Influence of micro-pore structure in tight sandstone reservoir on the seepage and water-drive producing mechanism—a case study from Chang 6 reservoir in Huaqing area of Ordos basin

Abstract

To study the influence of pore structure on the seepage and water-drive producing mechanisms, various methods were combined to describe the micro-pore structure in the Chang 6 tight sandstone reservoir in the Huaqing area, Ordos Basin, China. Nuclear magnetic resonance (NMR) was combined with displacement experiments to determine the distribution of oil and water in pores of different scales before and after water flooding. There are few micro pores in the reservoir. As permeability increases, the distribution of nano pores decreases, while sub-micro pores increase. Also, sub-micro pores are the main pathway within the reservoir. There is a negative power function correlation between the minimum starting pressure gradient of the oil (Swc) and maximum throat radius. Also, with a decrease of permeability, smaller pore throats become more abundant and the nonlinear section of the flow velocity-differential pressure curves increase. There is a large amount of crude oil gathering in the nano pores. As permeability increases, the main sources of movable oil are from the nano pores (kg< 0.4 × 10−3μm2), sub-micro pores (kg ≈ 0.4 × 10−3μm2-1.0 × 10−3μm2), and micro pores (kg > 1.0 × 10−3μm2). Displacement efficiency is always the highest in sub-micro pores.