Numerical simulation study of anisotropic velocities in fractured-vuggy carbonate reservoirs

Abstract

The characterization of carbonate reservoirs based on seismic and acoustic log data is difficult due to their anisotropy. Complex pore structures are one of the major factors causing this anisotropy. To solve this issue, a model considering pore structure is proposed. In this model, the differential Kuster–Toksöz model and Hudson’s model are used to add various types of pores in the rock matrix, and Thomsen’s anisotropy theory is used to estimate the anisotropic velocities of carbonate rocks with fractures of arbitrary dips. Based on this model, the effects of pore structure (including porosity, pore aspect ratio, fracture porosity and dip) on elastic wave velocities are discussed. The simulation results show that the pore aspect ratio and porosity are the dominant factors influencing the elastic wave velocities of carbonates. Pores with small aspect ratios (aspect ratios of less than 0.15) have an extremely significant influence on elastic wave velocities. For anisotropic carbonates with aligned fractures, fracture porosity and dip are important factors influencing elastic wave velocities, especially when they exceed critical values. The critical values for P- and S-waves are 0.3% and 30° and 0.3% and 20°, respectively. These rules provide good guidance for analysing the measured data and are helpful for characterizing carbonate reservoirs and optimizing production. The elastic wave velocities predicted by this model agree well with acoustic well log data in fractured-vuggy carbonate rocks with arbitrary dip fractures. This result demonstrates that the proposed model is valid for predicting anisotropic velocities in fractured-vuggy carbonate rocks.