Determination of Safe Mud Temperature Window for Drilling Operation in Hydrate Deposits in Shenhu Area, Northern South China Sea

摘要

Borehole instability is a frequently encountered problem during drilling in hydrate reservoir. In this paper, both the mechanism of borehole collapse and the safe mud temperature window during the drilling operation in hydrate reservoir were explored. It was found that borehole collapse area is an elliptical area with its long axis along the direction of minimum horizontal stress. Moreover, compared with low density drilling fluids, hydrate dissociation is the major contributor to the collapse of borehole in hydrate reservoir. Besides, the collapse ellipse becomes flatter and flatter as the difference of in-situ stresses increases. Finally, the effect of drilling fluid temperature on borehole collapse was conducted, and the relationship between borehole enlargement rate and drilling fluid temperature was obtained. Then the safe mud temperature window (actually, it is the upper limit of the temperature window) was determined with considering different borehole enlargement rate.