Anisotropy Models and Parameters of Fractured Rocks by Physical Modeling Data

Considered in this study the orthorhombic (ORT) medium is a more accurate model of fractured reservoir, which is superposition of the VTI- and HTI-media, that is, vertical fractures (HTI-model) embedded in thin-layered background rock (VTI-model). In our ultrasonic experiment, the ORT model is represented by synthetic vertical fractures embedded in laminated synthetic rock specimen. In the physical modelling, there are five ORT-rock samples with different number of cracks in them, but their VTI-background is the same in each ORT-rock. In the processing and interpretation of the velocity data we apply an effective-medium theory of the Linear Slip model of Schoenberg. Our physical-modeling approach demonstrates the consistency of the summation rule ORT = VTI + HTI for the Thomsen anisotropy parameters ε and γ, that is ε(ORT) = ε(VTI) + ε(HTI), and γ(ORT) = γ(VTI) + γ(HTI). However, for the δ-parameter, the summation rule is not confirmed, that is, δ(ORT) is not equal to the (δ(VTI) + δ(HTI))- sum. To estimate the anisotropy parameter δ, we lacked additional VP45-velocity measurements at an angle of wave incidence of 45°. We develop a way to predict the δ-parameters in the ORT-rocks from the available data.