Modeling fracture growth for EGS in foreland sedimentary basins

Abstract

Hydraulic fractures propagate in a form that depends on the forces acting on it, including the elastic forces of the rock, the viscous forces of the liquid, and the driving pressure gradient. Rock layering also needs to be accounted for, in which the rock properties can cause sharp changes in these forces. These factors influence the resulting surface area of a fracture, and therefore in the case of EGS, the ultimate productivity of a plant. The rising importance of EGS, and associated costs of drilling, bring a need for high quality models of fracture propagation, in order to assess plant productivity beforehand. We numerically simulate fracture propagation for a field site in the Llanos basin in Colombia, which has a monzogranite basement rock and overlying layers of sandstone and mudstone. We vary the fracture dip between models and keep the other parameters (material properties, injection rate, etc.) constant. Horizontally dipping fractures propagate radially and maintain a circular, penny-shape. Fractures with greater dips become vertically elongated due buoyancy forces, driving the propagation upwards. Fractures that propagate into overlying (softer) rock layers respond by reducing in horizontal breadth. We then assess the heat conduction into the fractures using 3D finite element modeling. Steeply-dipping fractures have larger surface areas (favorable for heat capture), but also propagate upwards into cooler rock. Horizontal fractures have smaller surface areas but remain at depth, in contact with hotter rock. There is a trade-off between these competing factors, so that fractures with dips of 30° maximize the heat capture. For the extensional tectonic environment of this site, we argue that a vertical fracture is likeliest to form. Therefore, in order for an EGS plant to be sufficiently productive, we recommend drilling an injection well that is deep enough to account for the upwards propagation of such a fracture, around 4 km depth.