Advances and Challenges in the Characterization of Micro-pore Structures in Tight Sandstone Reservoirs for Unconventional Oil and Gas Exploration

Abstract

Tight sandstone oil and gas exploration represents one of the pivotal areas in the field of unconventional hydrocarbon resources. The characterization of micro-pore structures within these reservoirs is crucial for understanding the seepage mechanisms and mastering the enrichment patterns of oil and gas. With the evolving focus of micro-pore structure research, significant advancements have been made in recent years in the development of characterization techniques for tight sandstone reservoirs. This paper reviews the progress in two major stages: (1) qualitative-semi-quantitative research, which emphasizes micro-fractures, reservoir properties, wettability, sensitivity, water flooding and oil-water relative permeability, pore and throat types, sizes, and distributions, primarily utilizing scanning electron microscopy (SEM), thin section observation, X-ray diffraction (XRD), and conventional mercury injection capillary pressure (MICP) tests; and (2) semi-quantitative-quantitative research, which shifts towards the quantitative characterization of pore and throat planar distributions, cement types, and cementation styles, employing techniques such as environmental SEM, nuclear magnetic resonance (NMR), constant-rate mercury injection, and digital core characterization based on two-dimensional SEM images and three-dimensional computed tomography (CT) scans. Despite these advancements, three primary challenges remain: (1) balancing the precision and scale in the characterization of pore structures in tight sandstone reservoirs; (2) achieving breakthroughs in the combined characterization of full-pore-size distribution; and (3) enhancing the integration of digital core technology with artificial intelligence (AI). Looking ahead, the characterization of micro-pore structures in tight sandstone reservoirs should prioritize the following aspects: (1) investigating the dynamic variations of pore-throat structures in response to decreasing fluid pressure during reservoir development, with a focus on the gradual contraction of fine and complex pore-throat systems; and (2) exploring methods to enhance oil recovery by lowering the threshold of movable throat radii, based on the control of pore-throat structures over seepage capacity and hydrocarbon mobilization in tight reservoirs.