Issue 45

Q.-C. Li et alii, Frattura ed Integrità Strutturale, 45 (2018) 86-99; DOI: 10.3221/IGF-ESIS.45.07 88 F UNDAMENTAL THEORY Borehole instability in hydrate reservoirs ellbore instability usually manifests as shear failure and/or tensile failure. Borehole collapse generally occurs in the direction of the minimum principal stress due to the stress concentration caused by the low drilling mud density [28, 29]. Hydrate reservoirs are usually sandstone reservoirs with high porosity, and borehole collapse is the main form of borehole instability in hydrate formations. Hydrate dissociation results in the changes of both pore pressure and the effective stresses. Therefore, hydrate dissociation within the near-wellbore region during drilling operation can aggravate the borehole collapse. And, borehole collapse in hydrate reservoirs depends on the initial formation conditions and the conditions of drilling mud within the wellbore. Schematic diagram of borehole instability while drilling in hydrate reservoirs is shown as Fig.1. Therefore, determining the collapse area during a drilling operation in hydrate reservoirs needs to consider the effect of hydrate dissociation caused by the invasion of drilling mud. During the determination of collapse area, the equivalent plastic strain is used as the parameter for judging the collapse of a wellbore. When the surrounding rock shows a positive equivalent plastic strain value, it is considered that the borehole may collapse. Figure 1 : Schematic diagram of borehole instability while drilling in hydrate reservoirs. Figure 2 : Schematic diagram of the relationship between various coordinate systems. Effective stresses around arbitrary borehole The effective stress is the key for wellbore stability, and the relationship between the total stress, the effective stress and the pore pressure can be expressed by the principle of effective stress, which is shown as the following equation. W