Abstract
In order to reveal the gas–water distribution and formation mechanism of the low-permeability sandstone gas reservoir, the gas reservoir distribution and the formation mechanism in a low-permeability sandstone gas reservoir are investigated using data obtained from a physical simulation experiment of gas percolation. The exploration and experimenting for petroleum in the upper Paleozoic gas pool of the Sulige gas field in the Ordos basin in this paper. Results showed that the gas reservoir is characterized by low gas saturation, a complex distribution relationship of gas–water, and weak gas–water gravity differentiation. The characteristics of gas distribution are closely related to permeability, gas flow, and migration force. The capillary pressure difference is the main driving force of gas accumulation. There exists a threshold pressure gradient as gas flows in low-permeability sandstone. The lower that permeability, the greater the threshold pressure gradient. When the driving force cannot overcome the threshold pressure (minimal resistance), the main means of gas migration is diffusion; when the driving force is between minimal and maximal resistance, gas migrates with non-Darcy flow; when the driving force is greater than maximal resistance, gas migrates with Darcy flow. The complex gas migration way leads to complicated gas- water distribution relationship. With the same driving force, gas saturation increases with the improvement of permeability, thus when permeability is greater than 0.15 × 10−3 µ m2, gas saturation could be greater than 50%.
Highlights
A low-permeability sandstone petroleum reservoir is usually characterized by low porosity, a tiny pore throat, low productivity, and poor permeability
Natural Gas Industry Standards (Oil–Gas Reservoir Evaluation) (SY-T6285-1997), low-permeability sandstone reservoirs containing oil are divided into low-permeability (50 × 10−3 to 10 × 10−3 μm2 ), very low-permeability (10 × 10−3 to 1 × 10−3 μm2 ), ultralow-permeability (1 × 10−3 to 0.1 × 10−3 μm2 ), Energies 2020, 13, 1774; doi:10.3390/en13071774
When gas migrates in the sandstone reservoirs, it is a process of the nonwetting phase displacing the wetting phase
Summary
A low-permeability sandstone petroleum reservoir is usually characterized by low porosity, a tiny pore throat, low productivity, and poor permeability. There are differences in the partitioned standard of low-permeability reservoirs in different countries. A low-permeability reservoir was defined by the former Soviet Union as one with permeability of less than (50–100) × 10−3 μm2 [5], and by the Federal Energy Regulatory Commission (FERC) in the United States as less than 0.1 × 10−3 μm2 [4]. Natural Gas Industry Standards (Oil–Gas Reservoir Evaluation) (SY-T6285-1997), low-permeability sandstone reservoirs containing oil are divided into low-permeability (50 × 10−3 to 10 × 10−3 μm2 ), very low-permeability (10 × 10−3 to 1 × 10−3 μm2 ), ultralow-permeability (1 × 10−3 to 0.1 × 10−3 μm2 ), Energies 2020, 13, 1774; doi:10.3390/en13071774 www.mdpi.com/journal/energies. A reservoir with permeability of less than 10 × 10−3 μm was deemed to be a low-permeability sandstone reservoir
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