Effect of cyclic formation-pressure changes on permeability

Unconsolidated sandpacks of various mesh sizes and consolidated Berea sandstone cores were subjected to repeated pressurization/depressurization cycles under constant confining pressure, after which their absolute and relative permeabilities were measured during the relaxation periods. The permeabilities of the sandpacks decreased as a result of the pressurization, and the reduction in permeability increased as the magnitude and duration of the applied net core pressure increased. The permeabilities continued to decrease with successive pressurization/depressurization cycles, albeit at a decreasing rate. After a finite number of cycles, no further reduction was observed. The number of cycles needed for stabilization was inversely proportional to the duration of the pressurization cycles and was found to be lower for fine sand than for coarser sand. Some recovery in permeability was achieved after the cores were allowed to relax; however, the cores were permanently damaged. Berea sandstone cores also exhibited reductions in permeability as a result of pressurization/depressurization cycling.