Beyond Hydrocarbon Extraction: Enhanced Geothermal Systems

Abstract

There are many forms of energy that exist subsurface. Mining typically extracts minerals that are energy rich and processes them to produce electricity. The oil and gas industry extracts hydrocarbons with high energy content that are suitable for energy production. This chapter illustrates the concept of “heat mining” that is a form of mining but has not caught much attention. This term “heat mining” was originally used to describe a general concept of mining heat from deep granitic rocks by injecting cold water and recovering it as steam or water/steam mixture to produce electricity. Beginning with the Fenton Hill Hot Dry Rock project developed by Los Alamos National Laboratory in 1971, there is a long history of development of different types of heat mining projects in Japan, the UK, China, Germany, France, Iceland, and Hungary. This chapter presents a simple conceptual model of a doublet reservoir that can be developed in a sedimentary basin at a depth below where the conventional hydrocarbon resources can be found. In addition, as we move forward with the development of the original heat mining concept from granitic rocks, it is important to understand the consequences of long-term heat extraction. Developing an enhanced geothermal system or engineered geothermal system (EGS) is a complex process and is dependent on a range of geological and operating variables. Stress distribution and re-distribution in and around EGS during the different phases of development may have a significant impact on the reservoir itself as well as surrounding rock masses. Thus, the second part of this paper addresses issues associated with stress redistribution during and after the working cycle of EGS and gives insights in understanding the behavior of stress redistribution in and around basement rock. As the basement rock is thermo-elastically connected to the country rock, newly generated stresses interact with the existing in situ stresses under prevailing conditions of geological, design, and operating variables. Inability to predict the timing and the degree of impact suggests a need of a consorted effort of investigations between oil/gas, mining and geothermal industry, and academic disciplines.