Acoustic Sounding of Hydraulic Fractures in a Low-Permeability Reservoir

Abstract

Theoretical models indicate the possibility of diagnosing the presence and the conductivity of the hydraulic fractures in reservoirs of the permeability of order milli Darcy by means of an acoustic TV set, which is a cylindrical probe of the length of several meters with a generator of impulse signals and pressure sensors. It is suggested to generate an impulse signal in a fluid, filling the gap between the probe body and the outer wall of the borehole. It is supposed that the impulse signal is generated in a fluid located in the gap between the probe body and an outer wall of the borehole. The signal evolution recorded by means of the pressure sensors as a damping of its amplitude and the appearance of the reflected burst of pressure allow one to estimate the presence and the conductivity of the fractures in the bottomhole zone. We consider fractures, which are radial or longitudinal to an open part of the borehole. The length of the impulse signal is less than the length of the probe, but exceeds the width of the gap between the probe body and the outer wall of the borehole. We take into consideration the damping of the impulse signal due to the viscosity effects in a boundary layer near the borehole walls. The width of a fracture is much less than the wavelength, and this is why the radial fracture is admitted by the reflecting surface. We provide the results of the dispersion analysis and numerical experiments on the influence of the filtration characteristics of the fractures, the width of the gap and the type of fluid on the evolution of the impulse signals in the channel.