Progress on Flow Mechanism in Low Permeability Formation

Some progress concerning low permeability flow in pore structure and interaction with formation fluid, micro-scale effect, threshold pressure gradient, relative permeability curve is reviewed. Pore structure and distribution patterns have no distinct difference in different permeability rocks while pore-throats have. In low permeability formation conventional Newtonian fluid showed non-Newtonian characteristics which performed as no-Darcy flow. The existence of threshold pressure gradient was questioned and denied because of the drawbacks of experiment procedure and apparatus, as well as great success of tight oil reservoirs development without considering threshold pressure gradient. Smaller diameter of microtubule leads to higher degree of nonlinearity. Effects of solid-fluid interaction to percolation should be noticed in low permeability formation. One well-accepted model indicated that solid liquid interface potential has a positive relationship with reciprocal of permeability or pore radius, thus interface force becoming too large to neglect when permeability or pore radius decreased to a certain value. Boundary effects fluid started to flow with overcoming surface molecule force only when displacement pressure drop increased to a certain degree, which accounts for the threshold pressure gradient. Interfacial tension was the key factor effecting fluid interface shape and thus having a significant influence on percolation mechanism.