Anisotropy parameter inversion from sonic and density logs in horizontally transverse isotropic media

A methodology of calculating anisotropy parameters in horizontally transverse isotropic (HTI) media using a Backus average-like algorithm is presented herein. Anisotropy parameters in HTI media are calculated by mapping the stiffness parameters that exist in HTI media and vertically transverse isotropic (VTI) media by tilting the Christoffel equations. Fast and slow transverse waves, compressional wave and density logs are used as inputs into the averaging algorithm and, from these, anisotropy parameters are calculated over a predefined averaging window. From the results, the horizontal compressional wave velocity in the direction of the symmetry plane of isotropy can be determined, as can the compressional wave velocity that is perpendicular to the symmetry plane. When the anisotropy is caused by a single set of vertical fractures, these correspond to the directions perpendicular to and parallel to the fracture plane respectively. In cases where the thickness of the bed of interest is thin, a workflow is presented to choose an averaging length that will allow for the calculation of anisotropy parameters and velocities in thin beds. This technique was applied to a coal seam gas well situated in the Surat Basin and anisotropy parameters were calculated over two horizons.