Gas Migration Mechanisms And Casing Pressure Evolution During Deep-water Well Cementing

Gas migration and casing pressure during well cementing are problems that almost every gas well will be faced with.The severity of the problem ranges from the most hazardous to the most marginal.However,in view of the special deep water environment,the mechanism of gas migration is more complex.Firstly,the cement slurry near the mudline is in low temperature,and the cement slurry located in deep part of wellbore is in relatively high temperature.The sensitivity of the hydration performance of cement slurry to temperature makes the hydration process more complicated.Secondly,hydrate layers are widely distributed in deep water areas of China.During the well cementing through hydrate layers,a large amount of hydration heat is released by cement slurry,which leads to the decomposition of hydrate,and then causes gas migration and casing pressure.This paper focuses on the mechanism of gas migration and evolution of casing pressure during deep-water cementing.Firstly,considering the complex interaction between the cement hydration and temperature,the transient temperature calculation method for deep-water cementing is established based on cement hydration kinetics.Then,considering the transient development of cement pressure,a prediction model for well cement exposed to a wide range of temperature and pressure is established.This paper establishes a model for short-term gas migration during deep water cementing,focusing on the influence of transient development of cement microstructures.Finally,the mechanism of gas migration and the evolution of casing pressure during well cementing through a hydrate layer is studied.Based on the established heat and mass transfer model in hydrate formation,the prediction models of gas migration and casing pressure are established.A series of conclusions and suggestions obtained from simulation analysis can provide safety guidance for deep water cementing design and construction:(1)Compared with field data,the maximum calculation error of the transient temperature prediction model established in this paper is only 5.6%.The temperature of cement slurry increases rapidly,while the pressure decreases gradually with the development of cement hydration.At the same time,the higher temperature leads to a faster hydration rate,the lower pressure results in a slower hydration rate.(2)The cement slurry located in deep wellbore has higher hydration degree under the influence of high temperature and high pressure.At the same time,the chemical shrinkage and cementing strength of the cement slurry show a similar trend with hydration degree,which increase with time.(3)The microstructure of cement sheath changes with time and space.With the developmement of cement hydration,the cement internal voids will be closed gradually,so the porosity and permeability of cement slurry will decrease with time.The spatial variation of cement microstructures is mainly influenced by temperature and pressure distribution.Under higher temperature and pressure environment,cement slurry has higher hydration rate,which leads to a lower porosity and permeability.(4)The pore pressure of cement sheath firstly decreases with the development of cement hydration.Gas migration occurs when the pressure is lower than the formation pressure,and the pore pressure of cement will increase continuously.The gas migration results in casing pressure,which is obviously affected by formation pressure.Higher formation pressure,porosity and permeability result in higher casing pressure.(5)According to our simulating,the transient temperature change of hydrate layer is a long-term process compared with the cement hydration process.This indicates that the potential safety hazard of gas migration caused by hydrate decomposition does not exist only during cementing,but also lasts for a long time after completion of cementing.(6)When the needle valve at the wellhead is closed,the gas will accumulate continuously,which will cause a higher pressure and prevent the further hydrate decomposition.But this will lead to higher annulus pressure.The casing pressure caused by hydrate layer decomposition can not be eliminated by simply opening the needle valve at the wellhead.On the contrary,the needle valve at the wellhead speeds up the decomposition of hydrate as a result of the pressure decrease in hydrate layer.