Abstract:

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.

Key words: dual media, start-up pressure gradient, numerical simulation, production and development index, volcanic gas reservoir