Shallow Gas and Shallow Water Flow Induced by Natural Gas Hydrate Dissociation in Deep Water Sediments

摘要

A heat transfer model in NGH sediments during drilling fluid deep-circulation and a dynamics modelrnof NGH decomposition, in which the temperature-difference driving process is mainly considered, arernestablished to evaluate NGH dissociation and gas and water production during deep-sediment penetration.rnA local overpressure generation model from NGH dissociation is constructed to calculate the inducedrnoverpressure and shallow gas (SG) and shallow water flow (SWF) formation under various conditions.rnInvestigated are the effects of drilling fluid circulating temperature and NGH saturation on NGHrndissociation and overpressure (SG and SWF) formation processes. The results indicate that the heatrnconduction from circulating drilling mud to NGH formation is so slow that local overpressure will onlyrnbe developed within about 1.5m around the wellbore for 9 days of drilling fluids deep-circulation underrntypical operation conditions. For the typical NGH bearing sediment, an overpressure as high as 2.4MPa willrnbe induced within the formation about 1.5m around the wellbore after about 9 days of drilling fluid deepsedimentrncirculation. This will exert great threat on the safety of cement sheaths and NGH sediment caps.rnDuring deep-sediment penetration and drilling mud circulation, the NGH saturation has more significantrneffects on NGH decomposition and local overpressure development than mud circulating temperature.rn100% increase of drilling mud circulating temperature will result in only 10% increment of the localrnoverpressure around the wellbore, while 100% increase of NGH saturation will result in 130% augmentrnof in-situ overpressure.