Phase Envelopes in Reservoir Fill Analysis

Abstract

Abstract Phase envelopes are routinely employed by reservoir engineers for fluid characterization. These envelopes are controlled by the reservoir PVT conditions, but fundamentally also by the hydrocarbon fluid type and composition. As a result of increasing source-rock maturation, fluids with decreasing average molecular weights, decreasing densities and increasing gas-to-oil ratios (and hence different phase envelopes) are generated, which are thus linked to fluid history. In addition to their importance for exploration, charge models can play a key role in constraining reservoir models and in optimising field development, particularly when PVT data are properly integrated with fluid geochemistry. Two distinct patterns of fluid phase evolution from two different fields are presented, and their relations to charge analysis and reservoir models are discussed. The first example discusses the identification, based on hydrocarbon geochemistry complemented by overlapping phase envelopes, of compartmentalised filling cycles in what was initially considered a single oil-rimmed gas accumulation. The second example presents an opposite scenario where two wet gas accumulations 20-km apart laterally and 400-feet average depth difference appear to represent a single more-expansive accumulation spread over areas of variable PVT conditions and reservoir qualities. The wet gas across both accumulations is characterised by a continuous phase evolution pattern that shrinks systematically (cricondentherm shifts to lower temperature and cricondenbar to lower pressure) suggestive of phase fractionation of a charge of single maturity. The proposed gas distribution model represents a discovery of a hybrid conventional and unconventional (tight sand) system, with potential for basin-centered gas.