Relationship between acoustic body waves and in situ stresses around a borehole

Abstract

This paper shows that there are three types of shape of acoustic body waves propagating inside competent and homogeneous formations penetrated by a borehole : simple, double, and resonant. This fact is connected to the modifications undergone by the area located around a well after drilling in relation to in situ state of stress. There are overstressed and understressed vertical cylindrical zones with "bubble-shaped" cross-sections, in which acoustic body waves are supposed to reflect. The horizontal size and the shape of the stress modified areas depend on the values of (K1+K2)/2 - with K1=SH/Sv (the overburden pressure) and K2=Sh/Sv -, that is to say on the in situ horizontal stress and the anisotropy of this stress. The calculation of the velocities of the P and S double waves allows us to evaluate the radial thicknesses of these stress modified areas. As their values are different according to whether they result from P or S waves, we propose an explanation based on their wavelengths. The comparisons with other works on the in situ stress of the studied boreholes allow us to propose a method for evaluating the horizontal stress and its anisotropy, thanks to the estimation of the value (K1+K2)/2.