A new approach to the estimation of reserves in a fractured limestone reservoir is presented and verified with a lookback analysis over the past five years of production from the field. This approach uses a filtered Monte Carlo method to integrate independent reserves calculations based upon volumetric, material balance, well pressure survey analysis, and well decline estimates of oil in place and recovery. For the Waihapa-Ngaere field, two estimates of oil in place are available: a volumetric estimate obtained from mapped gross reservoir volume and formation parameters; and a material balance estimate obtained from pressure decline and production data. Independent estimates of oil recovery can be obtained from estimation of recovery factors based upon areal and vertical sweep in the fractured reservoir, and recovery obtained from extrapolation of well decline. The approach taken, that is to integrate all of the available information and only accept parameter sets which are consistent, led to an estimate of reserves and production potential from the field which has proved remarkably accurate as a predictor of field performance and recovery over the past five years.

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