Mechanism Of Micro Flow In Low-Permeability Sandstone Reservoirs

Low-permeability reservoirs display such characteristics as narrow pore throats, complex pore structure, large specific surface and rich clay minerals. These characteristics will result in strong micro flow effects when fluid flow in low-permeability reservoirs. Because of micro flow effects, it’s difficult to describe flow in Low-permeability reservoirs using Darcy’s Law. Thus, a new theory which can accomplish better descriptions is in urgent need.Pore structure characteristics, interaction between fluids and pore walls are key factors in studying micro flow in Low-permeability reservoirs. The pore model reconstructed from Micro-CT images is the most accurate model. However, the pore segmentation process is difficult, for low-permeability sandstone Micro-CT gray level image showed a unimodal distribution. After analyzed Micro-CT imaging principle, reasons of unimodal distribution are believed to be low porosity, main core components of quartz and feldspar as well as noises. On this basis, a new image preprocessing method has been presented.Pore segmentation and representative element volume analysis based on porosity were carried out. Isolated void space in the three-dimensional image was defined and clear, which can reduce computer storage and computational capabilities in reconstruction and simulation process. Pore models of low-permeability sandstone were reconstructed by Marching Cubes method algorithm and Delaunay triangulation algorithm. STL and OFF format were selected to be interface files for numerical simulation.Because of Interactions between mineral and oil or water, boundary layer which changes fluid viscosity in low-permeability reservoirs has been produced. Fluid viscosity in low-permeability reservoirs is the function of bulk fluid, Porous media properties, and driving pressure gradient.Through the micro-channel flow experiments and theoretical analysis, boundary slip exists for non-wetting phase, and slip length increases with contact angle. However, the experiment did not show evidence that slip length has any relationship with contact angle for non-wetting phase.The occurrences of clay minerals have significant impacts on micro flow in low-permeability reservoirs. Dispersed particle occurrence can cause Local resistance which depends on the match relationship between Pore throat size and kaolinite size. Frictional resistance, controlled by size and shape of illite, and pressure resistance are produced by Bridge occurrence. Thin film occurrence can give rise to frictional drag which is affected by microstructures of chlorite such as size, shape, distribution, and topology.After discussed the scope of applications about molecular dynamics theory, Lattice Boltzmann Method, and Navier-Stokes Equations, it is believed Navier-Stokes Equations can describe liquid flow in low-permeability sandstone reservoirs. Considering the effects of boundary layer and clay minerals occurrences in low permeability sandstone reservoirs, the permeability of pore models in three directions is calculated on the basis of Navier-Stokes Equations with Finite Element Method. And then a numerical simulation study about the effects of boundary fluid relative thickness and viscosity in low-permeability reservoirs has been carried outAfter identifying the main influencing factors by using micro flow mechanism combined with pore structure characteristics, specific technologies can be used to develop low-permeability reservoirs with high efficiency.