Physical Properties and Seepage Characteristics of Unconsolidated Sandstone under Re-Compaction

The formation pressure of unconsolidated sandstone reservoir drops in the development process which damages the reservoir and affect oil well productivity. It is necessary to study the physical properties and seepage characteristics of unconsolidated sandstone under re-compaction. First, a continuous testing method was adopted to measure the physical property of pay zone core under re-compaction in the way of constant driven flow rate and variable inner pressure, the variation laws of physical properties of unconsolidated sandstone under compaction were discussed at the macroscopic level. Then the evolution characteristics of the pore structure were further elaborated under compaction from microscopic point of view through mercury penetration experiment. Finally, the seepage feature under compaction was studied by the fluid variation experiment. The permeability of unconsolidated sandstone was reduced continuously with the enhancement of re-compaction, which represented a decline of 53%. When the porosity was reduced 7% or so, a majority of the pore was transformed into pore throats under compaction, the pore-throat volume ratio increased from 1.5 to 1.96, and the pore radius corresponding to peak value reduced to 50% of the original value, which caused the permeability to decline permeability more than the porosity. With the continuous compaction, oil-water two phase permeability reduced to nearly 50%, the residual oil saturation increased from 17.8% to 19.2% and irreducible water saturation increased from 18.5% to 21.2%.The research results show that with the increase of compaction, the pore and throat of unconsolidated sandstone are all compressed, leading to poor reservoir physical properties, and while the porosity drops slightly, the permeability drops in a power function, and the permeability was not recovered in the process of formation pressure recovery. Irreducible water saturation and residual oil saturation increase exponentially with the increase of effective stress, and oil phase permeability is linear downward with the increase of effective stress. Consequentely, this is the cause of decrease of oil well productivity due to insufficient water injection or injection water delay.