1. The material balance for simple depletion defines the relation between average reservoir pressure and (a) cumulative wellstream produced, (b) cumulative surface gas produced, (c) cumulative condensate produced, and (d) producing condensate-gas ratio (CGR).
CCE & CVD laboratory data provide the necessary information to define the material balance relations of a gas condensate reservoir.
Assuming no water influx (i.e. zero water & rock compressibilities) the material balance relations are given directly by CCE data (above dewpoint) and CVD data (below dewpoint) — RFgw(pR), RFgd(pR), RFo(pR), CGR(pR); the CGR(pR) relation is independent of assumptions regarding water expansion & influx.
When water expansion and/or aquifer encroachment are important, the material balance calculation is more complex and may require an iterative solution to find the fundamental relation between cumulative production (RF) and pressure.
The “2-phase” Z-factor (Z2) is calculated assuming that the gas condensate reservoir depletes according to the material balance equation for a dry gas. 2-phase Z-factors should only be used in the traditional gas material balance equations, such as Havlena-Odeh, simplified Fetkovich, and pot-type aquifer.