Abstract
The uncertainties between reservoir quality and gas migration and accumulation in tight sandstone gas reservoirs are intrinsically attributed to complex microscopic pore structures. Integrated analysis including the physical simulation experiment of gas migration and accumulation, X-ray computed tomography (X-CT), and casting thin section (CTS) were conducted on core plug samples collected from the Upper Paleozoic Permian Lower Shihezi and Shanxi tight sandstone of the Daniudi area in the Ordos Basin to investigate the impacts of pore structure on the gas migration and accumulation. Physical simulation suggested that the gas flows in migration in tight sandstone reservoirs were characterized by deviated-Darcy linear flow and non-linear flow regimes. Minimum and stable migration pressure square gradients determined by application of apparent permeability were employed as key parameters to describe gas flow. Pore structure characterization revealed that the tight sandstone reservoir was characterized by wide pore and throat size distributions and poor pore-throat connectivity. The pore–throat combinations could be divided into three types, including the macropore and coarse throat dominant reservoir, full-pore and full-throat form, and meso-small pore and fine throat dominant form. Comparative analyses indicated that pore and throat radii determined the gas flow regimes by controlling the minimum and stable migration pressure gradients. Gas accumulation capacity was dominated by the connected effective porosity, and the gas accumulation process was controlled by the cumulative effective porosity contribution from macropores to micropores. Variations in pore structures resulted in differences in gas migration and accumulation of tight sandstone reservoirs. The macropore and coarse throat-dominant and the full-pore and full-throat reservoirs exhibited greater gas migration and accumulation potentials than the small pore and fine throat dominate form.
Highlights
Tight gas has attracted worldwide attention since the exploration of the large Cretaceous Balnco low-permeability gas field in the San Juan Basin in North America
The seepage curves revealed that gas flow in migration deviated from Darcy flow and was characterized by an evident “threshold pressure gradient”, which represented the additional pressure gradient required for overcoming the resistance and starting fluid flow (Figure 4)
Two flow regimes were found in the gas migration, including a concave seepage curve with non-linear flow in low-velocity stage and linear flow in relative high-velocity stage that can be described by piecewise curves revealed that gas flow in migration deviated from Darcy flow and was characterized by an evident “threshold pressure gradient”, which represented the additional pressure gradient required for overcoming the resistance and starting fluid flow (Figure 4)
Summary
Tight gas has attracted worldwide attention since the exploration of the large Cretaceous Balnco low-permeability gas field in the San Juan Basin in North America. Tight sandstone gas has been discovered in more than 70 basins worldwide, which are mainly distributed in North America, Europe, and Asia. The exploration of tight sandstone gas has developed rapidly in the past decades in China, and prolific tight gas reservoirs have been discovered in the Ordos Basin, Sichuan Basin, and Songliao Basin [6,7,8,9]. Tight sandstone gas is generally referred to as the tight sandstone reservoirs with complex gas and water distributions, low gas saturation, and continuous accumulation characteristics [7]. The tight gas sandstone reservoir is defined as gas-bearing sandstone reservoirs with porosity
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