Molecular Simulations Of Alkanes At High Temperature High Pressure

Technology advances in hydraulic fracturing and horizontal drilling made it possible to explore unconventional shale gas and oil,which are in the state of high temperature and high pressure(HTHP)because of the rapidly increased depth of drilling wells.It is critical important to accurately calculate the properties of gas and oil,such as densities and viscosities,in the design of drilling and production processes.However,most of the available methods,such as equation of states(EOSs),are developed at moderate temperature and pressure.It is shown that the current EOSs,both conventional cubic EOSs such as PR and SRK,and statistical mechanical based EOSs such as SAFT(Statistical Associating Fluid Theory),is not accurate enough to predict the densities of alkanes at HTHP states.This can be partly remedied by re-optimizing the model parameters by fitting the new HTHP data,while usually in compensation of the lost accuracy at low pressure/temperature state.Starting from the interactions between molecules,one can predict many fluid properties in virtue of the modern developments of molecular simulation techniques and accurate force fields.Currently,molecular simulations are playing an important role in both predicting the macroscopic properties and uncovering the microscopic mechanics.The accuracy of predictions heavily depends on the force fields used in the simulation.At present,various force fields have been successfully developed for wide range of materials,from small simple gas molecules to highly complicated protein and nucleic acid systems.However,it is seldom reported in concern of their performance at HTHP state.In this work,the TraPPE-UA force field was used to simulation alkane systems at both VLCC(Vapor Liquid Coexistence Curve)and HTHP states,including the three homologues of pentane,normal alkanes up to 20 carbon atoms and some alkane mixtures.Although very few state-independent parameters(three united atom types,each tow parameters)are used in the force field,good predictive power is shown by our simulation results.The deviations of densities are within 1%and 3%at VLCC and HTHP states,respectively.There is no detectable increase of the deviations when the simulation is extended to alkane mixtures at HTHP states.The results also showed the viscosities are under-estimated about 30%by TraPPE-UA force field.