SPE Reservoir Engineering, Vol.10, No.2, 82-87, 1995
RESERVES DETERMINATION USING TYPE-CURVE MATCHING AND EMB METHODS IN THE MEDICINE-HAT SHALLOW GAS-FIELD
Tight, shallow gas reservoirs in the Western Canada basin present a number of unique challenges in determining reserves accurately. Traditional methods such as decline analysis and material balance are inaccurate owing to the formation's low permeabilities and poor pressure data. The low permeabilities cause long transient periods that are not separated easily from production decline with conventional decline analysis, resulting in lower confidence in selecting the appropriate decline characteristics (exponential or harmonic), which effects recovery factors and remaining reserves significantly. Limited, poor-quality pressure data and commingled production from the three producing zones results in nonrepresentative pressure data and hence inaccurate material-balance analysis. This paper presents two new methods of reserve evaluation that address the problems described above for tight, shallow gas in the Medicine Hat field. The first method applies type-curve matching, which combines the analytical pressure solutions of the diffusivity equation (transient) with the empirical decline equation. The second method is an extended material balance (EMB), which incorporates the gas deliverability theory to allow selection of appropriate p/z derivatives without relying on pressure data. Excellent results were obtained when these two methods were applied to 10 properties that gather gas from 2300 wells. The two independent techniques resulted in similar production forecasts and reserves, confirming their validity. They proved to be valuable, practical tools in overcoming the various challenges of tight, shallow gas and in improving the accuracy in gas-reserves determination in the Medicine Hat field.