Study of void space structure and its influence on carbonate reservoir properties: X-ray microtomography, electron microscopy, and well testing

Many carbonate reservoirs containing significant hydrocarbon resources are characterized by complex pore structures and, as a result, heterogeneous distributions of fluids in their pore networks. Accurately assessing the microscopic pore structure characteristics of carbonate rocks and their influence on reservoir properties is essential for the successful development of oil fields. In the studied carbonate reservoirs of three oil fields in the Perm Krai, Russia, complex tectonic activity and multi-stage diagenetic modification have resulted in the for-mation of a heterogeneous pore network spectrum ranging from macropores to nanometer-scale (<1 mu m) pores. Thin sections and computed tomography were applied to obtain the petrography of carbonate rocks, 2D images of a wide range of pores, and a 3D representation of the pore network. Well testing was used to study the pore network structure at the macrolevel. Combining well testing and laboratory-based core studies allowed the structural features of the reservoirs of the three fields under consideration to be characterized in high detail. The presence of magnesium in the mineral composition of the studied limestone decreased its capacitive charac-teristics. We identify three fundamentally different void space scenarios: (i) void spaces formed only by primary intercrystalline pores, (ii) much larger secondary pores are also present, and (iii) primary and secondary voids connected by a network of partially healed fractures. Such variations in the structure of void space contribute to varying reservoir behavior. The presence of larger voids and, accordingly, higher initial permeability contributes to intense deformation of the void space and permeability decreases with decreasing pressure. The established patterns in this work explain the characteristic dynamics of well flow rates during field development.