Borehole acoustic measurements are commonly used to detect and quantify variations in rock elastic properties by comparing measurements acquired before and after fracturing. While shear-waveform slowness variations are routinely recorded in boreholes, analysis of the corresponding attenuation is mostly limited to laboratory experiments. Interpreting the effects of fractures on borehole acoustic measurements often requires numerical modeling. This paper quantifies the effects of hydraulic fractures perpendicular to the well axis on shear and Stoneley borehole acoustic measurements via numerical modeling. Dipole and low frequency monopole acoustic propagation is simulated in 2D models assuming a borehole elastic and viscoelastic model. We vary casing-cement-formation bonding considering models with open hole, perfect-bond cased hole, well-bond cement bonding and unbonded cased hole. Simulation results show that hydraulic fractures perpendicular to the well axis give rise to attenuation and reflected events for both shear and Stoneley propagation modes. For shear waveforms, casing presence obscures the detection of reflected events, but attenuation is still observed. For Stoneley waveforms, the presence of casing enhances the contrast between casing-formation and fluid-filled fractures. Well-bond casing models are the optimal cased-hole models required for fracture interpretation because fracture features can be observed for both shear-wave and Stoneley modes.
Borehole acoustic measurements are commonly used to assess the elastic properties of rocks around a well. Rocks can have either natural or hydraulically induced fractures. The presence of fractures often has measurable effects on the acquired borehole acoustic waveforms. Therefore, it is important to understand the role played by fractures on borehole acoustic measurements. Analytical and numerical models enable the estimation of the effect of single and multiple fractures on Stoneley and shear waves (Hornby et al., 1989; Tang and Cheng, 1989; Bakku et al., 2013; Minato and Ghose, 2017, Xu et al., 2021). Numerical modeling represents a technical challenge because analysis of complex geometries is usually required. Two-dimensional (2D) numerical models are powerful tools to understand waveform propagation along the borehole. However, they are limited to only axisymmetric geometries. Three-dimensional (3D) modeling presents technical challenges because it requires a great amount of computational resources. The finite-difference time-domain (FDTD) algorithm is the most extended method for numerical modeling of wave propagation in boreholes (Cheng et al., 1995; Liu et al., 1996; Wang and Tang, 2003; Guan et al., 2009; He et al., 2010). Other methods include finite-element method (FEM) (Michler et al., 2009, Wang et al., 2013, Pardo et al., 2021), frequency-domain hp-adaptative algorithms (Matuszyk et al., 2013), and spectral-element methods (SEM) (Komatitsch and Tromp, 1999; Xu et al., 2021).
