Safety Evaluation Of The Wellbore Integrity For Deepwater Well Considering The Shallow Gas And The Gas Hydrate

The global demand for energy continues to expand,the development of offshore oil and gas exploration has been an impetus towards deepwater.Compared to the conventional oil and gas wells,deepwater wells are confronted with the special marine environment and geological conditions,which have increased the complexity of deepwater well development,and might result in major accidents in recent years such as "The Deepwater Horizon Incident".Therefore,it is important to conduct the wellbore integrity safety evaluation of the deepwater wells.This paper have conducted full investigation of the study on the wellbore integrity,the failure causes of deepwater wellbore integrity has been done.The deepwater well barriers could be divided into two categories:physical barrier and operating barrier.This paper focus on the effects of the shallow gas and the methane hydrate dissociation on the deepwater wellbore safety due to there are no substantial differences of the barriers assessment methods between the deepwater wells and the normal wells.Firstly,the seepage equations of the shallow gas in the cement sheath and the annulus liquid were built;The calculation model of the annulus pressure caused by shallow gas channeling was established according to the volume change and the state equation.Based on the case well,effects of the cement sheath permeability,TOC,formation pressure and temperature etc,on the annulus pressure were analyzed.Then,the calculation model of the deepwater well temperature and pressure fields was built on the basis of the heat transfer mechanism,momentum conservation and energy conservation laws.Effects of the production and time on the temperature were conducted.Based on the wellbore temperature field and methane hydrate phase equilibrium theory,the influence factors on the gas velocity and the cumulative gas production caused by the methane hydrate dissociation combined with the case well were discussed.In addition,the excess pore pressure generated by the methane hydrate dissociation was established according to the volume compatibility,the sensitivity analysis of the excess pore pressure was carried out.Finally,the mechanical calculation model of the casing-cement sheath-formation was developed,the deepwater wellbore integrity safety evaluation was conducted.The effects of the annulus pressure and the excess pore pressure on the stresses and the safety factor of the surface casing and the cement sheath were discussed.And the influence of the production on the safety factor of surface casing and the cement sheath with/without considering the excess pore pressure was conducted.These are helpful for improving the wellbore integrity of the deepwater wells,and extending the service life of the deepwater well.