Effects of Drilling Mud Properties on Hydrate Dissociation Around Wellbore during Drilling Operation in Hydrate Reservoir

Abstract

Natural gas hydrate is a potential energy source in the near future, and its commercial development can alleviate the global energy crisis. Disturbance of drilling mud invasion on hydrate reservoir can lead to hydrate dissociation, affecting wellbore stability while drilling in clayey silt hydrate reservoirs. In this work, the coupled thermo-hydro-chemical finite element model was developed, and influences of drilling mud properties on hydrate dissociation were investigated. The obtained results show that the hydrate dissociation range around wellbore widens as the mud temperature increases. The final dissociation range caused by drilling mud invasion nonlinearly increases from 3.83cm to 10.57cm when the mud temperature has increased from 17.25℃ to 21.25℃. Therefore, the drilling mud needs to be cooled during preparation in platform. In addition, dissociation range narrows as the bottom-hole pressure increases. Dissociation range decreases from 12.18cm to 7.46cm when the bottom-hole pressure is increased from 14.50MPa to 17.00MPa. Thus, the overbalanced/near-balanced drilling operation is preferred during drilling in hydrate reservoirs, and the underbalanced drilling operation is not recommended. Moreover, the increase of mud salinity exacerbates hydrate dissociation in the nearwellbore region. In view of the prevention of hydrate dissociation in the near-wellbore, it is necessary to confect the drilling mud that with appropriate salinity while drilling in hydrate-bearing sediments.

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