Applicability of full waveform inversion to sonic logging

Sonic logging has been widely used for many years to acquire physical properties of hydrocarbon reservoirs. In the early years, the detection of first breaks for acoustic signals travelling along wellbore, and, later, the method was replaced by slowness time coherence (STC) method using full acoustic waveform to accurately estimate the velocity of elastic waves in formation, since the estimation of velocity is important for the identification of fluid contacts such as OWC (Oil-Water Contact), GOC (Gas-Oil Contact), etc. The resolution of existing methods is restricted to 6 inch defined by neighboring receiver distance of logging tool. However, there are significant needs to gain the resolution higher than the current detection of fluid contact locations as well as their variations in time. We, therefore, tried to introduce the method of full-waveform inversion (FWI) as an innovative technique to estimate the elastic velocities with much higher resolution in the subsurface using sonic logging. Although the FWI method has been proven to provide seismic velocities with higher resolution than the other conventional seismic reflection methods, the applicability of FWI to the sonic logging has not been investigated yet, and we first examine if the FWI could be used for the objective to estimate velocities finer than the current limitation of receiver spacing using numerical experiments. Our results show that both GOC and OWC become visible with the resolution higher than the conventional sonic methods whose resolution is 6 inches. We conclude that FWI would be applicable to sonic logging as a high-resolution method