Figure 1.  Methane partial pressure in seals at steady diffusion.  A.  Homogeneous seal. Maximum capillary pressure occurs at the base of the seal and decreases up section.  B.  Two layer heterogeneous seal, where the lower seal layer has lower diffusivity. Pm is lower than the single layer case.  C. Two layer heterogeneous seal, where the lower layer has higher diffusivity. Pm is higher than the single layer case, and may exceed Pd of the lower layer. D.  As the number of alternating higher and lower diffusivity layers increase, deviations from the average Pm trend are less, so it is less likely for Pm to exceed the capillary displacement pressure.