ABSTRACT:

Fluid flow in an Enhanced Geothermal Systems (EGS) is mainly controlled by the network of induced and preexisting fractures. The permeability of the network is affected by mechanical, thermal and chemical processes. Access to the thermal energy stored in the reservoir rocks strongly depends on the connection between the drilled and boreholes and the natural fractures. As the water circulates through the reservoir it interacts with the rocks, often leading to mineral precipitation and dissolution reactions as well as large gradient in temperature and pressure. In this work we develop analytical solution for thermoelastic reactive chemical transport and calculate the resulting fracture aperture change. Analytical solutions are useful in testing more complex numerical solutions and also allow one to readily gain insight into the fundamental issues that are involved. We focus on single component reactive fluid transport in a fracture. Both solute reactivity along the fracture and its diffusion into the rock matrix are considered using temperature dependent reaction kinetics.

1. INTRODUCTION

Fluid flow in an Enhanced Geothermal Systems (EGS) is mainly controlled by the network of induced and preexisting fractures. The permeability of the network is affected by mechanical, thermal and chemical processes. Access to the thermal energy stored in the reservoir rocks strongly depends on the connection between the drilled and boreholes and the natural fractures. As the water circulates through the reservoir it interacts with the rocks, often leading to mineral precipitation and dissolution reactions as well as large gradient in temperature and pressure. Certain aspects of the thermal and mechanical processes have been studied in EGS [1, 2, and 3]. Poroelastic, thermoelastic and silica reaction kinetics with their combined effects also have been considered to explain the change in fracture aperture [4, 5]. It has been shown that thermoelastic effects dominant near the inlet when compared to those of poroelasticity. The effect of super-saturated fluid on fracture aperture is to close the fracture faster than under-saturated fluid and in some situation, under-saturated fluid has tendency to open the closure. Deposition of amorphous silica is more dominant when compared to quartz. Chemical reactions between the rock and the circulating fluid have been studied [5, 6, 7] and shown to significantly affect fracture aperture by precipitation and dissolution of minerals [8, 9, 10, 11, 12]. The combined effects of thermoelastic stresses and silica precipitation in hydrothermal systems has been studied experimentally in [13], indicating that a rapid initial decrease in permeability is caused by a thermoelastic stresses and a further decrease is resulting from silica precipitation. The study of combined influence of thermoelasticity and silica reactivity in [3] shows an interesting interaction between these processes. In particular, it has been observed that under certain conditions, silica reactivity tends to dominate permeability. Different studies have been conducted to simulate reactive transport in the fracture system such as: an equivalent porous medium approach e.g. in [14], single fractures or fractures sets and multi-component transport e.g. in [15]. But here we focus on the single component reactive transport in a fracture as proposed in [15, 16, and 17].

This content is only available via PDF.
You can access this article if you purchase or spend a download.