Experimental study of permeability of salt rock and its application to deep underground gas storage

In order to provide reliable data for evaluating the integrity of gas storage caverns constructed in deep salt formations(≥1 500 m), experiments have been conducted to assess the effect of deformation on the permeability of the impure salt rock samples from Jiangsu province, China. The transient method of step-decay of Argon gas was used in conjunction with the conventional triaxial compression tests with the samples incrementally deformed and the permeability measured at the same time. The tests were performed at room temperature and at the effective confining pressures of 20 MPa in Utrecht University, Netherlands. It was found that the permeability of relatively pure salt samples decreased from 10^-16 m² to below 10^-21 m² with the increasing of differential stress(0-40 MPa), presumably due to the crack closure and the porosity reduction. The permeability of the samples with high impurity content was 1-2 orders higher than that of the pure salt rock, which was interpreted to be due to the more effective compaction occurred in the pure salt. Using the concept of dilatancy boundary, the stress state around the spherical storage caverns in the deep subsurface is analyzed;and it is found that the stress state in the rock wall remained completely in the non-dilatant field(region of compression). It could be deduced that even under low-pressure conditions of cavern operation, dilatancy would not easily occur in the cavern walls, implying the high sealing integrity of deep storage cavern in salt rock.