A Novel Approach for Reservoir Simulation and Fluid Characterization of Unconventional Fields:Phase 1 Molecular Level Considerations

摘要

In spite of the Shale Gas revolution,that has changed the dynamics in US supply and while today’s combination of hydraulic fracture and horizontal drilling has made certain shales economic.This study presents that the fundamentals for fluid behavior in Unconventional reservoirs remain unknown.As the amount of reserves in the world is massive,today’s technologies and workflows need to evolve more and systematically apply the correct science to extract full value of every shale. Unconventional fields contain hydrocarbon fluids that are under strong influence of their pore-walls. The reason for such behavior is attributed to pore-size distribution being at nanometric level(10-9m), widely demonstrated by scanning electron microscopy(SEM),simulation and adsorption data(Diaz- Campos et al.,2008;Ambrose et al.,2009).At this tiny scale,fluid behaves different to what it is known conventionally. Current methods for calculating gas reserve in a typical unconventional field attempt to take such complexity into account by modifying methods used in conventional fields without definitive and clear success.The modified method utilized today is known as modified“Langmuir isotherms”(M.Gasparik et al.,2012).Such approaches require detailed isotherm analysis in the laboratory and iterative calcula- tions to estimate its unknown constants Langmuir volume(VL)and Langmuir pressure(PL).It has added variables to the Langmuir’s equation,adsorbed methane density,adding complexity to the original equation and making its application difficult if not impossible.Furthermore the constants and added variable are representative to a single well level only,utilizing such values at a reservoir level are prone to compromise the accuracy of the results.Novel approaches to generate datasets for unconventional gas reservoirs are therefore needed. In this study,it is used a molecular-modelling approach to analyze how hydrocarbon fluids accumulate in such nano-pores and how the pore-size affects its mobility.Molecular modelling is a form of computer simulation(Allen M.P and Tildesley D.J.,2007)that is been used in the biomedical industry,material science,chemistry,chemical engineering and has been evolved to be an essential means with which to explore chemical and physical interactions in unconventional reservoirs. We characterize methane behavior when confined to a nanoporous medium,by means of Computer Simulation at molecular level.From the results of the fluid characterization at molecular level we observe that methane adsorbed density changes with reservoir pressure.Furthermore we conclude that the methane permeability is reduced with pressure,specifically when the reservoir is above~5700psia methane shows a liquid-like behavior,such findings generate a fundamental insight about fluid behavior when confined to shale type of reservoirs.The experimental method for validating the MD simulation results is also discussed.