Mud Density Optimization for Horizontal Well System in Clayey Silt Hydrate Reservoir with Considering Borehole Collapse

Although horizontal well system has great advantages in methane production from clayey silt hydrate reservoir, borehole collapse is easy to occur during the drilling operation, which can seriously affect the drilling safety and efficiency. However, previous investigations have failed to thoroughly explore the mechanism of borehole collapse in horizontal well system. In this study, numerical investigation on collapse behavior of different boreholes within the horizontal well system in hydrate reservoir was conducted. It is found that borehole enlargement rate of both injection wells and production wells in horizontal well system decreases with the increase of drilling fluid density. However, due to the faster dissociation rate of natural gas hydrates, borehole enlargement rate is often higher in injection wells as compared to production wells. Based on the investigation results of borehole stability, lower limit of the safe mud weight window for horizontal well system was determined by considering different acceptable borehole enlargement rates. After analysis, it was found that the lower limit of the safe mud weight window depends on the mud density of the injection well for a specific acceptable borehole expansion rate. Moreover, the lower limit of safe mud weight window needs to be designed larger with improvement of the requirement for wellbore stability (i.e., decrease of acceptable borehole enlargement rate). For example, when acceptable borehole enlargement rate changes from 20 to 5%, the lower limit of the safe mud weight window needs to be increased from 1.007 to 1.047. Investigations in this paper will play a significant role in preventing serious borehole instability during drilling operation in hydrate reservoirs.