An analytical and numerical model to determine stresses in a Rock Melt Drill produced glass liner for potential use on Mars

Abstract

A numerical and analytical model was constructed to determine the resultant stress state imposed on a Rock Melt Drill produced liner. The purpose of this study was to determine if the liner produced would possess the strength required to prevent a failure in the borehole wall. Derivation of energy, mass and momentum conservation equations was performed to aid in the identification of applicable loads acting on the melted material that will form the liner. A finite difference model was coded to produce a temperature profile in the liner thickness. An analytical stress model, using the results of the derived equations and the numerical thermal model, was constructed to determine the magnitude of the stresses the liner is subjected to after operation of the Rock Melt Drill. By using values from the Rock Melt Drill design and formation material properties taken from the literature, from experiment and through calculations, a baseline resultant stress was able to be determined for the liner. Utilizing the thermal and stress model, a parametric analysis of the stresses and temperature profile was conducted over a range of Rock Melt Drill operational parameters and formation material property parameters. This parametric analysis was conducted to determine trends between the above parameters and the resultant temperature and stress profiles. It was determined that the liner would have enough strength to prevent failure by collapse up to the required depths of 5 km, under ideal conditions. Additional loads applied to the liner in the form of an increase pressure gradient or formation fluid load may be enough to cause collapse of the liner. Also, the resultant tensile stress in the liner at shallow depths is great enough to cause crack propagation near the surface. It is a conclusion of this study that the Rock Melt Drill may be a potentially viable drilling system for use on Mars, in terms of preventing borehole collapse. Further study is necessary to determine the state of the liner in more realistic conditions, such as including pressurized fluids in the models, and additional work is needed to optimize the Rock Melt Drill system.