Multiscale and diverse spatial heterogeneity analysis of void structures in reef carbonate reservoirs

The complex structure of carbonate reservoirs significantly impacts the development of oil reservoirs. Most published studies focus on void systems in sandstones and shales, while carbonates have received little attention. In this study, we investigated five carbonate oil reservoirs located in Perm Krai, Russia. Using thin-section observation, SEM, XRD, XRF spectroscopy, and computed microtomography (μ-CT), we evaluated the microscopic structure of the reservoirs' void space and assessed its impact on the performance of acid stimulation and hydraulic fracturing. The examination of core samples shows that they are monomineral, composed predominantly of calcite (≈99%). The core samples have similar lithological compositions, but the structure of their void space differs significantly, presenting a variety of shapes, sizes and distributions in the sample volume. SEM analysis reveals the presence of nanofractures and microfractures that are open, partially filled, or completely filled with calcite. With an average of 6.5%, the porosity of the carbonate objects shows little variation; however, their permeability varies substantially, from 11.5 to 628.9 ∙103 μm2. In addition, our study established that the structure of the void space has a considerable effect on the fluid inflow into the well and on the efficiency of acid stimulation and hydraulic fracturing. During acid stimulation or hydraulic fracturing in reservoirs with nano-fractured zones, a sharp deviation in the development of wormholes or fractures is possible. Therefore, the effectiveness of these treatments cannot reliably be predicted when nanofractures are present.