Demystifying Wettability Alteration in Kerogen as a Function of itsGeochemistry and Reservoir Temperature and Pressure Using MolecularDynamics Simulations

摘要

Development of reliable models for hydrocarbon-in-place and water saturation estimation requiresknowledge about wettability of mudrocks and the parameters(including rock properties and reservoircondition)affecting it.A significant volume fraction of organic-rich mudrocks is composed of kerogen.Therefore,wettability of kerogen affects the overall wettability of organic-rich mudrocks.The chemicalcomposition and structure of kerogen varies with kerogen type and thermal maturity,which affectsthe surface properties of kerogen such as wettability.In a recent publication,we demonstrated usingexperimental techniques that kerogen could be water-wet at low thermal maturities and oil-wet at higherthermal maturities.However,the impacts of kerogen type and reservoir temperature/pressure conditions onkerogen and mudrock wettability is yet to be quantified.Therefore,the objectives of this paper include(i)quantifying the impacts of kerogen molecular structure and composition on water adsorption capacities,(ii)quantifying the impacts of reservoir pressure and temperature on water adsorption capacity of kerogenusing molecular dynamics(MD)simulations.In order to achieve the aforementioned objectives,we use a combination of molecular dynamicssimulations and experimental work.The inputs to the molecular dynamics simulations include realisticmodels of kerogen,which are condensed to porous kerogen structures.Water molecules are filled inkerogen pore structure and MD simulation is performed.The outputs of the simulations include radialdistribution function(RDF),and adsorption isotherms of water on kerogen for different kerogen types,thermal maturities,and temperature conditions.The adsorption processes are modelled for pressure andtemperature conditions ranging from 0 to 35 MPa and 320 to 370 K,respectively.The outcomes of moleculardynamics simulations demonstrated that the water adsorption capacities of kerogen vary significantly withkerogen type,thermal maturity,and temperature and pressure conditions.The RDF results showed thatthe water adsorption capacity decreased from type I to type III kerogen.The water adsorption capacity ofkerogen was found to increase by 128% with 38% increase in oxygen content.The increase in the adsorptioncapacity was attributed to the strong attraction between oxygen containing functional groups in kerogen andwater.The adsorption isotherms of water and kerogen samples showed that the water adsorption capacitydecreased by 0.19 mmol/g as the temperature increased from 320 K to 370 K.The average water adsorption capacity of kerogen was found to increase by 20% with increase in pressure by 34 MPa.The results obtainedfrom molecular dynamics simulations were found to be in good agreement with experimental results.Theresults of this paper can be used to predict the adsorption capacities of any kerogen with the availability ofgeochemical information.This important property of kerogen is required for estimating kerogen wettabilityand can enhance understanding of fluid-flow mechanisms in organic-rich mudrocks.