Molecular dynamic simulation of the impact of thermal maturity and reservoir temperature on the contact angle and wettability of kerogen

Understanding the interfacial interactions between fluids (oil and water) and kerogen is imperative for efficient development and production of hydrocarbons in unconventional shale reservoirs. Kerogen is one of the major constituents (up to 30 vol%) of organic-rich mudrocks. Thus, wettability of kerogen can directly affect the multi-phase fluid-flow properties, water production, and resistivity of organic-rich mudrocks. The chemical composition of kerogen varies with kerogen type and thermal maturity. Reservoir temperature is another factor that can have an impact on thermodynamic processes such as interfacial interactions. Therefore, it is important to fundamentally understand the impact of kerogen geochemistry and reservoir conditions on the wettability of kerogen at the molecular-scale domain. The objectives of this paper are to quantify the impacts of reservoir temperature as well as kerogen molecular structure on the three-phase air/water/kerogen and methane/water/kerogen contact angle on the kerogen surface using molecular dynamics (MD) simulations. The MD simulations are performed on kerogen molecules of types I, II, and III at different thermal maturity levels and at a temperature range from 300 to 360 K. Simulation results showed that the air/water/kerogen contact angle formed on kerogen surface is highest for type III kerogen at 45.5 degrees and lowest for type II kerogen at 20 degrees. For type II kerogen, the contact angle increases from 20 degrees to 75.5 degrees as the thermal maturity increases from immature stage to over-matured stage. Increase in temperature from 300 to 380 K decreases the contact angle measurements by 62%. The methane/water/kerogen contact angle showed similar trend with type III kerogen having the highest contact angle (58 degrees) among the three kerogen types. The impact of thermal maturity on methane/water/kerogen contact angle shows an increase from 25.5 degrees to 88.5 degrees from kerogen II-A to kerogen II-D, which is consistent with the observations from air/water/kerogen contact angles. The documented results and workflows can potentially contribute to improving formation evaluation of organic-rich mudrocks by providing information about wettability of kerogen. The outcomes of this paper can also potentially enhance the understanding the role of organic content and its geochemical properties in fluid-flow mechanisms, which can be used to predict water production in organic-rich mudrocks.