| Title | Review on the reservoir engineering and operation of 16 MW Yangyi geothermal project |
|---|---|
| Authors | Haibing Shao, Feng Luo, Chaofan Chen, Shanmin Wang, Xiaolin Chen |
| Year | 2023 |
| Conference | World Geothermal Congress |
| Keywords | Yangyi; Geothermal Power Production; Organic Rankine Cycle; Binary Plant; Operation and Maintenance |
| Abstract | The Yangyi geothermal field is located in Tibet, China. It was first explored in the late 1980s and early 1990s and was estimated to be a medium- and high-temperature reservoir. By 2012, 2x500 kW screw expanders were installed on-site by the previous owner-operator. In 2016, 2x2MW screw expanders units were added additionally. However, due to a failed 72h pilot-scale power production test, the two big units were not connected to the grid. In 2015, Hangzhou Jinjiang Group became the new owner-operator of the plant, bringing in additional financial investment along with a new operation team. Due to the low system efficiency with the old units and severe calcite scaling in production wells, the new owner ceased the operation of the screw expanders. From 2016 to 2017, a series of reservoir exploration and development work was carried out, including gravity and magnetic-telluric surveys, well repair and logging, production and re-injection tests. Based on a comprehensive evaluation and interpretation of all available data, ZK203 and ZK208 were chosen as the production wells, while ZK403 was converted to a re-injection well. After confirmation of the resource, a 16 MW Organic Rankine Cycle (ORC) based binary power plant was purchased from Ormat Inc and installed on site for the Phase I project. In October 2018, the power plant was connected to the grid, producing just a few MW of power at the beginning, and gradually increasing its output. In Feb 2019, after resolving several technical issues, the power plant reached its 16 MW full capacity output. Before the start of the production, a downhole inhibitor injection system was installed in both production wells, and the calcite inhibitor is continuously injected and mixed with the geothermal fluids. After overcoming several technical difficulties, the injected inhibitor completely resolves the calcite scaling problem, as no scaling minerals are observed in either the wellbores or the separator. Shortly after the start of power production, a long-term tracer injection and monitoring program was conducted. Based on the one-year observation data, it is confirmed that the re-injected geothermal brine flows back into the production well after just a couple of days through the 200~400m deep shallow reservoir. Despite the short retention time, the thermal decay in production temperature is very limited. From 2019 to 2022, a minor temperature decrease is observed in the production wells. Although the power output of the plant is only marginally influenced, a deeper re-injection well is recommended to completely remove the risk of thermal breakthroughs in the future. On average, the geothermal brine and steam are produced from the two wells at a combined total flow rate of ca. 650 t/h. With a dryness factor of 10%, the separated fluids enter the plant at 143 dC. At the outlet of the preheater, the re-injection brine temperature is kept around 60 dC to prevent potential silica scaling. With this setting, a gross power output of 15.92 MW can be achieved on average. With about 11% auxiliary power consumption, a net production of 14.17 MW can be sold to the grid. From 2019 to 2022, the annual net power sale accounts for 88.9, 110.4, 113.1, and 96.8 GWh. Since the feed-in tariff scheme for geothermal power production has not been finally determined yet, Yangyi currently only receives a price of 0.25 RMB/kWh from the state grid. This price is considerably lower than other renewable power sources such as wind and photo-voltaic. Although the plant has had a positive cash flow in the last 4 years, when the depreciation of equipment is calculated, the Phase I project still has a negative investment return. Considering the strong will of reducing carbon emissions by the Chinese government, the owner of the plant is expecting a more favorable sale price for geothermal power. The Yangyi Geothermal Power Plant remains confident to extend its capacity and has made a detailed drilling plan for a new 15~16 MW plant in the Phase II project. The successful deployment and operation of the Yangyi project show that geothermal energy can make a considerable contribution to achieving China’s carbon neutrality goal by 2060. |