Inventory forecast of underground gas storage based on modified material balance equation

In view of the gap between the gas storage inventory forecasted by the traditional material balance equation and the actual inventory, experiments of displacing gas by water and displacing water by gas were carried out, and the modified material balance equation considering pore volume loss due to water invasion and pay heterogeneity was deduced based on the percolation mechanism inferred from the experiments above, moreover the equation was verified by example analysis. The experimental results show that: the irreducible water saturation and residual gas saturation in underground gas storage (UGS) domain should be evaluated through water-gas seepage experiments because these parameters are different from the process of gas field development; the pore volume in the water flooded reservoir decreases after conversion for gas storage, and the pore volume loss increases rapidly when the reservoir permeability decreases. The net pay was divided into flooded zone, transitional zone and gas zone with the equations for pore volume loss calculating due to water flooded. Then one parameter describing reservoir heterogeneity was defined based on numerical simulation and eventually the traditional material balance equation was modified with the consideration of pore volume loss due to water invasion and reservoir heterogeneity. Example analysis result shows that the actual UGS inventory differs greatly from the inventory predicted by the traditional material balance equation, but agrees with that calculated by the modified one.