Sound velocity of drilling mud saturated with reservoir gas

Knowledge of the in-situ sound velocity of drilling mud can be used in mud- pulse acoustic telemetry for evaluating the presence and amount of gas inva sion in the drilling mud. We propose a model for calculating the in-situ de nsity and sound velocity of water-based and oil-based drilling muds contain ing formation gas. Drilling muds are modeled as a suspension of clay partic les and high-gravity solids in water or oil, with the acoustic properties o f these fluids depending on pressure and temperature. Since mud at differen t depths experiences different pressures and temperatures, downhole mud wei ghts can be significantly different from those measured at the surface. Tak ing this fact into consideration, we assume constant clay composition anti obtained the fraction of high-gravity solids to balance the formation press ure corresponding to a given drilling plan. This gives the in-situ density of the drilling mud, which together with the bulk moduli of the single cons tituents allow us to compute the sound velocity using Reuss's model. In the case of oil-based muds, we take into account the gas solubility in oil. Wh en gas goes into solution, the mud is composed of solid particles, live oil and, eventually, free gas. A phenomenological model based on a continuous spectrum of relaxation mechanisms is used to describe attenuation due to mu d viscosity. The calculations for water-based and oil-based muds showed tha t the sound velocity is strongly dependent on gas saturation, fluid composi tion, and drilling depth.