Paper: Multi-zone Methods To Predict Gas Well Performance

This paper explains the contributing elements of a new formula developed for more accurately predicting the performance of those gas wells which include a high permeability zone interbedded with one or more low permeability zones. The theory assumes the existence of three conditions: that the well depletes without water encroachment; that each zone remains discreet from every other --that is, without cross flow among zones when the well is producing; and that each zone has either a hydraulic fracture or some skin effect. As a practical matter in using the model, however, only one of these reservoir conditions need be strictly met: freedom from water encroachment. The model developed herein does adapt to reservoirs that have limited cross flow between zones; it also adapts to those with a hydraulic fracture in only some of the zones. Finally, it includes equations which help to calculate matrix permeability whenever a known hydraulic fracture does exist. We illustrate the functions of this model by assuming the existence of a shaley-sand, six-zone reservoir and by ascribing to it certain characteristics. We examine how the model uses this data and then discuss the results.