Non-Darcy flow in oil accumulation (oil displacing water) and relative permeability and oil saturation characteristics of low-permeability sandstones

Petroleum Science, Feb 2010

Hydrocarbon resources in low-permeability sandstones are very abundant and are extensively distributed. Low-permeability reservoirs show several unique characteristics, including lack of a definite trap boundary or caprock, limited buoyancy effect, complex oil-gas-water distribution, without obvious oil-gas-water interfaces, and relatively low oil (gas) saturation. Based on the simulation experiments of oil accumulation in low-permeability sandstone (oil displacing water), we study the migration and accumulation characteristics of non-Darcy oil flow, and discuss the values and influencing factors of relative permeability which is a key parameter characterizing oil migration and accumulation in low-permeability sandstone. The results indicate that: 1) Oil migration (oil displacing water) in low-permeability sandstone shows non-Darcy percolation characteristics, and there is a threshold pressure gradient during oil migration and accumulation, which has a good negative correlation with permeability and apparent fluidity; 2) With decrease of permeability and apparent fluidity and increase of fluid viscosity, the percolation curve is closer to the pressure gradient axis and the threshold pressure gradient increases. When the apparent fluidity is more than 1.0, the percolation curve shows modified Darcy flow characteristics, while when the apparent fluidity is less than 1.0, the percolation curve is a “concaveup” non-Darcy percolation curve; 3) Oil-water two-phase relative permeability is affected by core permeability, fluid viscosity, apparent fluidity, and injection drive force; 4) The oil saturation of low-permeability sandstone reservoirs is mostly within 35%–60%, and the oil saturation also has a good positive correlation with the permeability and apparent fluidity.

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Non-Darcy flow in oil accumulation (oil displacing water) and relative permeability and oil saturation characteristics of low-permeability sandstones

Pet.Sci. Non-Darcy flow in oil accumulation (oil displacing water) and relative permeability and oil saturation characteristics of low-permeability sandstones Zeng Jianhui 1 2 Cheng Shiwei 1 2 Kong Xu 0 2 Guo Kai 1 2 Wang Hongyu 1 2 0 CNPC Greatwall Drilling Company , Beijing 100101 , China 1 School of Natural Resources and Information Technology, China University of Petroleum , Beijing 102249 , China 2 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum , Beijing 102249 , China Hydrocarbon resources in low-permeability sandstones are very abundant and are extensively distributed. Low-permeability reservoirs show several unique characteristics, including lack of a definite trap boundary or caprock, limited buoyancy effect, complex oil-gas-water distribution, without obvious oil-gas-water interfaces, and relatively low oil (gas) saturation. Based on the simulation experiments of oil accumulation in low-permeability sandstone (oil displacing water), we study the migration and accumulation characteristics of non-Darcy oil flow, and discuss the values and influencing factors of relative permeability which is a key parameter characterizing oil migration and accumulation in low-permeability sandstone. The results indicate that: 1) Oil migration (oil displacing water) in lowpermeability sandstone shows non-Darcy percolation characteristics, and there is a threshold pressure gradient during oil migration and accumulation, which has a good negative correlation with permeability and apparent fluidity; 2) With decrease of permeability and apparent fluidity and increase of fluid viscosity, the percolation curve is closer to the pressure gradient axis and the threshold pressure gradient increases. When the apparent fluidity is more than 1.0, the percolation curve shows modified Darcy flow characteristics, while when the apparent fluidity is less than 1.0, the percolation curve is a “concaveup” non-Darcy percolation curve; 3) Oil-water two-phase relative permeability is affected by core permeability, fluid viscosity, apparent fluidity, and injection drive force; 4) The oil saturation of lowpermeability sandstone reservoirs is mostly within 35%-60%, and the oil saturation also has a good positive correlation with the permeability and apparent fluidity. Non-Darcy flow; relative permeability; oil saturation; low-permeability sandstone 1 Introduction Actually, low-permeability sandstone is a relatively indistinct concept, which has not been strictly and precisely defined internationally yet. Berg (1975) suggested that the upper limit of low-permeability sandstone was 1×10-3 μm2-10×10-3μm2. The classification standards for reservoir properties of clastic rock and non-clastic rock issued by the Chinese National Committee on Mineral Reserves define a reservoir with porosity between 10% and 15% and permeability between 5×10-3μm2 and 50×10-3μm2 as lowporosity and low-permeability reservoir, while that with porosity less than 10% and permeability less than 5×10-3μm2 as super-low-porosity and super-low-permeability reservoir (Li, 1997) . The oil and gas resources preserved in low-permeability sandstone are very abundant and are extensively distributed all over the world. They can be found in almost every oilproducing country. A big quantity of low-permeability sandstone deep-basin gas reservoirs were discovered in the Los Angeles Basin and Powder River Basin of the US, Carpathians, Krasnodar, Urals-Volga and West Siberian oil province of the former Soviet Union, and the Alberta Basin in the west of Canada (Surdam, 1995; 1996; Law, 2002; Ayers, 2002; Williams et al, 1998) . In China, low-permeability sandstone reservoirs are common in almost all the oil and gas bearing basins, especially in the Ordos, Sichuan, Junggar, Tarim and Songliao Basins. According to the statistics of Jiang et al (2004 ), the petroleum resources in China amount to 940×108 t, of which 210.7×108 t (22.41%) occur in low-permeability sandstone. Among the newly increased proven reserves in recent years, low-permeability reservoir hydrocarbon reaches 70%, and the proportion is increasing. Low-permeability sandstone reservoirs have many different characteristics from conventional reservoirs. They are mainly distributed in the slope and syncline locations in basins without a definite trap boundary or caprock. Their reservoir rocks have low porosity and permeability or super low porosity and permeability (porosity less than 10% and permeability between 10-3 and 10-12 μm2). Hydrocarbon resources are extensive but unevenly distributed inside or near the source area with no migration or primary migration due to limited buoyancy effect. Under abnormal pressure (high or low pressure) and with no clear gravitational differentiation, oil-gas-water distribution is complex and hydrocarbon is locally accumulated with relatively low oil (gas) saturation (Law and Curtis, 2002; Ayers, 2002; Williams et al, 1998; Pang et al, (...truncated)


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Jianhui Zeng, Shiwei Cheng, Xu Kong, Kai Guo, Hongyu Wang. Non-Darcy flow in oil accumulation (oil displacing water) and relative permeability and oil saturation characteristics of low-permeability sandstones, Petroleum Science, 2010, pp. 20-30, Volume 7, Issue 1, DOI: 10.1007/s12182-010-0003-2