A Study Of Performance Optimization And Degradation Mechanism Of Self-Degradable Temporary Sealing Cement For High Temperature Geothermal Wells

In this paper,a study of self-degradable temporary sealing cement for high temperature geothermal wells was carried out.The cement will play an important role in plugging during the drilling process,and its self-degradation will provide a production channel for geothermal fluids after the drilling.The ideal material should not only obtain high strength at 85°C,but also self-degrade after contacting water at high temperature of 200?300°C.Degradable polymers:polylactic acid(PLA),starch,polyvinyl chloride(PVC)and polyvinylidene chloride(PVDC)were selected to improve the rheological properties of carboxymethyl cellulose modified alkali-activated cement(CAC),by introducing or replacement.The influence of polymers on the performances of cement and the self-degradation were experimentally studied.The self-degradation mechanism of the materials was investigated by theory and test,and the foam self-degradable temporary sealing cement was successfully developed.The main conclusions of the study are as follows:The rheological properties of cement can be considerably improved by adding PLA or using three other polymers to replace CMC.The temporary sealing cement for different temperatures was developed,according to the different thermal degradation temperature of polymers.The optimal dosages of polymers were determined by testing the reduction of compressive strength and the properties of cement.PLA and starch were used to develop temporary sealing cement for 200°C,2%PLA and 3%carboxymethyl starch were determined as the optimal dosages of PLA and starch respectively.PVC and PVDC were used to develop the cement for 300°C,and both of the optimal dosages of them are 20%.Considering the problem of more fractures,lower formation pressure and fracture pressure in geothermal reservoirs,the polymers and sodium dodecyl sulfate(K12)were compounded to prepare foam temporary sealing cement.The effective self-degradation standard of the materials was set,and the optimum cement formula of different temperature was determined by direct comparison,as followed:(1)200°C,2%PLA+5%PVC+3%CMS+1.5‰K12+71.88%slag+17.97%fly ash;(2)250°C,1%PLA+20%PVC+2.25%CMS+1.5‰K12+61.28%slag+15.32%fly ash;(3)300°C,1.5%PLA+20%PVC+1.5%CMS+0.75‰K12+61.54%slag+15.385%flyash.Compared with the alkali activated cement,the density of the three selected slurries are all lower than 1.20g/cm~3 at room temperature,which preliminarily achieved the purpose of controlling the density of cement slurry.The mechanism of the degradation of the cement with polymers was further analyzed and summarized:the thermal degradation of polymers leads to the increase of internal pores of the material,during the heating process at high temperature;the material is subjected to the effect of thermal shock,at the moment of contacting with water;the solid products dissolve in water generating in situ exothermic,at the early age of immersing in water;as the immersing time prolongs,the continuous dissolution of solid products in the cement causes the continuous reduction of the strength.Among the polymers,the thermal degradation of PVC and PVDC produces HCl to corrode cement,which destroys cement structure in essence.The key to the degradation mechanism of foam temporary sealing cement is:the high temperature causes the thermal degradation of polymers between independent foams;the polymers degrade with generating gas products,corroding and deteriorating the cement structure.The independent foams are connected in cement,and thus the cement structure is destructed.