Experimental study of fracture structure effects on acoustic logging data using a synthetic borehole model

Existing methods of well-logging interpretation often contain errors due to the presence of subsurface fracture networks within carbonate reservoirs. The differences of frequency ranges and measurement methods between the acoustic well-logging and indoor ultrasonic test cause inconsistent results. This study establishes a new method of devising a borehole model with controlled fracture structures, which is more suitable for large-scale experiments, such as borehole drilling and laboratory hydraulic fracturing. A self-build acoustic logging system is used to measure critically refracted waves in different directions and investigate the effects of fracture structures (porosity, aspect ratio-AR, and size) on the acoustic logging signals in the time and frequency domains using the Hilbert-Huang transform (HHT). Experimental results show that the velocity of the Stoneley wave is less sensitive to fracture structures than the velocity of the P- and S-waves, while strong effects are observed on the attenuation of the Stoneley wave. The best-fitting power and logarithmic functions are used to quantify the relationships between the properties (velocity and attenuation) and fracture structures. Predictions of the conventional rock-physical models used on high frequencies always overestimate the velocities at the well-logging scale. Intrinsic mode functions (IMFs) and marginal spectra show energy attenuation in different frequency ranges. Furthermore, fracture AR and size have similar mechanisms of scattering attenuation, which differ from the absorption attenuation of fracture porosity, and the effect of size is much stronger. The high precision of the borehole model has better implications than the rock-physical models for reservoir evaluations and predictions using acoustic logging or microseismic data at lower frequencies.