What happened to all my Gas? P/z plots in Layered Reservoirs

P/z plots for low permeability or layered reservoirs do not yield a straight line which extrapolates to the IGIP; they are curved and deviate strongly from the theoretical straight line. Consequently, the interpretation of the P/z plot is often grossly misleading and typically significantly underestimates the IGIP, leading to erroneous production forecasts and reserve estimates. The problem with the simple P/z material balance plot is that the pressure measured using a pressure build up test is not representative of the drainage volume of the well. When P/z plots cannot be analysed quantitatively, typically a numerical reservoir simulator is used to match the shut-in bottom-hole pressures and the continuously measured wellhead rates and pressures. However, a simple multi tank material balance method can provide valuable insights into the reservoir behavior and can be used either as a precursor to, or to replace numerical simulation. This paper describes a technique that can be used for medium to high permeability reservoirs.