A porothermoelastic wellbore stability model for riserless drilling through gas hydrate-bearing sediments in the Shenhu area of the South China Sea

Abstract

Wellbore instability during the gas hydrate drilling process frequently occurs due to the metastable character of sediment-hosted hydrates and weakly consolidated sediments. An accurate prediction of wellbore stability in gas hydrate drilling is very important. In this paper, a transient heat transfer model between the wellbore and sediment in a gas hydrate reservoir is proposed to simulate temperature profiles inside the drilling string and annulus. Using these temperature profiles, a porothermoelastic model is proposed to compute the temperature and pore pressure field in the entire formation during riserless drilling. Then, the thermodynamic stability of the gas hydrate and wellbore stability are analysed. The findings indicate that it is not necessary to use low-temperature drilling mud during riserless drilling. The risk of in situ hydrate dissociation is very low with a drilling mud density of 1.1 g/cm3. Then, a safe mud density window is obtained for future riserless drilling in the gas hydrate reservoirs in the Shenhu area.