Gas Material Balance - Tight gas

The material balance equation, for a gas reservoir, can be described graphical by a linear relation between P/z and cumulative production (Dake, Fundamentals p 25 -29). Key underlying assumptions, for tight gas reservoirs, are;

• The reservoir has been produced long enough for the wells to be influenced by the reservoir boundaries and can be described as being under semi-steady state conditions
• Pressure measurements used in the P/z plot are acquired from pressure build-ups of sufficient duration that they are reasonably representative of the ‘average’ reservoir pressure.

The time required for these assumptions to be satisfied increases with decreasing permeability. Thus, in tight gas reservoirs it can take a considerably length of time for steady conditions to be established and for representative pressure measurements to be acquired.

In the SPE paper 36702 the impact of heterogeneity  on the P/z plot of a tight gas reservoir was investigated using numerical modelling. The results show a significant impact on the plot for the 5 scenarios considered. The P/z plots are consistently non-linear. In the case of a random distribution of high and low permeability grid blocks there is an initial dramatic high gradient line.

The overall character;

has been observed in UKCS Tight gas reservoirs.