Physical Properties Response of Hydrate Bearing Sediments near Wellbore during Drilling Fluid Invasion

At present, the laboratory experiment of drilling fluid invasion problem mostly focus on small-scale, carrying out large-scale experiment based on physical parameters more similar with actual sediment would provide guidance for drilling fluid formulation during actual drilling process in hydrate-bearing formation and accurate well logging identification and hydrate reservoir evaluation. This experiment were based on artificial cores which were made according to the physical properties of hydrate-bearing formation in the Gulf of Mexico. Results indicate that the temperature and pressure rise when hydrate is heated to decompose, while the resistivity firstly increases and then decreasse, in which, hydrate equilibrium conditions are not only affected by temperature and pressure, but also by pore-water salinity. During drilling fluid invasion, the pressure spread rate is much faster than heat, hence it is probably that in-situ pore water and gas continue to form hydrate for pressure increase while temperature doesn't change. The high drilling fluid temperature is the main factor controlling hydrate decomposition, and pressure difference between drilling fluid and pore pressure can help improve the pore water pressure, which is beneficial to hydrate stability. Though high salinity drilling fluids are conducive to higher pressure difference and will inhibit hydrate formation in drilling fluid, it can also lead to gas hydrate dissociation. Therefore, in order to reduce the drilling risks in the hydrate-bearing formation, the density of drilling fluids should be increased during the safe density window range, but the density increase also increases the drilling fluid invasion. Therefore, a certain amount of kinetic inhibitors and fluid loss control agent leak loss control agents should be added in the low temperature drilling fluids. At the same time, the logging while drilling method or deep laterolog data should be chosen so as to avoid the distortion caused by drilling fluid invasion and hydrate decomposition.