## Blog #1: Getting TOUGH

I am now finishing the third week of my internship, and so far everything has gone swimmingly. The first day, I met with my host, Megsu Hu, to discuss the scope of my position and what I should begin to work on. Up until now, I have spent most of my time reviewing slides, manuals, and a textbook to get a better grasp of the theory and physics behind TOUGH2. For those of you unfamiliar, TOUGH2 is a numerical simulator for flows and heat transfer through porous and fractured media, that media most generally being rock. It can be used for many simulations related to energy including CO2 storage, nuclear waste storage, groundwater flow, and most relevant to me: geothermal energy generation. A large part of my first few weeks have been reading and working written problems to understand what TOUGH is as a numerical model and the equations behind it. By using the physics and properties of various materials and systems, TOUGH2 is able to thoroughly simulate a wide variety of processes within energy geosciences. Although physical experiments are typically the most reliable source of data, they can be very time and resource expensive, but by using a simulator such as TOUGH2, you can run multiple scenarios quickly and adjust parameters to understand relationships and optimize performance.

The most important input and output of the simulator is the mesh. A mesh takes a continuous volume and breaks it up into a finite number of discrete elements. Typically, each of these elements has some initial conditions and some set of relationships with its neighboring elements. These relationships are algebraic expressions derived from the differential equations that govern the phenomena at play. The primary differential equations are those of conservation of mass, energy, and momentum. Mass for example states that the time rate of change of mass in a given volume must be equal to the mass flux across the element's surface plus the rate any mass is being generated/ taken away within the volume. The net mass flow into a given volume must be equal to the sum of the flow into or out of its neighboring elements. By breaking down these relationships into solvable equations, TOUGH2 is able to simulate flow or heat transfer across a mesh and produce an updated mesh at some desired time step later.

Although so far, most of my work has been reading about theory and deriving discretization equations to solidify my understanding of what TOUGH2 does and where the ideas come from, I am going to begin running simulations next week. These simulations will first be very simple, just so that I get a hang of the software and what all the input and output blocks look like. After I feel confident with how it all works generally, I am going to come up with a problem specific to my interests and set up various relatively simple scenarios relating to it. This will allow me to further familiarize myself with the program while gaining a slightly better understanding of a problem I want to solve and some additional questions to ask. I am not yet sure what my specific problem will be, but as I am primarily interested in geothermal generation it will likely be something centered around heat transfer between rock and water.

Already, I feel as if I have learned so much and my understanding of numerical methods and discrete element analysis has grown greatly, but I am excited to begin to apply it and learn even more. Additionally, I am so excited to be choosing my own problem and seeing what I can learn of something that I am truly passionate about. Going into this, I did not think I would have nearly as much free reign as I do now, and I am thrilled to be able to ask my own questions and begin something that might become a long term project.