The Mechanisms Of Formation Damage Removal By Using Ultrasonic-assisted Chemical Stimulation Technique

During the oil field development,the nearby wellbore is usually blocked due to the interaction of injected fluid with the subterranean formation.As a result,the fluid percolation will decrease which will lead to the reduction in oil productivity and escalation in well injection pressure.Although there are various kinds of formation damage,but the most common types are;clay swelling and fine migration upon exposure to injected water,inorganic scaling formation and calcium carbonate plugs,and polymer plugging.Currently,chemical deplugging and physical plugs removal techniques are the main methods to ameliorate the damaged permeability nearby the wellbore region.The latter is characterized by efficient,simple and convenient,and environmentally secure method.Due to these distinguished characteristics,it widely attracts the researchers in petroleum industry.This dissertation is emphasized on the treatment of three most common causes of formation damage,such as water sensitivity,inorganic scale,and PHPAM plug.The influence of elementary ultrasonic parameters,for instance,frequency,power,and irradiation time on aforementioned formation damages is systematically studied.In addition,the synergetic mechanism of ultrasonic waves and chemical stimulation technique is further investigated.Interesting results are listed below:（1）Refer to the water sensitivity,the optimum ultrasonic parameters are found to be 20 KHz,1000 W,and 100 mins.Maximum 25.3%and 34.2%damaged permeability recovery was witnessed by applying optimum ultrasonic parameters and chemical agent（mud acid）respectively.It was further increased to 45.8%by the simultaneous application of chemical agent and optimum ultrasonic waves.However,further irradiation adversely affected the damaged permeability recovery.The mechanism of permeability recovery is attributed to the shocking waves and thermal energy.The ultrasonic waves produced thermal energy during wave propagation in porous media.Besides,heat energy would also be generated due to cavitation and boundary friction.The boundary friction is proportional to the ultrasonic frequency.Moreover,higher the ultrasonic power,more would be the heat generation.Beyond optimum irradiation time,the detachment of comparatively big particles could cause plugging at later stage.This is main reason behind the first increase and then decline in permeability recovery.（2）Refer to the CaCO₃ deplugging from the core sample,the most suitable ultrasonic parameters are measured to be 20 KHz,1000 W,and 100 mins.The damaged permeability recoveries of HCl and ultrasonic waves were 44.5%and 37.6%respectively,but the permeability recovery was escalated to 61.5%when HCl and ultrasonic techniques were applied together.The identical pattern was followed beyond optimum irradiation time which is attributed to the particle-bridge formation at the later stage.（3）Refer to the PHPAM gel degelation and deplugging,three ultrasonic transducers with three different frequencies,18 KHz,20 KHz,and 25 KHz and same power（1000 W）were used to degrade the PHPAM gel sample.Results showed that transducer with frequency 20 KHz worked well by reducing the gel’s viscosity from 2495 mPa.s to 1.4 mPa.s under 10 mins exposure.As far as the chemical treatment is concerned,H₂O₂ among other chemicals remained prominent where it effectively reduced the original viscosity（2495 mPa.s）to 2.6 mPa.s after 24hours of reaction.Similarly,NaOCl and ClO₂ were applied to bring down the PHPAM gel sample’s original viscosity to 6.5 mPa.s and 159 mPa.s respectively.The Scanning Electron Microscopy（SEM）of original and treated samples was conducted which further provided evidence of degraded viscosity through significant changes in micromorphology of PHPAM gel.Compared to chemical treatment（24 hours）,high efficiency of ultrasonic waves（10 mins）is attributed to the three ways of degradation,such as thermal energy,shocking waves,and hydroxyl radical（HO?）.While,only hydroxyl radical（HO?）is responsible for degelation of PHPAM gel in the case of chemical treatment（H₂O₂）.In addition,the maximum proportions of PHPAM gel deplugging from core sample(30×10^-3μm² gas permeability)were 23.5%and19.80%by using ultrasonic waves（20 KHz,1000 W,and 100 mins irradiation）and chemical agent（H₂O₂）respectively.The permeability recovery was further increased to 40.90%by the simultaneous application of ultrasonic waves and chemical agent.（4）In a nutshell,ultrasonic-assisted chemical recovery technique has the different approaches towards the different kinds of permeability damages.Specifically,the combined effect of ultrasonic waves and chemical agent（compared with chemical treatment）was the most effective on PHPAM gel deplugging（average damaged permeability recovery is over 200%）which is followed by water sensitivity damage（average damaged permeability recovery is over143%）and inorganic scale deposition（average damaged permeability recovery is over 131%）.