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 nu mber of unique challenges in determining reserves accurately. Traditio nal methods such as decline analysis and material balance are inaccura te owing to the formation's low permeabilities and poor pressure data. The low permeabilities cause long transient periods that are not sepa rated easily from production decline with conventional decline analysi s, resulting in lower confidence in selecting the appropriate decline characteristics (exponential or harmonic), which effects recovery fact ors and remaining reserves significantly. Limited, poor-quality pressu re data and commingled production from the three producing zones resul ts in nonrepresentative pressure data and hence inaccurate material-ba lance analysis. This paper presents two new methods of reserve evaluat ion that address the problems described above for tight, shallow gas i n 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 m ethod is an extended material balance (EMB), which incorporates the ga s deliverability theory to allow selection of appropriate p/z derivati ves without relying on pressure data. Excellent results were obtained when these two methods were applied to 10 properties that gather gas f rom 2300 wells. The two independent techniques resulted in similar pro duction 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 de termination in the Medicine Hat field.