After flowback, the residual fracturing fluid will reduce the gas seepage space and influence natural gas production, which attracts widespread attention. In this study, the irreducible water saturation was investigated, and its controlling factors were clarified. We target the Upper Paleozoic Taiyuan and Shihezi Formations, which belong to a tight gas reservoir in the eastern Ordos Basin. The main experiments include porosity, permeability, mineral composition, nitrogen adsorption, mercury intrusion porosimetry, nuclear magnetic resonance, and high-speed centrifugation. The specific surface area is very low and varies from 0.95 to 4.03 m2/g, and the median pore-throat diameter ranges from 28.6 to 698.6 nm. Through the T2 cutoff value, the water saturation can be divided into movable water saturation (Smov) and irreducible water saturation (Sirr). Furthermore, the Sirr can be divided into water saturation in large pores controlled by the small throat (Sirrl) and water saturation controlled by the capillary force (Sirrc). In both formations, the Sirr has a negative relationship with porosity, permeability, and average pore diameter and exhibits a positive relationship with the specific surface area. The Sirrl has a positive relationship with the median pore-throat diameter in Taiyuan Formation, but the Sirrl has a weak relationship with the median pore-throat diameter in Shihezi Formation. The Sirrc has a negative relationship with porosity, permeability, and average pore diameter and displays a positive relationship with the specific surface area in Taiyuan Formation, but the Sirrc has a weak relationship with these parameters in Shihezi Formation. The relationship difference between Taiyuan and Shihezi Formations was mainly caused by the pore structure, demonstrated by the amplitude ratio in three peaks. Based on the above analysis, this study is conducive to understanding the mechanism of water occurrence and its controlling factors. 

Rock Fabric of Tight Sandstone and Its Influence on Irreducible Water Saturation in Eastern Ordos Basin

Review of volcanic reservoir geology in China

Pore-scale modeling of pore structure properties and wettability effect on permeability of low-rank coal

Diverse effects of intragranular fractures on reservoir properties, diagenesis, and gas migration: Insight from Permian tight sandstone in the Hangjinqi area, north Ordos Basin

Paleo-Environment Reconstruction of the Middle Permian Lucaogou Formation, Southeastern Junggar Basin, NW China: Implications for the Mechanism of Organic Matter Enrichment in Ancient Lake

Fluid mobility has been important topic for unconventional reservoir evaluation. The tight sandstones in Chang 7 Member of the Ordos Basin has been selected to investigate the fluid mobility based on the application of core flooding-NMR combined method and core centrifugation-NMR combined method, and the porous structure is studied using optical microscope, field emission scanning electron microscope (FE-SEM), CT and mercury injection. Our results include: (i) Feldsparrock fragments dissolution pores, calcite dissolution pores, clay mineral dissolution pores, intergranular dissolution expansion pores, inter-granular pores, intra-kaolinite pores, and intra-illite/smectite mixed layer pores are developed in Chang 7 tight sandstones; 3D CT pore structure shows that the pore connectivity is positively related to physical properties, and the overall storage space is connected by the throat with diameter between 0.2 and 0.3 µm. The percentage of storage space connected by throats with diameter less than 100 nm can reach more than 35%. (ii) Movable fluid saturation of Chang 7 tight sandstones is between 10% and 70%, and movable oil saturation is between 10% and 50%. Movable fluid saturation may cause misunderstanding when used to evaluate fluid mobility, so it is recommended to use movable fluid porosity in the evaluation of fluid mobility. The porosity ranging from 5% to 8% is the inflection point of the fluidity and pore structure. For samples with porosity less than 8%, the movable fluid porosity is generally less than 5%. Moreover, the movable fluid is mainly concentrated in the storage space with a throat diameter of 0.1 to 1 µm. For samples with porosity greater than 8%, the porosity of the movable fluid is more than 5%, and the movable fluid is mainly concentrated in the storage space with a throat diameter of 0.2 to 2 µm. (iii) The movable fluid saturation measured by core flooding-NMR combined method is generally higher than that measured by core centrifugation-NMR combined method. The former can evaluate the mobility of the oil-water two-phase fluid in samples, while the latter can better reflect the pore structure and directly evaluate the movable fluid in the pore system controlled by different throat diameters. All these results will provide valuable reference for fluid mobility evaluation in tight reservoirs.

Fluid Mobility Evaluation of Tight Sandstones in Chang 7 Member of Yanchang Formation, Ordos Basin

Unconventional volcanic reservoir is different from conventional reservoir in reservoir space, diagenesis, pore formation and evolution. The Carboniferous volcanic reservoir was selected in Junggar Basin, Northwest China because based on sediment/rock cores and outcrop data, diagenesis and pore evolution were studied by elemental measurements, thin section observations, and diagenetic analyses. These analyses shows that the reservoir lithology is predominantly intermediate-basic volcanic, and the reservoir storage space is composed mainly of secondary dissolved pores and fractures. The reservoir displays great heterogeneity, and has experienced a great variety of diagenetic alteration during various diagenetic stages including: (1) eruption fragmentation, crystallization differentiation and condensing consolidation at consolidated diagenetic stage; (2) metasomatic alteration, filling, weathering and leaching, dissolution by formation fluids and tectonism at the epigenetic modifications stage. The formation and evolutionary process of the pores is extremely complicated. The primary pores were formed during the consolidated diagenetic stage, and laid a foundation for the late development and alteration of effective reservoir. During the epigenetic modifications stage secondary reservoir storage space was developed via the formation of secondary pores and the development of fractures through weathering and leaching, dissolution by formation fluids and tectonism.

Characteristics of Diagenesis and Pore Evolution of Volcanic Reservoir: A Case Study of Junggar Basin, Northwest China

Three-dimensional imaging of fracture propagation in tight sandstones of the Upper Triassic Chang 7 member, Ordos Basin, Northern China

Isotopic geochemical characteristics of two types of carbonate concretions of Chang 7 member in the middle-upper Triassic Yanchang Formation, Ordos Basin, Central China

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Fluid Mobility Evaluation of Tight Sandstones in Chang 7 Member of Yanchang Formation, Ordos Basin

Characteristics of Diagenesis and Pore Evolution of Volcanic Reservoir: A Case Study of Junggar Basin, Northwest China

Three-dimensional imaging of fracture propagation in tight sandstones of the Upper Triassic Chang 7 member, Ordos Basin, Northern China

Isotopic geochemical characteristics of two types of carbonate concretions of Chang 7 member in the middle-upper Triassic Yanchang Formation, Ordos Basin, Central China