Understanding the permeability of burrow-related gas reservoirs through integrated laboratory techniques

Abstract

This study integrates results from petrographic investigations, spot permeability measurements, computed tomography (CT) scans, and core water flooding experiments to investigate the potential correlation between the spot permeability and the bulk permeability of burrow-related reservoirs. These results are obtained from samples collected from Thalassinoides-bearing strata of the upper Jurassic Hanifa Formation, central Saudi Arabia, and provide insights to select the appropriate permeability values that represent the bulk permeability of burrow-related reservoirs. The petrographic results show a mud-dominated matrix with isolated moldic pores and grain-dominated Thalassinoides passive fills with substantial interparticle pores and minor moldic and interskeletal pores. The spot permeability of the passive fills of the Thalassinoides network ranges from 6 mD to 628 mD, whereas the spot permeability of the matrix surrounding the Thalassinoides network ranges from 0.1 mD to 5 mD. These results indicate that the passive fills of the Thalassinoides network, which is fully represented in the CT scans of the 9.75-cm diameter core, are substantially permeable compared to the tight matrix, and suggest a dual-porosity system in the studied strata. The bulk permeabilities estimated from the harmonic, arithmetic, and geometric means vary considerably (0.7 mD, 28.9 mD, and 1.8 mD, respectively). The CT scan observations of the investigated core suggest a low to moderate bioturbation intensity (mean = 15.8% ± 6.2%), while the core water flooding results suggest that the bulk permeability of the core is 17.3 mD. Previous research has indicated that harmonic and geometric means should be considered the best estimates for bulk permeability at low bioturbation intensities in Thalassinoides-related reservoirs. Nevertheless, the core water flooding results are close to the arithmetic mean, indicating that after establishing burrow connectivity, the fluid flow behavior is similar to that in a continuous parallel permeability domain, irrespective of bioturbation intensity. Therefore, this study suggests that the burrow connectivity should be considered in bulk permeability estimation when linking such estimation to bioturbation intensity.