Tight shallow gas reservoirs in the Western Canada Basin present a number ofunique challenges in accurately determining reserves. Traditional methods suchas decline analysis and material balance are inaccurate due to the formations'low permeabilities and poor pressure data. The low permeabilities cause longtransient periods not easily separable from production decline usingconventional decline analysis. The result is lower confidence in selecting theappropriate decline characteristics (exponential or harmonic) whichsignificantly impacts recovery factors and remaining reserves. Limited, poorquality pressure data and commingled production from the three producing zonesresults in non representative pressure data and hence inaccurate materialbalance analysis.
This paper presents the merit of two new methods of reserve evaluation whichaddress the problems described above for tight shallow gas in the Medicine Hatfield. The first method applies type curve matching which combines theanalytical pressure solutions of the diffusivity equation (transient) with theempirical decline equation. The second method is an extended material balancewhich incorporates the gas deliverability theory to allow the selection ofappropriate p/z derivatives without relying on pressure data.
Excellent results were obtained by applying these two methodologies to tenproperties which gather gas from 2300 wells. The two independent techniquesresulted in similar production forecasts and reserves, confirming theirvalidity. They proved to be valuable, practical tools in overcoming the variouschallenges of tight shallow gas and in improving the accuracy in gas reservesdetermination in the Medicine Hat field.
Reserves determination for tight shallow gas reservoirs in the WesternCanada Basin has relied on traditional methods such as conventional declineanalysis and material balance. However, there are characteristics of thesereservoirs and their operations which potentially result in inaccuratepredictions of future rates and remaining reserves using conventionaltechniques.
The main characteristic of tight shallow gas reservoirs which impactsconventional decline analysis is its poor permeability of typically less than 2md. Due to this low permeability, wells experience long transient periodsbefore they begin experiencing pseudo steady state flow which represents thedecline portion of their lives.