| The drilling operations in the Gulf of Mexico and the North Sea have indicated that shallow geological hazards such as shallow water flow(SWF)greatly influence deepwater drilling safety.The further deepwater drilling in the South China Sea also tend to encounter SWF hazards.It has significant meaning to identify and characterize SWF sediments in the target area,clarify SWF hazard evolution mechanisms and controlling factors,its influence on drilling safety and propose effective controlling measures of SWF during deepwater drilling.The influences of SWF on deepwater drilling safety are described.The reservoir numerical simulation method is applied in this study.A typical geological model of SWF and a SWF blowout model are established on ths basis of characterization of SWF in target area.The models are used to simulate water and sand blowout process from SWF sediment during deepwater drilling.Furthermore,investigated are the effects of various influencing factors,including SWF overpressure factor,SWF size,porosity and permeability and penetration rate,on SWF blowout.Moreover,based on basic characteristics of natural gas hydrate(NGH)sediment in deepwater basins,constructed are heat transfer model of NGH formation during deep-sediment penetration,dynamics model of NGH thermal dissociation and model of local overpressure evolution due to NGH decomposition.The models are used to calculate shallow NGH dissociation and overpressure development around the wellbore during drilling fluid deep-sediment circulation.Also,the effects of different factors such as mud circulating temperature and NGH saturation on NGH dissociation are investigated.Finally proposed are SWF prevention and control measures in deepwater drilling.The results indicate that the cumulative water and sand blowout will keep continuously increasing during drilling through SWF formations.When drilling through a certain SWF zone with the area of 3,600m2,thickness of 100 m,porosity of 0.4,permeability of 3μm2 and formation pressure of 24.3MPa at the depth of 500 m below the mudline at the water depth of 1,500 m,the cumulative water blowout and sand blowout will be as high as 100m3 and 1m3,respectively,when the whole SWF sediment is penetrated.SWF formation pressure has significant influences on SWF hazards.When penetrating SWF zones,10% increase ofoverpressure factor of SWF formation will result in around 90% increase of cumulative water blowout and 150% increase of cumulative sand blowout after the SWF zone is totally penetrated.Also,the porosity and permeability of SWF zone have important effects on SWF hazards.10% increase in porosity and 20% elevation of permeability will result in 14% increment in cumulative water and sand blowout.Higher penetration rate will lead to less water and sand blowout when penetrating the whole SWF sediments.10% increment of penetration rate can generate 4% decline of ultimate cumulative water blowout and 3% decline of ultimate cumulative sand blowout as soon as the SWF zone is penetrated.Raising penetration rate is an effective measure to control SWF blowout during deepwater drilling.Under typical operation conditions,NGH dissociation will occur within the formation with the radius of about 1.5m around the wellbore for 9 days of drilling fluids deep-circulation.The NGH dissociation will be constantly triggered and the overpressure buildup will be continuously induced during deep-circulation of drilling fluids and further oil and gas production operations.The overpressure around the wellbore should be timely monitored to evaluate its potential risks.The induced overpressure increases with mud circulating temperature and NGH saturation.100% increase of drilling fluid effective temperature can lead to 10% elevation of the local overpressure and 100% increase of NGH saturation will result in 130% augment of in-situ overpressure.The NGH saturation has more significant effects on NGH decomposition and local overpressure development than mud temperature in the cases of deep-sediment penetration. |
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