Most production and observation wells are cased. Hydraulic fracturing of unconventional coalbed methane and shale gas reservoirs and acidization of carbonate reservoirs are essential production steps to enhance hydrocarbon productivity. Cased-hole radial profiling of shear slownesses can help to assess the effectiveness of such production processes by determining the radial extent of near-wellbore alteration caused by open fractures that contribute to production. In addition, estimation of depletion- or injection-induced changes in reservoir stresses using time-lapse sonic data requires cased-hole radial profiling of shear slownesses to confirm that the estimated shear moduli are not affected by near-wellbore effects.
Sonic data processing and interpretation of cased-hole data are more challenging than they are for openhole data because of the presence of a steel casing bonded to the cemented annulus. The steel casing is a strong waveguide, and its associated modes interact with the formation modes of interest. In addition to the steel casing, a cemented annulus is required to bond the casing and the formation. A new processing method for cased-hole sonic data accounts for the presence of these additional layers in the inversion.
The new technique estimates radial variation of the three shear moduli using the Stoneley and cross-dipole dispersions provided by the acoustic scanning platform in cased holes. The inversion algorithm is based on the Backus-Gilbert method, and it solves a volume integral equation that relates fractional changes between the measured and reference borehole dispersions to differences between the radially varying shear modulus from that in a radially homogeneous reference state. The inversion algorithm is applicable to both concentrically and eccentrically placed steel casing in an open hole. In the case of eccentrically placed casing, radially varying thickness of the cement in the annulus is considered to be part of the near-wellbore alteration of formation properties outside the casing surface. Processing parameters internal to the inversion algorithms have been optimized for different casing sizes. The accuracy and robustness of radial profiles have been validated using a synthetic model for different types of radially varying formations. A field example illustrates the method with cased-hole radial profile changes of the three shear slownesses from sonic data acquired before and after hydraulic fracturing.