Establishment of productivity equation for ultra-deep gas reservoirs considering threshold pressure gradient and pressure-sensitive effect

Ultra-deep gas reservoirs generally have low-permeability characteristics,so there will be threshold pressure gradient.Meanwhile,the permeability of reservoir rock changes as the gas reservoir pressure or effective overburden pressure changes.Therefore,based on Darcy′s formula and considering the influence of threshold pressure gradient and pressure-sensitive effect on the seepage law of ultra-deep gas reservoir,this paper established a new productivity equation.The established equation was used in a gas reservoir.The calculation result shows that the production of gas well decreased when considering the threshold pressure gradient and pressure-sensitive effect of gas reservoir.When considering one of two effects,the production of the gas well decreased even more and the ratio of the AOF(absolute open flow) is about 71.44% of that when neglecting these two effects.This study also shows that the threshold pressure gradient and pressure-sensitive effect can not be ignored when examining the seepage law and inflowing performance of ultra-deep gas reservoir.This study is significant to deeply understand the productivity,predict the production performance and the recovery factor of ultra-deep gas reservoir.