A prediction method for safe mud density of natural gas hydrate reservoirs under temperature-pressure dual control

Wellbore instability is one of the most encountered issues during hydrate exploration, which correlates with mud density and temperature. However, current studies ignore the effect of mud temperature on safe mud density windows, leading it fails to meet requirements of sufficient drilling in hydrate reservoirs. Herein, we propose a new model to predict safe mud density windows by introducing temperature-pressure dual control, which is more accurate and applicable for hydrate reservoirs. Collapse pressure equivalent density decreases 6.19% while fracture pressure equivalent density increases 4.52% with hydrate saturation rising from 0 to 50% under normal conditions. Hydrate formation can narrow the safe mud density window significantly. Equivalent density of collapse pressure presents temperature sensitivity, especially mud temperature ranging from 287.9 to 289.3 K. Hydrate dissociation can narrow the safe mud density window by ~27% on average with hydrate saturation of 50%, which will be magnified with increasing intermediate principal stress. On this basis, an integrated control system involving temperature-pressure dual control is established to deal with potential drilling risks of multi-layer hydrate reservoirs. This work provides new insights into safe mud density windows dependent on temperature and presents design principles for sufficient drilling in natural gas hydrate reservoirs.