钻井液在海洋含水合物地层的侵入机理与特征

Formations containing gas hydrates are encountered both during ocean drilling for oil or gas, as well as gas hydrate exploration and exploitation. Because the formations are usually permeable porous media, inevitably there are energy and mass exchanges between the water-based drilling fluids and gas hydrates-bearing formation during drilling, which will affect the log response, borehole’s stability and reservoir evaluation. In conditions of close-balanced or over-balanced drilling, the water-based drilling fluids will invade into hydrate-bearing formation and at the same time hydrates will decompose induced by heat transfer. So the whole invasion process can be described as an anisothermal and unstable displacing and diffusing which coupled phase change. Therefore, the differences and characteristics of the drilling fluids invading into the hydrate-bearing formation and its influence rule were preliminarily discussed. The paper proposed an idea of establishing a prototype to simulate the invasion process. On the assumption that hydrates were a portion of pore fluids and their dissociation was taken as a continuous water and gas source with no uniform strength, a mathematical model was built and could be used to describe the dynamic process of drilling fluids invasion by coupling modified kinetic equation of heated hydrate dissociation into mass conservation equations.