Simulation Method for Variable Permeability of Delay Interbed Based on Stress Variation Principle

The compaction simulation of compressible delay interbed is an important part of land subsidence simulation. Currently, the most widely used MODFLOW software has two modules, SUB and CSUB, both of which can simulate compressible delay interbed. The difference lies in that the head diffusion equation of the SUB module is based on the principle of head change, while CSUB can use either head change or geological stress variation principles. When based on the principle of geostress variation, the CSUB method is more physically reasonable. However, its limitation lies in the fact that, when solving the diffusion equation for compressible delay interbeds, it does not account for the effects of variations in the discrete nodal cell thickness and hydraulic conductivity of the interbed. This study improves the solution method for the head diffusion equation of compressible delay interbeds based on the principle of geostress variation. The Kozeny-Carman equation was introduced to establish a relationship between the hydraulic conductivity and porosity of the interbeds, while variations in the thickness of discrete nodal cells were also incorporated into the solution process. Collectively, these improvements lead to a more rigorous approach. To verify the effectiveness of the proposed simulation method, three representative test cases were developed and comprehensively compared with the CSUB results. The results indicate that notable discrepancies emerge between the two approaches when the interbed undergoes substantial compression, whereas the method proposed in this study effectively prevents the occurrence of "overcompaction" within the interbed.