A comparison study of density-driven flow in fractured porous media

Density-driven flow occurs in many geologic settings, including seawater intrusion into a freshwater aquifer, carbon sequestration in deep sediments, and glacial cycles altering the salinity in fractured crystalline bedrock. A problem commonly used to benchmark density-driven flow simulations is the Elder problem. This problem originated in the 1960s as a buoyant flow experiment where convection was initiated by heating the bottom of a water-filled, rectangular Hele-Shaw cell. To benchmark porous media flow and transport simulators, the Elder problem was modified in the 1980s to use density changes that drive convection arising from variations in solute concentration rather than variations of temperature. The modern Elder problem is set in a rectangular domain with zero-flux boundaries on all sides, where density-driven convection is initiated by an elevated, constant salt concentration along its upper boundary. We extend the Elder problem to three dimensions, and investigate the effect of varied fracture networks on density-driven flow. Slight variations in each timestep solution can have an outsized effect on the ultimate solution that is reached, and these effects are even more pronounced in fractured systems. To examine the bounds of these effects, we compare the results reached by two flow and reactive transport simulators: FEHM and PFLOTRAN. FEHM is a groundwater flow and transport simulator that uses a control volume finite element approach to ensure conservation of mass. PFLOTRAN is a massively parallel flow and transport simulator that solves a coupled system of equations using a finite volume discretization. The fracture network is discretized using dfnWorks, a parallelized computational suite capable of generating stochastic fracture networks in three dimensions. We find that both simulators are in good agreement with the Elder problem in an unfractured two-dimensional system, and examine the origin of the variations in simulation output of the fractured media simulations between FEHM and PFLOTRAN.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. SAND2022-10477 A. This work was supported by the USDOE Office of Nuclear Energy and the Office of Environmental Management through the Los Alamos National Laboratory. The Los Alamos National Laboratory is operated by Triad National Security for the National Nuclear Security Administration of the USDOE (Contract no. 89233218CNA000001).