Study On The Mechanical Properties And Modified Mechanism Of Oil Well Cement-Based Composites Toughened By Asphalt

The quality of cementing directly affects the safe production life of oil and gas wells and production efficiency of oil and gas in oil and gas exploretation.The quality of cementing is closely related to the mechanical properties of cement sheath.The cement sheath is typically brittle.Under the action of downhole working condition and high-temperature and high-pressure environment at the bottom of the well,its mechanical integrity is easily destroyed,and the ability of interlayer sealing is lost,resulting in annulus channeling,which will cause huge economic loss and endanger life and safety.Therefore,how to improve the toughness of cement paste is the key to improve the cementing quality of oil and gas wells.Existing researches indicate that cement-based materials toughened by asphalt is feasible.However,the softening point of the emulsified asphalt for toughening in the concrete industry is low.If it is directly used for the toughening of oil well cement paste in cementing engineering,there are two drawbacks.The first is that the wellbore environment is complicated,and it is difficult to control the demulsification of emulsified asphalt.The second is that under the action of high temperature and high pressure in deep wells and ultra-deep wells,the asphalt may soften and become fluid,and the toughening effect may lose.In this study,sulfonated asphalt and natural rock asphalt with high softening point were used as modified toughening admixture for oil well cement.The composition,structure,particle size,temperature stability and acid and alkali resistance of the two types of asphalt were first studied.Low temperature plasma technology was used to improve the hydrophilicity of the rock asphalt surface.The modification results were characterized and the modification mechanism was explored.From the perspective of material composition,1?4%sulfonated asphalt and rock asphalt were added into well cement slurry.The effect of two types of asphalt on the properties of cement slurry was studied.The compressive strength and tensile strength of pure cement paste,sulfonated asphalt cement matrix composites,rock asphalt cement matrix composites,uniaxial compressive stress-strain curve of cement paste,stress-strain curve under triaxial static load and stress-strain curve under cyclic load were tested and analyzed.Scanning electron microscopy(SEM),X-ray diffraction analysis(XRD),Nitrogen adsorption(BET),Mercury intrusion(MIP),Thermogravimetric analysis(TG)and solid nuclear magnetic methods(NMR)and other analytical methods were used to study the microstructure of asphalt cement matrix composites,and the relationship between microstructure and mechanical properties was investigated.The method of calculation analysis and SEM microscopic observation of crack propagation were used to explore the toughened mechanism of oil well cement paste by asphalt.The results show that the asphalt has good alkali resistance and slight corrosion in high concentration H2S solution.Poar C-OH bond,C=O bond and C-O bond appeared on the surface of rock asphalt treated by low temperature plasma,which reduces the contact angle between the surface of rock asphalt and water,and improves the hydrophilicity of the surface of rock asphalt.Both sulfonated asphalt and rock asphalt resulted in a slight decrease of density of cement slurry.With the increase of the content of sulfonated asphalt,the fluidity of cement slurry first increases and then decreases sharply.Rock asphalt slightly increases the fluidity of cement slurry.The addition of sulfonated asphalt reduces the free fluid of cement slurry,while the addition of rock asphalt can increase the free fluid of cement slurry.The addition of sulfonated asphalt is helpful to improve the rheological properties of cement slurry.The incorporation of rock asphalt is detrimental to the rheological properties of cement slurry.Both sulfonated asphalt and rock asphalt reduce the compressive strength of cement paste,but the reduction caused by sulfonated asphalt is even greater.Sulfonated asphalt and rock asphalt have the effect of increasing tensile strength.When the sulfonated asphalt content is 3%,the tensile strength at 7 and 28 curing ages at 90? increased by 45.5%and 17.3%,respectively.When the content of rock asphalt is 3%,the tensile strength at 7-days increases by 22.6%,and the strength at 28-days increases by 21.0%.From the results of the uniaxial and triaxial stress-strain curves of the samples,the sulfonated asphalt content of 1%,2%and 3%is beneficial to improve the toughness of the oil well cement paste.When the rock asphalt content is 2%and 3%,it is beneficial to improve the toughness of the oil well cement paste.The degree of toughness improvement of cement paste with sulfonated asphalt and rock asphalt is comparable under the same preparation conditions.X-ray diffraction analysis shows that the addition of sulfonated asphalt and rock asphalt inhibit the hydration of oil well cement to some extent,but there is no effect on the product category of oil well cement hydration.When curing temperature is 90?,hydration products mainly are in the form of CS2H3,which forms a continuous network structure in cement paste.The structure is good for the development of the strength of cement paste.When the temperature is 120?,CS2H3 is no longer be stable,and transformed into C2SH.This crystal with plate-like morphology reducs the strength of cement paste.SEM analysis shows that the addition of 3%sulfonated asphalt reduces the amount of calcium hydroxide hydration products of cement paste at the same curing age,which is related to the slurry retardation caused by sulfonated asphalt.Scanning electron microscopy analysis of asphalt cement matrix composites shows that the interface between untreated rock asphalt and cement paste is poor.However,the interface between treated rock asphalt and cement paste is relatively dense,and C-S-H gel appears on the interface asphalt.Pore structure analysis shows that less than 4%content of sulfonated asphalt has little effect on the number of harmful pores in cement paste,but the number of harmful pores increases when the content of sulfonated asphalt is 4%.The pore structure test results of rock asphalt cement matrix composites show that rock asphalt has no adverse effect on the number of harmful pores.The results of thermogravimetric analysis and hydration heat test show that both sulfonated asphalt and rock asphalt delay the cement hydration,but the sulfonated asphalt causes the retardation effect of the cement slurry to be more obvious.Infrared analysis shows that there is a tendency to hinder the formation of silicon tetrahedral and O-Si-O bonds because of the addition of asphalt.The results of nuclear magnetic analysis show that both rock asphalt and sulfonated asphalt reduce the degree of polymerization of silicon tetrahedron in C-S-H gel,and reduce the average chain length of C-S-H,and result in the decrease of the compressive strength of asphalt cement composites.The sulfonated asphalt has a greater influence on the degree of polymerization of the siloxane tetrahedron and the decrease of the C-S-H chain length than the rock asphalt.The correlation between the pore structure and the toughness of the asphalt oil cement matrix composites was calculated by the grey correlation analysis method.The order of correlation was:average pore size>cumulative pore volume>pore size distribution.The order of correlation of pore size distribution to the toughness of cement paste is:(<20nm)>(>50nm)>(20-50nm).The test results show that the sulfonated asphalt and rock asphalt content are 3%,which will reduce the average pore size and cumulative pore volume distribution of the cement paste,and also change the pore size distribution and reduce the number of harmful pores.This is one of the reasons why the toughness of 3%sulfonated asphalt cement composites and 3%rock asphalt cement composites can be improved.The experimental results agree well with the grey correlation calculation results.Two mechanisms have been proposed to explain the reasons of toughening oil well cement stone by asphalt.The first is interface enhancement mechanism.The microscopic test results and the asphalt-cement interface model show that two sublayers of physical adsorption layer and chemical bonding layer are formed on the interface between asphalt and cement paste,which forms a strong bonding interface and effectively prevents crack propagation.The second is crack deflection,bending and bridging toughening mechanism by SEM micro-morphology observation.