Integrated model of combining hydrogen blended natural gas storage and carbon sequestration in fractured carbonate underground gas storage: Engineering analysis of gas mixing.

Integrating hydrogen blended natural gas into underground gas storage (UGS) systems poses challenges due to gas mixing with carbon dioxide (CO2) as cushion gas, which may degrade extracted gas quality and complicate operations. An integrated multicomponent gas model for fractured UGS formed from depleted oil and gas reservoirs is proposed, accounting for non-Darcy flow, reservoir deformation and high-velocity gas mixing. A new pressure-density coupling method is introduced from incremental and holistic formats. A hybrid solution algorithm is presented by combining direct and iterative solution algorithms. A global mass conservation correction method is introduced, deriving from discretization equations. Simulation results demonstrate that pressure-density coupling method is more accurate than existing methods. The hybrid solution algorithm significantly reduces computation time by up to 63%. The global mass conservation correction method eliminates cumulative mass errors in long-term simulations. Gas mixing in the construction, injection and extraction processes is analyzed. The hydrogen volume fraction affects gas properties like density and viscosity, it does not significantly influence gas mixing within the studied range. These findings offer valuable insights for optimizing operations using CO2 as cushion gas and hydrogen blended natural gas, contributing to more efficient and sustainable energy storage solutions.