Abstract: Material balance equation for gas reservoir is an important method in gas reservoir engineering. In gas reservoir there exist elastic energy of rock, imeducible water, gas and aquifer, which are determined by critical parameters that consist of static parameters such as reservoir and fluid quality, gas reserves, aquifer size and dynamic parameters such as average pressure and accumulative production, etc., but these parameters are of uncertainty at the early stage of gas reservoir development. This paper presents the new equation for aquifer evaluation of elastic waterflood gas reservoir derived from material balance equation and the analysis of uncertainty of aquifer size in early stage of gas reservoir de velopment.

Key words: gas reservoir, material balance equation, edge water, bottom water, uncertainty