Effect Of Nano-materials On Properties Of PVA High Temperature Self-degrading Cement

During the development of high temperature geothermal wells,a large number of fractured formations are often encountered,or artificial fractured formations are used to increase production.In order to prevent drilling fluid leakage during drilling,it is necessary to plug the fractures.After drilling,it is necessary to unblock the fractures and increase the contact area of geothermal fluids to achieve the purpose of increasing production.This requires that the temporary plugging material used can maintain a certain strength in the drilling process and crack again at the production stage after drilling.In this paper,based on alkali slag/fly ash cement,a new polymer-polyvinyl alcohol?PVA?was added,and the self-degradation effect of cement was promoted by its swelling property and phase transformation from gel state to sol state at high temperature.However,the setting time of the composite polymer cement is too long?Initial setting time is 280min?and the compressive strength is too small under the curing condition of 85??4.27MPa?.Therefore,Nano-materials?Nano-bentonite,Nano-silica,Nano-cellulose?are added to the composite cement,and the physical filling effect,large specific surface area and high surface activation energy of Nano-materials are utilized to improve the polyvinyl alcohol?PVA?composite cement,and Nano-materials are explored the influence of material on other properties of PVA composite cement.The experimental analysis shows that Nano-materials can reduce the density of PVA composite cement,increase the apparent viscosity of cement,reduce the setting time of cement,reduce the filtrate loss at medium pressure and keep the pH value of filtrate basically unchanged.Nano-silica and Nano-cellulose have better performance in improving the compressive strength of cement cured at 85?.When the dosage is3%,the compressive strength of cement cured at 85?can be increased by 70.5%and77.5%,respectively.Because of super absorbency,Nano-bentonite can't be added into cement too much,so the strength improvement is not obvious.When the content of Nano-silica is 3%,the strength of Nano-silica decreases by 56.2%compared with the same period?1.27MPa?,while the strength of Nano-cellulose can't effectively reduce the original strength.The hydration products and mechanism of cement were studied by XRD,FTIR and polarizing microscope.It was found that the hydration products were C-S-H gel,a small amount of C-A-S-H gel and a small amount of C-N-A-S-H gel.The addition of PVA and Nano-materials will not change the hydration products of cement,but Nano-materials can increase the density of hydration products,and Nano-silica can also promote the formation of hydration products.The experimental results show that the main reasons for the self-degradation of the composite cement are the swelling of PVA particles at 85?,the phase transition of PVA from gel state to sol state at high temperature,and the decomposition of CO₂ gas to form gas chambers and airways.