The Darcy flow theory suitble to conventional sandstone gas reservoirs with a single medium can not be applied to volcanic gas reservoirs

because they are characterized by the dual media of matrix and fracture and the higher original water saturation

resulting in an impact of star-up pressure gradient on a percolation process.According to the theory of mass conservation

a mathematical model of two-phase(gas and water) flow in dual media was built under consideration of start-up pressure gradient

for which the finite difference approch was used to obtain numerical solution and a calculation program of numerical simulation was compiled.In combination with laboratory experiment results

the values of this model parameters are determined

and it is shown that the impact of start-up pressure gradient has to be considered when permeability is lower than 0.1×10-3 μm2 and irreducible water saturation is over 20%.The numerical simulation results of a single well in XS gas field are consistent with the actual situation

and several indices of production and development

such as production plateau time

gas output during production plateau

recovery degree of production plateau and final recovery efficiency

are lower when the impact of start-up pressure gradient is considered than not.It is obvious that the impact of start-up pressure gradient has to be considered during the production of volcanic gas reservoirs.