| When drilling in gas hydrate bearing formation with the deep-water drilling fluid,the deep-water drilling fluids invaded into the hydrate bearing formation,the hydrate will decompose because of the heat change between drilling fluids and hydrate bearing formation.The decomposition of hydrate will produce a lot of gas and a small amount of water,it may increase the pore pressure and reduce the rock intensity,and it may cause the instability of the borehole wall.Especially when hydrate acting as the cement of sediment formation,the decomposition of hydrate cause wellbore collapsed for the reason of loss cementing support.The release of a large number of gas may cause well kick or wellblowing,the gas will regenerate hydrate on suitable conditions at the bottom of the sea section,the solid hydrate may cause plugging.Therefore,it is necessary to carry out the theories and methods research of drilling fluid inhibit hydrate decomposition.The results will provide drilling fluid technology support to stabilize the hydrate bearing formation,and improve the safety and efficiency in drilling engineering.Therefore,this paper simulated the hydrate decomposition process of drilling fluid invading into the hydrate formation,and analyzed the influence of hydrate decomposition kinetics under the physical and chemistry effect of the drilling fluid.The research results will guide us to choose some chemical reagent which can inhibit hydrate decomposition.First,we use the improved hydrate decomposition inhibitory experiment device to carry out test,the parallelism and repeatability of hydrate formation are guaranteed by optimize the temperature,pressure,surfactant and other conditions.In this paper,the change of the number of gas molecules in the calculation of peng-robinson equation is introduced to describe the two-phase equilibria of the gas liquid in the binary system,for a more intuitive understanding of the formation and decomposition of gas hydrate.By combining actual working conditions and indoor means,we established a complete set of experimental methods for inhibitive evaluation of hydrate decomposition.By using the new evaluation method,we investigate the characteristic of hydrate decomposition under the effect of the three kinds of inhibitors: ethylene glycol(EG),polyvinylpyrrolidone(PVP K90),Lecithin.Combining with the theories of molecular dynamics simulation,quantum chemistry,and laser Raman spectroscopy,we discussed the mechanism of inhibition of hydrate decomposition by inhibitors.The results showed that EG in low concentration(10wt%)inhibited the decomposition of hydrate by strengthen the hydrogen-bond network structure.With the increase of EG concentration,the inhibit hydrate decomposition effect weakened,and it gradually transformed into promoting hydrate decomposition.PVP-K90 uses the five-membered lactam ring in its molecular structure,it can adsorb on the surface of solid hydrate by hydrogen bond,and the process of heat transfer and mass transfer of solid hydrate by external fluid is weakened,and the decomposition of hydrate is inhibited.Lecithin can hydrate to phospholipid bilayer lipid membrane in the water phase,it can adsorb on the surface of solid hydrate by hydrogen bonding effect,and it may form a layer of film coating to inhibit hydrate decomposition.Under appropriate conditions,it can promote the decomposition of gas and water formation of hydrate again.Lecithin can effectively inhibit hydrate decomposition when concentration is higher than 0.5wt%,the inhibition effect is better than the EG and PVP.Finally,the compatibility of hydrate decomposition inhibitor and deep-water drilling fluid was analyzed.A set of water-based drilling fluid system suitable for drilling of natural gas hydrate formation is constructed.The drilling fluid can effectively inhibit hydrate decomposition and generation,and it have low temperature rheological property,good inhibitory and lubricity.Furthermore,it can satisfy the basic drilling fluid requirement for hydrate bearing formation drilling. |
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