USE OF NUCLEAR MAGNETIC RESONANCE AND NEW CORE ANALYSIS TECHNOLOGY FOR DETERMINATION OF GAS SATURATION IN PRETTY HILL SANDSTONE RESERVOIRS, ONSHORE OTWAY BASIN

Abstract

The failure of conventional log interpretation of low resistivity gas-bearing reservoirs in the Lower Cretaceous Pretty Hill Sandstone, onshore Otway Basin, has led to the use of the saturation versus height, Leverett J function as a basis for predicting hydrocarbon saturation.

The recent application of a new method of proprietary core analysis (corEVAL™*) in the 1998 gas discovery well Redman–1, allowed the derivation of a more realistic Leverett J function to water saturation transform for the Pretty Hill Sandstone. Furthermore, this transform could be applied beyond the cored interval to the remaining reservoir section by calibrating the core with its nuclear magnetic resonance response. An algorithm, which converts Schlumberger's combinable magnetic resonance (CMR*) cumulative T2 distributions into a pseudo- capillary pressure curve, has been derived enabling the calculation of gas saturation directly from this log. The CMR derived permeability log also assisted in facies differentiation of the reservoir section and in the selection of wireline pressure and formation fluid sampling points.

The combined application of nuclear magnetic resonance technology and proprietary core analysis, independently validated by formation sample and test data, resulted in a 30% increase over previous methods, in average gas saturation in the reservoir being calculated. This has lead to a predicted increase in estimated gas in place at the Redman Field