Critical Porosity in Understanding Acoustic Porosity Anomalies. A Case Study in Complex Carbonate Reservoir in Indonesia

The difference between acoustic porosity to total porosity logs has been used to infer the secondary porosity. Apart from gas and/or organic matter, any acoustic porosity with higher reading than total porosity would be considered as an anomaly response. This paper discusses anomalous acoustic porosity in a complex carbonate reservoir, where the acoustic porosity shows higher values than those of the total porosity. The investigation reveals that this anomaly correlates with changes of rock type quality and can be corrected using the critical porosity concept. The critical porosity links the rock quality to change of the acoustic velocity as a function of critical porosity-bulk modulus for each rock type (Akbar, 2019). Niu et al (2009) proposed the use of shear modulus to determine the critical porosity value. By combining Niu et al (2009) with Akbar (2019) approaches, the critical porosity and critical bulk modulus can be calculated for all rock types. The result indicates that the critical porosity for each rock type is lower than the proposed value from Akbar (2019). Further analysis from the corrected acoustic porosity suggests that the secondary porosity is low, and this observation is supported by low anisotropy data from dipole sonic. The use of shear as opposed to compressional wave data is better suited in this case study, with a clearer trend in determining the critical porosity. The case study highlights the importance of the critical porosity concept in understanding anomalies in the acoustic porosity as an effect of rock quality and explaining the preferred method when determining critical porosity values in a gas-bearing complex carbonate reservoir. Further studies may be needed to investigate the critical porosity relationship for poorer rock types in complex carbonate reservoirs.