Abstract
The Chang 81 reservoir in the Maling Oilfield, Ordos Basin, China, is featured by complex microscopic pore structure and obviously different fluid distribution characteristics. It is a typical low-porosity and low-permeability reservoir. To analyze the occurrence characteristics and main controlling factors of movable fluids in Chang 81 reservoir, this study carries out NMR-based quantitative analysis, which is combined with several microscopic experiments involving analyses of conventional physical properties, image granularity, casting thin sections and electron microscope scanning. There are mainly three types of T2 spectrum curves in terms of the shape, namely left-high-peak–right-low-peak, left-low-peak–right-high-peak, and unimodal shapes. Permeability is well correlated with movable fluid saturation, and larger physical property variation results in more significant movable fluid saturation variation. The main controlling factors that contribute to the obviously different occurrence characteristics of movable fluids are demonstrated to be pore type, pore–throat radius, pore–throat radius ratio, sorting coefficient, effective pore–throat volume, and clay mineral filling. Among them, pore and throat radii and pore–throat radius ratio are more dominant factors, followed by effective pore–throat volume and sorting coefficient. The clay mineral filling, pore type and physical property have the least influence on the movable fluid saturation.
Highlights
Movable fluid saturation, referring to a critical saturation value measured by nuclear magnetic resonance (NMR) experiments, can royally reflect occurrence characteristics of fluids in the pore structure (Gao et al 2015; Yang et al 2013; Zhang et al 2015)
Nuclear magnetic resonance experiments show that movable fluid saturation values of 13 core samples range from 18.85 to 80.59% with an average of 42.06%
Through analysis of the main controlling factors that influence the occurrence characteristics of movable fluids, it is found that physical properties have certain influence on the movable fluid saturation in the lowpermeability reservoir
Summary
Movable fluid saturation, referring to a critical saturation value measured by nuclear magnetic resonance (NMR) experiments, can royally reflect occurrence characteristics of fluids in the pore structure (Gao et al 2015; Yang et al 2013; Zhang et al 2015). NMR technology is mainly applied in two fields, namely the indoor rock sample analysis and the logging interpretation (Bai et al 2016; Zhou et al 2016) The former application can reflect NMR response characteristics, NMR porosity and NMR permeability of different types of reservoirs (Cao et al 2016). It can be combined with centrifugal experiments to calibrate T2 cut-off value of movable fluids, while it can be combined with displacement experiments to study oil–water and gas–water seepage mechanisms (Ren et al 2015) The latter application can precisely provide information about reservoir physical properties, including effective porosity, permeability, bound water saturation, and pore size distribution, according to the resonance signals generated by the hydrogen atoms in the reservoir fluid in the magnetic field. In the evaluation of low-permeability tight sandstone reservoirs, the conventional parameters (e.g. reservoir thickness, continuity, distribution, porosity and permeability) and the movable fluid properties should be highlighted
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