The variation mechanism of petrophysical properties and the effect of compaction on the relative permeability of an unconsolidated sandstone reservoir

Firstly, triaxial mechanical experiments, CT scans, and mercury injection experiments were performed to analyze the microstructural characteristics and the macroscopic mechanism of changes in the petrophysical characteristics under different pressure. Secondly, a modified permeability test approach was adopted on reservoir cores in which a constant flow rate was maintained by changing the pore fluid pressure. The factors which affect the porosity and permeability of unconsolidated sandstone under compaction were investigated. Finally, two-phase displacement tests were performed to assess the influence of compaction on oil production. The results demonstrate that the porosity, permeability, and pore-throat size of loose sandstone are reduced with increasing compaction. Although the relative rigidity of the grains makes the reduction in porosity comparatively smaller, the permeability shows a sharp decline and the change is irreversible. The irreducible water saturation and residual oil saturation increase and the position of two-phase flow zone narrows with decreasing porosity and permeability. These changes act to weaken the percolation flow capacity and lead to a decline in oil-well productivity in areas where water injection is not timely or is insufficient. The understanding of compaction mechanism can be used to optimize the development of compacting reservoirs to take advantage of the compaction.