Gas hydrate risks and prevention for deep water drilling and completion: A case study of well QDN-X in Qiongdongnan Basin, South China Sea

Taking a deep-water exploration well of natural gas located in the Qiongdongnan Basin in the South China Sea as an example, the hydrate risks of the well under operational conditions during drilling and testing processes were analyzed, and the corresponding hydrate prevention solutions were presented and verified by lab experiments and field application. Based on the predicted gas hydrate equilibrium curves and the calculated wellbore pressure-temperature fields, the hydrate risks were analyzed. The maximum sub-cooling temperature is 6.5 °C during normal drilling with a small hydrate stability zone in the wellbore; when the drilling or testing stops, the hydrate stability zone in the wellbore becomes larger and the maximum sub-cooling temperatures are 19 °C and 23 °C respectively; the maximum sub-cooling temperature at the beginning of testing is no more than that when testing stops; when the tested production rate of natural gas increases, the hydrate stability zone in the wellbore decreases or even disappears if the gas rate is more than 25×104 m3/d. The designed hydrate prevention solutions include: adding sodium chloride and ethylene glycol into drilling fluid during normal drilling and when drilling stops; adding calcium chloride/potassium formate and ethylene glycol into testing fluid; applying downhole methyl alcohol injection when the production rate of natural gas is lower than 25×104 m3/d; filling the testing string with testing fluid when the test shuts down for a long time. Lab experiments and field operations have indicated that all the designed solutions can meet the requirements of hydrate prevention. Key words: gas hydrate, drilling fluid, wellbore temperature, sub-cooling temperature, hydrate inhibitor, deep water drilling