| Marine natural gas hydrate exists mainly in shallow unconsolidated mud in deep water,where there are sever drilling problems,like shallow gas,shallow stream,narrow pressure window,wall collapse etc.Moreover,natural gas hydrate is such sensitive to temperature and pressure,largely prone to decompose,that well control is notoriously difficult.Conventioanl drilling method plays main role in gas hydrate research,but has not been comercial.Subsea mudlift drilling,which is a potentional and achievable dual gradient drilling technology,combines the advantages of MPD technology.And it is able to monitor overflow faster,circulate kick fluid without stopping mud pumps,and remarkably reduce the NPT.These characters make subsea mudlift drilling technology prominent in marine natural gas hydrate drilling.This thesis is mainly focused on following aspects:1.Design of subsea mudlift drilling for marine natural gas hydrate systemAnalyse the characteristics and hydrate decomposition condition,kick fluid volume and bottom hole pressure adjustment range,together with the complexity of the system are the main criterions to adopt the RMR drilling system as the original scheme.Based on the principles of RMR and depressurization method,two drilling and producing test system schemes for natural gas hydrate,RMR natural gas hydrate drilling system and RMR natural gas casing drilling system were proposed.2.Calculation of pressure and temperature of the drilling fluid in circulatation systemThe mathematical model of pressure and temperature of drilling fluid in circulating system containing the friction factor had been established to study the distribution rules and effective factors like the injection temperature and flow rate,etc.as well as the necessarity of drilling fluid cooling system.In the end,according to the hydrate phase transition condition,the depressurization position of natural gas hydrate in mud return line was predicted.3.Deployment and configration of subsea mudlift drilling for marine natural gas hydrate systemRefering to conventional offshore drilling standards and processes,drilling process and casing structure for the two schemes mentioned above were suggested.Afterwards,installation process of some key equipment was presented.In the end,the parameters of mud pumps,equipment of mudlift system,drilling ship and drilling rig,etc.were calculated with some equipment options offered.4.Kick inspection and well control techniqueThe early kick monitoring method by recording the changes in the subsea pump power,flow rate,mud pump pressure,pit gain was given.To efficiently conduct well control measures,established a precisely mathematical speed-control model of a single mudlift pump.After comparing different speed control method for series mudlift pumps,optimized pump speed control sheme was selected.At last,the well control method and the experimental scheme based on SMD experimental platform were designed. |
