Design And Performance Research Of Low-density Cement Slurry With High Solid Volume Fraction In Deepwater

Under the situation that China’s dependence on foreign oil and gas demand continues to rise rapidly and the situation in Ukraine is leading to a sharp rise in international oil prices,the development of deepwater oil and gas resources is of great significance for China.It is an essential part of ensuring national energy security,safeguarding national marine rights and interests,practicing the strategy of becoming a maritime power,and realizing the great rejuvenation of the Chinese nation.With the intensification of exploration and development of deepwater oil and gas reservoirs,the geological conditions faced by cementing operations are becoming more and more complex.Cementing in deepwater geological areas with low formation fracture pressure gradients(e.g.,shallow formations,depleted oil reservoirs,highly permeable formations)can easily lead to formation fracture,causing serious leakage and gas channeling.In this event,low-density cement slurries with high solid volume fraction,stable performance and rapid strength development are commonly used,aiming to balance the formation pressure of the target formations.However,there are few reports involving cement slurries with high solid phase content and low density in deep water in China.Considering the broad prospects of deepwater oil and gas development and the particularity of deepwater low-density cementing,this paper conducts a systematic study on deepwater cement slurries with high solid volume fraction and low density.The effects of water-cement ratio,dispersant agent(F45 and F1010),defoamer agent(X60),anti-filtrating agent(G86),accelerator agent(A95),hollow glass microspheres(HGMs),and temperature on the rheological properties,compressive strength and porosity were investigated,aiming at developing low-density cement slurry with high solid volume fractions,stable performance,and rapid early strength development.Moreover,the effects of four different suspension stabilizers with anti-channeling and strengthening properties,namely silica fume(SF),nano silica fume(NSF),nano silica(NS)and colloidal nano silica(CNS),on hollow glass microspheres low-density cement slurry were investigated and compared.Combined with the test results of micro-computed tomography,X-ray diffraction analysis,and scanning electron microscopy,the action mechanism and strengthening mechanism of different suspension stabilizers were revealed microscopically.Based on the above experimental test and analysis,a set of low-density cement slurries with a high solid volume fraction suitable for cementing in deepwater low-fracture pressure gradient formations was formed.Meanwhile,the quaternary numerical relationship of water-cement ratio,the density of cement slurry,the SF content,and the hollow glass microsphere content is constructed based on a large number of calculations.Relationships between compressive strength and porosity and relationships between compressive strength and rheological parameters of cement slurry are conducted through linear regression analysis.In addition,considering the mineral composition,particle size distribution,water-cement ratio of cement slurry,and the curing temperature,an oil well cement hydration model and a composite Portland cement hydration model were established based on the cement hydration software HYMOSTRUC3D.Through the above research work,the following conclusions and understandings are obtained:(1)Increasing the water-cement ratio and adding dispersants can effectively improve the rheological properties and enhance the pumping performance of cement slurry.The ability of the above agents to improve the rheological properties of cement slurry is as follows:F1010>F45>A95>0>X60>G86.(2)The addition of F45,F1010,G86,X60,A95,HGMs into cement slurry leads to the increase of porosity and decrease of compressive strength of cement stone.In terms of the degree of negative influence on 1-day compressive strength:F1010>F45>X60>G86>HGMs>0>A95(promoting effect);in terms of the degree of negative influence on 7-day compressive strength:F1010>F45,X60>A95>G86>HGMs>0;in terms of the negative influence on 28-day compressive strength:F1010,F45,X60>HGMs>A95>G86>0.(3)The incorporation of suspension stabilizers(e.g.,SF,NSF,NS,CNS)with anti-channeling and enhancement properties will deteriorate the rheological properties of cement slurry,resulting in the reduction of the flow behavior index and the increase of the consistency coefficient of cement slurry.In terms of the degree of negative influence on the rheological properties of cement slurry:NS>CNS>NSF>SF.(4)The mechanical strength of cement slurry develops slowly under low-temperature conditions.Adding SF,NSF,NS,and CNS to cement slurry can effectively exert the suspension stabilization effect,physical filling effect,nucleation site effect,and volcanic ash effect to promote early strength development and improve the final strength while improving the stability of the slurry.In terms of improving the1-day compressive strength:NS>CNS>NSF>SF;in terms of improving the 7-day compressive strength of cement stone:CNS>NS>NSF>SF;in terms of improving the 28-day compressive strength of cement stone:CNS,SF,NSF>NS.(5)Through a large number of experimental studies,a low-density cement slurry system with high SVF suitable for cementing in deepwater low-fracture pressure gradient formation cementing is optimized and determined:100%G-grade oil well cement+17%HGMs+25%SF or 20%NSF or(2%～3%)NS or(15%～20%)CNS+1%F45+1%X60+3%G86+water,the density of cement slurry is adjustable around1.48 g/cm~3.The system has a high solid volume fraction(45%～50%),good rheological properties and stability properties,fast early mechanical strength development(8.39MPa/24 h),and final compressive strength can reach 49.53 MPa/28 days.In the later stage,the compounding and synergistic effect of NS,NSF,NS,and CNS can be considered to improve the rheological properties of the cement slurry while promoting the development of early mechanical strength and improving the final compressive strength.It is recommended to choose one of NS and CNS to be compounded with SF or NSF.(6)The numerical relationship of water-cement ratio,HGMs,SF,and cement slurry density in the ternary solid-phase cement slurry systems was found and constructed.The established derivation and calculation method is applicable to similar binary and ternary solid-phase cement slurry systems.It has a particular theoretical reference value for clarifying the intrinsic connection between the components of quaternary solid-phase or even more multi-solid-phase cement slurry systems.(7)Through regression analysis,it is found that the compressive strength of cement slurry has a strong negative linear correlation with the flow behavior index and porosity of cement slurry.The greater the flow behavior index of cement slurry,the greater the porosity of cement stone,and the lower the compressive strength of cement stone.When designing deepwater low-density cement slurry,on the premise that the basic rheological performance and pumping performance of cement slurry meet the requirements,the water-cement ratio should be reduced as much as possible.Meanwhile,the amount of dispersant agent,defoamer agent,and anti-filtrating agent should be controlled to reduce the flow behavior index of cement slurry,avoid the generation of macropores and achieve rapid strength development.(8)Based on the HYMOSTRUC3D hydration software,the oil well cement hydration model and the composite Portland cement hydration model were established.The evolution laws of compressive strength,porosity,pore size distribution,hydration heat,and CH content of cement slurry were obtained with the hydration model.The hydration process and microstructure evolution of cement slurry can be reproduced.In addition,the performance of the cement slurry can be preliminarily predicted,and the design of the cement slurry can be aided.