Combining petrophysical properties and ultrasonic velocity for improved prediction of tight carbonate reservoir

Modeling of the petrophysical properties in tight carbonate reservoirs is important for better characterization of reservoir quality. This study aims to investigate the relationship between reservoir properties (porosity and permeability) and ultrasonic P-wave velocity of carbonate samples from different sedimentary lithofacies. In this context, different carbonate lithofacies of a wide range of rock lithification were selected in order to examine and model their reservoir properties and ultrasonic velocity under multiphase confining pressure simulation. The carbonate lithofacies were selected from the Miocene Dam Formation carbonates in Eastern Saudi Arabia. Three lithofacies were selected for this purpose, those are, (I) The stromatolite boundstone, (II) The burrowed wackestone, and (III) The quartz fossiliferous wackestone-packstone. The nature and field setting of these lithofacies reflects wide range of sedimentary texture, lithification, and therefore reservoir properties. Besides porosity and permeability models, ultrasonic P-wave velocity models were generated for each carbonate lithofacies under a wide range of confining pressure. In addition to the petrography, further investigations were used using SEM, and micro-CT scan imaging in order to demonstrate the effect of lithofacies patterns on the petrophysical properties. The petrographic analysis, SEM, and micro-CT scan revealed that the sedimentary fabric, composition, internal structure, and diagenesis have influenced the porosity and permeability patterns of the different lithofacies, and therefore the ultrasonic p-wave velocity. The homogeneous composition of the burrowed wackestone in terms of grain sorting, shape and stability clarify its steady petrophysical relationships, however, its weakest lithification has affected this relationship in higher confining pressure (3MPa and 6MPa). In contrast, the highly lithified stromatolite boundstone and quartz fossiliferous wackestone-packstone revealed better relationship with higher confining pressure. This study showed that integration of petrophysical properties and ultrasonic P-wave velocity under multiphase confining pressure can provide guides that might lead to better understanding and prediction of the tightness of carbonate reservoir with less degrees of uncertainty.