| During the exploitation of heavy oil reservoirs by using the thermal recovery techniques of in-situ combustion(ISC),the cement sheaths needs being served in long-term elevated temperature and CO2 corrosion environment,which results in the failure of the isolation zone,casing damage and other problems.These have become one of the main factors affecting the production efficiency of heavy oil thermal recovery wells.Therefore,in order to improve the durability and integrity of the cement sheath,to ensure the quality of cementing and the life cycle of the wells,the paper studied the cementing material and their resistance to high temperature and corrosion.From the perspective of material science,this paper takes into account the working conditions of heavy oil thermal recovery of ISC.The mechanical properties,porosity and permeability,X-ray diffraction(XRD),scanning electron microscopy(SEM)were used to investigate the high temperature resistance and Failure evolution law of the oil well cement mortar.Particle size distribution,chemical composition,mineral compositionthe,and the development of strength of cement stone at high and low temperature were tested to investigate the performance of three types of aluminate cement.Cement slurry basic properties,thickening time,early strength and long-term strength of cement paste were tested.In this paper,the modification of the high alumina cement slurry system is studied,which is based on the optimization of the mixture and admixture.The mechanical properties,porosity and permeability,X-ray diffraction(XRD),scanning electron microscopy(SEM)were used to investigate explore the performance changes and high temperature resistance,corrosion mechanism of the modified CAC.The results show that the change of the phase and microstructure caused by the high temperature environment is the essential reason for the failure of the oil well cement.When the temperature is 300 0C,the hydration products C-S-H gel and Ca(OH)2 slightly dehydrated and decomposed.The strength of the cement paste decreases and the permeability increases,and the cement stone is at the critical point of the deterioration.When the temperature is 400℃,the cement mortar is slightly expanded,the hydration products dehydrated and decomposed.The deterioration of the cement stone becomes worse,which can not meet the requirement of the integrity of cementing.On the other hand,the modified calcium aluminate cement(CAC)slurry exhibits excellent basic performance,controllable and adjustable thickening time.The early strength of hardened cement paste is higher,the long-term strength is stable,and the permeability and porosity are lower.The high temperature resistance of CAC is better than that of the sand oil well cement.Under the low temperature environment,after the slag was added,the formation of six plate hydration products such as C2ASH8 made up for micro cracks and pores caused by the crystal transformation,regulated the temperature sensitivity of strength development.Under the high temperature of 500℃,the formation of C12A7 crystals caused by the recrystallization reactions contributes to the stability of the compressive strength of the modified CAC.The cement stone showed the strong elastic deformation ability.In addition,the modified CAC exhibits outstanding resistance to CO2 corrosion.Under the condition of low temperature and water soluble CO2 corrosion environment,the compressive strength of the modified CAC paste does not decrease but increases,only a small amount of corrosion products CaCO3 and CaAl2(CO3)2(OH)4·3H2O were found in the corrosion layer.However,the corrosive effect of the high temperature and gaseous CO2 environment on the modified cement paste is negligible,and the micro corrosion zone and the carbonization product are not found in the inner and outer layers of cement.The field tests of two wells were carried out using modified aluminate cement slurry system.Among them,the cementing quality of Du 813-37-48 well and Jin 45-13 is better,and it has good application prospect. |
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