A review on pore structure characterization in tight sandstones

Tight sandstone reservoirs typically contain a wide pore throat sizes ranging from the nano-scale to micro-scale, and have complex pore geometry and pore throat structure. Microscopic pore throat structures are the most important factors affecting the macroscopic reservoir quality and fluid flow in tight sandstones. Evaluation and characterization quantitatively the microscopic pore structures, including pore geometry, pore size distribution, and pore connectivity, are of great importance for maintaining and enhancing petroleum recovery. This paper critically reviews the pore throat structures of tight sandstones, as assessed from peer reviewed papers in the literature as well as from the authors' personal experiences, in the particular contexts of comprehensive characterization and description of the entire pore throat structure using various complementary techniques. The depositional controls and diagenetic imprints on reservoir quality and pore structure are firstly discussed. The pore systems including pore throat type, pore geometry, pore size and connectivity, which are related to the depositional attributes and diagenetic modifications, are summarized. Then the theories and procedures of various testing techniques commonly used for pore structure characterization of tight sandstones are reviewed. Additionally, the pore throat structure characteristics in tight sandstones are obtained from various techniques such as MICP, NMR, N(2)GA and XCT. Pore throat distribution and capillary parameters of tight sandstones are examined, and the relationship between pore throat size distribution and permeability is overviewed. The pore size distribution and 3D pore connectivity are evaluated from NMR and XCT analysis. The NMR spectrum is also linked to the macroscopic performance, and the pore network is determined from N(2)GA. Then fractal theory is introduced to explain the irregularity and heterogeneity of pore throat structure characteristics, and the models for fractal dimension calculation through various techniques are summarized. Lastly the integration of various techniques is encouraged to fully characterize the entire pore size spectrum in tight sandstones by considering the complex pore structures and limitations of a single experiment in pore throat structure evaluation. This review will provide important insights into the microscopic pore structure characteristics of tight sandstones, and address the gap in comprehensive and quantitative evaluation of the heterogeneity in tight sandstones with complex microscopic pore structures.