Experimental study of sealing performance: Effects of particle size and particle‐packing state on threshold pressure of sintered compacts

Threshold pressure Pcth (kPa), a parameter controlling rock's sealing performance in cases of CO2 geological sequestration, was studied in a supercritical CO2-water system under conditions of 1000 m depth (10 MPa and 40°C). With respect to correlation between Pcth and another important parameter of permeability, k (mdarcy), the closest-packing structure of spherical particles is defined theoretically as a line having slope of −0.5 on a double logarithmic plot. This study found this line by measuring Pcth of a capillary plate with known throat diameter as Pcth = 377.7 k– 0.5. This function is directly applicable to different depth and salinity conditions, although it requires minor correction at some temperature conditions. As a first step in determining the range of variation of rocks' Pcth and k from internal particle structures, we used sintered compacts of uniform spherical silica particles with diameters of 0.1–10 µm. Results show that measured values of sintered compacts were scattered around the closest-packing line, with the difference of packing state depending on different sintered additives and sintering temperatures. The change of packing state, occurring independently of resultant changes of porosity, varied sensitively according to Pcth. Therefore, Pcth is inferred to depend strongly on the local structure within rocks. The Knudsen number estimated for supercritical CO2 at 1000 m depth indicated that CO2 transmuted into a noncontinuum at k < 0.1 µdarcy in the closest-packing structure. Future studies should consider whether the concept of Pcth is applicable in this situation.