Analysis of the theoretical model of drilling fluid invading into oceanic gas hydrates-bearing sediment

Oceanic gas hydrate-bearing sediment is usually porous media, with the temperature and pressure closer to the curve of hydrate phase equilibrium than those in the permafrost region. In the case of near-balanced or over-balanced drilling through this sediment, the water-based drilling fluid used invades into this sediment, and hydrates decompose with heat transfer between drilling fluid and this sediment. During these processes, there are inevitably energy and mass exchanges between drilling fluid and the sediment, which will affect the logging response, borehole stability and reservoir evaluation. When drilling fluid invades into this sediment, solid and liquid phases of drilling fluid permeate into the wellbore and displace original fluids and solids, and water content of formation increases. With the temperature and pressure changing, gas hydrates in the sediment decompose into gas and water, and water content of formation further changes. When the filter cakes form, the invasion of drilling fluid is weakened. This process is accompanied by the heat and mass transfer within the range from wellbore to undisturbed area, including heat conduction of rock matrix, the convective heat transfer of fluids invaded, the heat absorbing of hydrate decomposition and the mass exchange between fluids invaded and the gas and water generated by hydrate decomposition. As a result, dynamic balance is built up and there are generally four different regions from wellbore to undisturbed area, i.e. filter cakes region, filter liquor region, water/free gas region, and water/free gas/hydrate region. According to the analysis on the invasion of drilling fuild into sediment, the whole invasion process can be described as an anisothermal and unstable displacement and diffusion process coupled with phase change. Refering to models of drilling fuilds invasion into normal oil and gas formation and natrual gas production from hydrate deposit by heating, the model of the invasion of drilling fluid into hydrate-bearing sediment has been preliminarily discussed based on kinetics of hydrate dissociation , with the assumption that hydrates were viewed as a portion of pore fluids and their decomposition was taken as a water and gas source without a uniform rate. A mathematical model was built up, and key parameters used for solving the kinetic equation of hydrate dissociation, such as the coefficient of effective porosity and permeability, absolute permeability, the synthetic specific heat and heat conductivity of hydrate-bearing sediment, are discussed. This model could be used to describe the dynamic process of drilling fluids invasion by coupling modified kinetic equation of heated hydrate decomposition into mass conservation equations, and also be used to study the evolution of pore water pressure, temperature, salinity, saturation of water/gas/hydrate with the depth of invasion and time. Key words: gas hydrates-bearing sediment, drilling fluid, hydrate dissociation, invasion process, model