探討以二氧化碳做為工作流體的地熱增進系統

發表於台電工程月刊，第715期，民國97年3月

探討以二氧化碳做為工作流體的地熱增進系統

A Study on the Enhanced Geothermal Systems Using CO₂ as Working Fluid

賴正義*

Lai, Cheng-I

摘　要

地球蘊藏著豐富的地熱資源，但是目前此種資源的開採僅在於一些特定的熱液系統，事實上，大多數的地熱資源是存在於缺乏破裂滲透性及流體循環的熱乾岩中，地熱增進系統的目的即在於從這些資源抽取地熱能。早期發展地熱增進系統係以水做為工作流體，不過，水雖具有許多符合地熱增進系統目的的特性，但是也會產生一些嚴重的缺失。目前國際上已有利用二氧化碳來增進石油、煤床甲烷開採之應用實例，即石油增進回收法與煤床甲烷增進回收法，近年來亦有研究者嚐試將此技術推廣至地熱增進系統。基於此，本文首先介紹熱乾岩地熱特性及其開發與利用現況，其次說明國際上應用二氧化碳於增進石油、煤床甲烷等能源開發之現況與效益，最後綜整國際上以二氧化碳做為工作流體的地熱增進系統之研發成果。結果顯示，二氧化碳應用於地熱增進系統時，不僅能開發熱乾岩地熱而增加能源效益，且能提供二氧化碳地質封存而降低溫室效應，甚至有避免水資源浪費的衍生效益。考量台灣具有良好熱乾岩地熱發展潛能，本文建議我國未來宜加強此相關技術之研發。

Abstract

There are huge amounts of geothermal resources in the earth. However, such resources have been tapped currently limited to some specific hydrothermal systems. In fact, most geothermal resources are reserved in the hot dry rocks which lack fracture permeability and fluid circulation. The purpose of enhanced geothermal systems, therefore, is to extract geothermal energy from these resources. In the early periods, water was employed as working fluid for enhanced geothermal systems. Although water possessed a lot of characteristics meeting the purpose of enhanced geothermal systems, it also had some detrimental effects. Currently, the application cases of carbon dioxide on the exploitation of oil and coalbed methane can be found in the world, that is, enhanced oil recovery and enhanced coalbed methane recovery. Recently, a few researches have tried to apply such technologies to enhanced geothermal systems. As a result, the paper first introduces the characteristics and the status quo of development along with utilization with regard to hot dry rocks. And then it addresses the status quo and benefits for applying carbon dioxide to the enhanced recovery of oil and coal-bed methane. Finally the research results in terms of enhanced geothermal systems running carbon dioxide are synthesized. The research results show that applying carbon dioxide to enhanced geothermal systems can mine hot dry rocks resources to advance energy benefits. In addition, it would achieve geologic sequestration of carbon dioxide to reduce greenhouse effect. Moreover, it is an ancillary benefit to save the consumption of water resource. Taking into account the promising potential of hot dry rock in Taiwan, it is suggested that we should reinforce the research and development on such technologies in the future.