The study of unconventional resources requires unconventional approaches. One of them is the use of molecular simulations to elucidate what laboratory equipment cannot see.
The development of unconventional shale plays requires more than a simple extension of technology and knowledge developed for conventional reservoirs. The complexity arises from the fact that the porosity and permeability in these systems are orders of magnitude smaller than conventional rocks, necessitating a paradigm shift in how wells are drilled and completed.
The technological changes that made oil and gas extraction possible in these nanoporous shale plays are horizontal drilling and hydraulic fracturing. In general, however, in spite of having nearly two decades of experience with shale plays, we are still far from understanding what the main flow and storage mechanisms of oil and gas are in these rocks. The study of these phenomena continues to present a huge challenge and an opportunity. The challenges arise because of experimental constraints and limited optical resolution of laboratory equipment. But it also presents an opportunity for molecular simulations to investigate and shed light on the behavior of confined fluids, their interaction with the rocks, and fluid transport through shale.
Molecular simulations are relatively new techniques for modeling fluids but not by very much. The first computer simulation of a liquid was carried out at Los Alamos National Laboratories in the United States by Metropolis et al.1 in 1953 using MANIAC, which was at that time one of the most powerful computers in the world. The earliest work of Metropolis et al. laid the foundations of modern Monte Carlo (MC) simulations (so-called because of the role that random numbers play in the method). Of course, the original models were highly idealized representations of molecules such as hard spheres and disks. Molecular dynamics (MD), another molecular simulation technique in which the equations of motion of thousands of atoms are integrated numerically, was first accomplished for a system of hard spheres by Alder and Wainwright 2,3 in 1957 and 1959. To this date, the models have gained realism that brings about complexity as they capture more physical details. For instance, the structure and dynamics of macromolecules such as proteins and nucleic acid have been widely studied since the 1970s using MD. The use of molecular simulations in petroleum engineering is relatively recent and spans the last ten years.
One of the goals is to study fluid behavior in confined conditions in organic nanopores. In the early days, gas storage and density were inferred from rather simplistic MD simulations relying on the use of graphene sheets -carbon atoms organized in a honeycomb pattern- as a surrogate for kerogen (the main component of organic matter) in shales. Since then, there have been more attempts at creating more realistic molecular models of kerogen that more closely represent the true chemical composition, thermal maturity and pore structure. Similarly, better sets of parameters to describe organic fluids have been developed over the last few years, leading to more accurate simulations to study behavior of hydrocarbons at reservoir conditions. These advances have led to breakthroughs in modeling fluid transport in the subsurface and highlighted the importance of molecular diffusion as the transport mechanism in organic nanopores 4, 5, 6. All this progression has made possible the design of simulations to study the feasibility of enhanced oil recovery in shale formations, as shown in Figure 1.
It is uncertain what the next step will be. What is certain is that, as a response to the unconventional challenges, more and more research groups are now implementing molecular simulations to investigate the vast phenomenology that has emerged as shale resources are produced.