-based amorphous coating/Ni60 interlayer interfaces.Based on the optimal preparation processes,the effects of KCl concentration,pH value,temperature and hydrostatic pressure on the corrosion behaviors of Febased amorphous coatings were investigated,and the corrosion mechanisms of Febased amorphous coatings were also discussed.It is found that due to the decrease of the compactness of passive films,the corrosion resistance and pitting resistance of Fe-based amorphous coatings decrease with the increase of chloride ion concentration,hydrogen ion concentration,hydroxide ion concentration,temperature and pressure.Under high temperature and high pressure,due to the uneven passive films and potential differences,the crystallized splats and intersplat regions in the Fe-based amorphous coatings are preferentially corroded,and the corroded intersplat regions serves as a bridge between pores and corroded crystallized splats,leading to the formation of corrosion channels and the failure of Fe-based amorphous coatings.In order to further improve the corrosion resistance of Fe-based amorphous coatings,epoxy resin was used to coat the coatings.Organic coatings can effectively prevent the corrosion solution from penetrating into Febased amorphous coatings,significantly prolonging their service lives.When the solution temperature is about 90℃,the service lives of the coated Fe-based amorphous coatings can reach 300 h,longer than those of the uncoated Fe-based amorphous coatings(203 h).In this study,the effects of impact angle,flow velocity and sand concentration on the erosion resistance of the Fe-based amorphous coatings on dissolvable magnesium alloy substrates were studied,and the erosion wear mechanisms of Febased amorphous coatings were also discussed.The results show that when the impact angle is less than 60°,the mass loss rates of Fe-based amorphous coatings increase with the increase of impact angle.However,as the impact angle is greater than 60°,the mass loss rates decrease with the increase of impact angle.At a low impact angle(<45°),the erosion wear mechanisms of Fe-based amorphous coatings are mainly cutting and delamination.When the impact angle is between 45° and 60°,the erosion wear mechanisms are mainly splat fracture and delamination.At a high impact angle(>60°),the erosion wear mechanisms are mainly plastic deformation and delamination.The mass loss rates of Fe-based amorphous coatings increase with the increase of flow velocity and sand concentration,but as the sand concentration exceeds 15 wt.%,the mass loss rates decrease with the increase of sand concentration because of the "shielding effect".At a low impact angle,the erosion wear mechanism of dissolvable magnesium alloys is mainly cutting,and the erosion resistance of Fe-based amorphous coatings is about 4.16 times higher than that of dissolvable magnesium alloy substrates.In the present work,the galvanic corrosion between coatings and dissolvable magnesium alloy substrates at various KCl concentration and temperature were studied,the degradation processes of dissolvable magnesium alloy ball seats were numerically simulated,and the degradation time of dissolvable magnesium alloy ball seats was evaluated.During the fracturing operation,the frac balls extrude the conical surfaces of ball seats under the action of high-pressure fracturing fluids,leading to the deformation of conical surfaces and the fracture of Fe-based amorphous coatings.After coating failure,galvanic corrosion occurs among Febased amorphous coatings,the Ni60 interlayers and the dissolvable magnesium alloy substrates.In addition,the dissolvable magnesium alloy substrates are used as the anode,and the Fe-based amorphous coatings and the Ni60 interlayers are used as the cathode.Corrosion takes place at the places where the coatings fail,and gradually extends into the interior of ball seats.With the increase of KCl concentration and temperature,the degradation rates of dissolvable magnesium alloy ball seats increase,and the degradation time of dissolvable magnesium alloy ball seats decreases.The evaluation of degradation time can provide a guidance for hydraulic fracturing operations. |
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