Analyses of Sonic Data in an Indonesian Well for Formation Damage, Stresses, and Bedding

Abstract

Abstract To help design the completion of an Indonesian development well, we have carried out cross-dipole dispersion analyses over a depth interval of approximately 500 ft. Dispersion analyses provide estimates of radial extent of formation damage and indicators of stress- and bedding-induced anisotropies. Most of these sections of this vertical well exhibit sand and shale lithology with shear slowness anisotropy ranging from 5 to 10%. Above the angular unconformity in the deeper section of the well, a clean sand interval (Sand A) shows evidence of cross-dipole dispersion crossovers. Crossing dipole dispersions are indicators of stress-induced anisotropy dominating the sonic data. We have developed a new technique for estimating the maximum horizontal SH, and minimum horizontal Sh stress magnitudes using multi-frequency inversion of wideband cross-dipole dispersions. At the mid-point of sand A, we estimate the overburden stress SV=-2278 psi; the maximum horizontal stress SH=-1843; and minimum horizontal stress Sh=-1698. The fast shear direction is NW5 in this depth interval. Radial profiling of formation shear velocity indicates varying degrees of mechanical alteration extending from one to two borehole diameters in the entire depth. A second clean sand interval B, 300 ft above sand A, shows dipole shear anisotropy on the order of 10%. However, cross-dipole dispersions appear to merge together at high frequencies instead of showing a cross-over. Radial profiling of shear velocity in the two orthogonal directions confirms mechanical damage extending to about 2x the borehole diameter. The near-wellbore region in this interval appears to have deformed (material creep) in an attempt to reduce shear stresses and attain hydrostatic equilibrium. Using the same stress sensitivity coefficients as estimated in the lower sand A, the differential stress (SH-Sh) is estimated to be about 20% larger in the upper interval, than the corresponding value in the lower interval. The fast-shear direction varies abruptly across the angular unconformity, changing from NW5 to NW75. Below the unconformity, this section exhibits beds with dips ranging from 10° to 30°. Cross-dipole dispersions show significant anisotropy and are non-intersecting at higher frequencies. Non-intersecting dispersions indicate bedding-induced anisotropy dominating the cross-dipole data. We have inverted borehole sonic velocities for four combinations of the TI-shale anisotropy, which can be combined with walk away VSPs to obtain all the shale anisotropy constants. These constants are needed in generating synthetic AVO gathers in anisotropic shale formations. Quantitative estimates of the radial extent of near-wellbore damage in this well suggest that perforations should penetrate deeper than twice the borehole diameter to avoid potential permeability impairment caused by near-wellbore mechanical damage.