Effect Of Nano-MgO/CNTs On The Properties Of Oil-well Cement Cured At 80?

The exploitation of shale gas is a major goal of the 13^th Five-Year Energy State Plan.As a main methodology of shale gas exploitation,the liquid pressure in fracturing method can reach as high as several tens of megapascal,it puts forward higher requirements on the performance of cement sheath(the cement between cast and rock)in cementing engineering.Moreover,the micro-annulus induced by autogeneous-shrinkage of oil well cement is the essential cause of the loss of sealing capacity and integrity of cement sheath.The addition of nanomaterials was proved to significantly improve the mechanical,physical and shrinkage properties of cement-based materials.However,scarce studies on the influence of nanomaterials on the mechanical,physical and shrinkage properties of cement paste cured at high temperature.In this dissertation,three types of nanomaterials,i.e.,single nano-Mg O,single CNTs and nano-Mg O+CNTs are incorporated into the oil well cements,which are cured at 80?and at 1-day,3-day and 7-day curing ages,according to rheological tests,mechanical tests and autogenous-shrinkage tests,the influence of the nanomaterials on the mechanical,physical and shrinkage properties of oil well cement paste is investigated in this study,the optimal mixing ratios of nanomaterials were also determined in this work.Multiple micro-techniques,i.e.,Mercury intrusion porosimetry(MIP),X-Ray diffraction-Rietveld(XRD-Rietveld),Thermo gravimetry-differential thermal analysis(TG-DTA)and Scanning electronic microscopy(SEM),are employed to investigate the micro-mechanism of the nanomaterials on the oil well cement paste and the impact of nanomaterials on the hydration behaviors of oil well cement paste.Several conclusions can be drawn in this thesis:(1)The mechanical and shrinkage properties test demonstrated that,the addition of nano-Mg O improves both the mechanical and shrinkage properties of oil well cement paste at 3-day curing age,however,the mechanical properties of 7-day curing oil well cement paste regress.The SEM results of 7-day curing oil well cement paste explain the regression of the strength of oil well cement paste:the micro-cracks appear in the cement matrix around the nano-Mg O particles,this is because of the reaction of the nano-Mg O with liquid water and the formation of expansive Mg(OH)₂,which induces the formation of micro-cracks and is responsible for the strength regression of 7-day curing oil well cement paste.The rheological tests reveal that the addition of nano-Mg O greatly decreases the rheological properties of oil well cement paste slurry,to recover the rheological of nano-Mg O mixed oil well cement paste slurry,proper content of dispersing agent should be added in the oil well cement paste slurry.(2)Experimental results show that 0.6‰of CNTs is the optimal content.The compressive strength and flexural strength test results show that the addition of 0.6‰CNTs can essentially enhance the mechanical properties of oil well cement paste;the autogenous shrinkage test results reveal that the addition of CNTs can greatly improve the shrinkage properties of oil well cement paste,compared with referenced oil well cement paste,the addition of 0.6‰CNTs can decrease about 70.2%of autogenous shrinkage.Both TG-DTA and XRD-Rietveld results demonstrate that the addition of CNTs accelerate the hydration of oil well cement,facilitate the formation of hydration reactants as well as the formation of early strength of oil well cement paste.SEM results show that CNTs play a role of bridge to inhibit the formation and propagation of micro-cracks.Compared with referenced oil well cement paste slurry,the addition of CNTs significantly improve the rheological properties of oil well cement paste slurry.(3)For the case of nano-Mg O+CNTs composites,experimental results show that 0.6‰CNTs+2%800?calcined nano-Mg O is the optimal content(C2M1),such optimal content can greatly improve the shrinkage of oil well cement paste and the autogenous shrinkage remains to be approximately 0 after 7-day curing age.Moreover,both compressive strength and flexural strength are not regressed and it show that addition of 0.6‰CNTs+2%800?calcined nano-Mg O in oil well cement paste exhibits perfect mechanical performance.The experimental results show that addition of 0.6‰CNTs+2%800?calcined nano-Mg O in oil well cement paste not only improves the shrinkage properties of oil well cement paste but also ensures no regression of its mechanical strength,which demonstrate its promising potential use in oil well cementing.