Geothermal energy is a desirable alternative to fossil energy. It can be exploited continuously and does not require any storage. For example, hot springs and other hot water reservoirs are conventional geothermal resources, while magma systems and hot deep rocks are considered unconventional resources with potential for geothermal exploitation.
Surface facilities as well as existing wells can be used for heat mining from depleted gas reservoirs. This comes with an economic advantage and extends the economic life of the reservoir. However, injecting water into high temperature reservoirs causes formation damage along with wellbore. Therefore, using water for heat transmission may not be applicable for geothermal exploitation. Researchers led by Professor Shaoran Ren from China University of petroleum proposed the use of supercritical carbon-dioxide as an alternative heat transmission fluid owing to its excellent thermal characteristics and unique mobility. Their work is published in journal, Applied Energy.
While using supercritical carbon-dioxide for heat mining, the authors dissolved the injected carbon dioxide in the water phase. They also vaporized water in the carbon-dioxide phase, which a continuous flow of the carbon-dioxide, salt precipitation could occur. Therefore, it wouldn’t be a good idea to ignore formation water evaporation. To simulate salt precipitation, the authors adopted mathematical models of precipitation and salt dissolution.
The authors implemented commercial software to simulate the geothermal exploitation process via carbon-dioxide injection, featuring permeability, salt precipitation, and water evaporation models. The software calculated heat transfer between formation rocks and fluids.
They observed that a change in the reservoir properties as well as the injection pressure affected the rate of carbon-dioxide flow and salt precipitation. This can affect the rate of heat production. They, therefore, used a sophisticated model that would offer guidance on screening depleted gas reservoirs for geothermal exploration.
The effects of salt precipitation were moderate and depended on reservoir conditions and back flow of formation water. However, high saline formation water can initiate reservoir damage. An increase in the injection pressure difference minimized the back flow of formation water, thus lessening the effects of salt precipitation. Also, injecting low saline water before injecting dry carbon-dioxide or after precipitation, are a few methods the authors proposed to reduce the effects of salt precipitation.
This study succeeded in developing a comprehensive model for the exploitation of geothermal resources using supercritical carbon-dioxide. Authors used the model to simulate the effects of formation water evaporation and salt dissolution on the rate of heat mining. They found out that depleted gas reservoirs have huge potential for geothermal exploitation.
Guodong Cui1, Liang Zhang1, Bo Ren2, Chioma Enechukwu1, Yanmin Liu1 and Shaoran Ren1. Geothermal exploitation from depleted high temperature gas reservoirs via recycling supercritical CO2: Heat mining rate and salt precipitation effects. Applied Energy, volume 183 (2016), pages 837–852.Show Affiliations
- College of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, China
- Department of Petroleum and Geosystems Engineering, The University of Texas at Austin, Austin, TX 78712, USA
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