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Predicting Hydrocarbon Composition in Unconventional Reservoirs with a Compositional Generation/Expulsion Model


Estimation of hydrocarbon compositions (GOR, CGR and gas wetness) is critical to appraise unconventional reserves and to determine the production strategy. Unfortunately, estimation from basin modeling is challenging because it relies on a detailed and accurate description of hydrocarbon generation and migration processes. Based on state of the art knowledge obtained by recent exploration and production in shale plays, we have developed a practical model to predict the hydrocarbon compositions in shale plays. In this model, we applied a compositional reaction network for hydrocarbon generation. The compositions include specific gas components (methane, ethane, propane, butane and pentane isomers), light oil (C6-C14 hydrocarbons) and heavy oil (C15+ hydrocarbons, resin and asphaltene). The precursor fraction of these components and the kinetic parameters of precursor cracking are calibrated by the laboratorial high-pressure pyrolysis of the source rock in closed reactors. The retained and expelled hydrocarbon amounts are modeled by coupling the hydrocarbon generation and expulsion processes. The generated/cracked amount of hydrocarbons and their precursors are calculated from the retained precursor amount and temperature-dependent kinetic parameters. Hydrocarbon expulsion occurs when the retained hydrocarbon exceeds the storage capacity. The storage capacity is determined by pore volume, surface area, fluid density and adsorption affinities; whereas the fluid density is determined by fluid composition, pressure and temperature. Thermal history, pore volume and pressure history are modeled separately as input. The expulsion of oil and gas is assumed to be through microfractures as Darcy flow. Resin and asphaltene components have such a high viscosity that they can only migrate when dissolved in oil. This mechanism brings about the compositional fractionation in oil during expulsion. The concentrations of mobile species in the instantaneous expelled fluid are proportional to the concentrations in the residual fluid. Retained hydrocarbons crack with increasing maturity (secondary cracking). Field data indicates that secondary cracking under geological conditions is self-accelerated, and the products are more enriched with methane than the laboratorial products. The model results are consistent with the production data from different shale plays, and provide details to help understand production behavior of tight reservoirs.