Analysis on the thermal efficiency improvement process for geothermal well in porous sandstone

In China, the wellhead water temperature of geothermal well in porous sandstone is lower and pilot tests on thermal effciency improvement process are defcient. To solve these problems, the adaptability between the thermal effciency improvement process and the wellhead water temperature was investigated in this paper. The water temperature curves of wellbore under various conditions were calculated from two aspects (i.e., geothermal well completion parameters and wellbore heat insulation technology) respectively by using the steady-fow thermodynamic model of open system. Then, different processes were compared from the aspects of warming effect, cost and operability. It is indicated that when the water is pumped at low rate, the wellhead water temperature is lower due to the joint effect of on-way heat loss and water production of low-temperature permeable hole section in the upper part. When the water is pumped at high rate, the wellhead water temperature increases with the daily water pumping capacity, but its increasing speed slows down. When the daily water pumping capacity is higher than 3 500 m³/d, the temperature increasing speed is quite low and tends to fatten. The deeper the main water producing hole section, the higher the wellhead water temperature. The wellhead water temperature can be increased by 8-18 ℃ if the high-temperature thermal reservoir at the bottom is produced by designing rationally the installation depthof water stopping device and perforated pipe in the geothermal well. If only the pump setting depth is increased, the increase of wellhead water temperature is less than 0.2 ℃. If annulus nitrogen heat insulation or aerogel heat insulation is adopted, the thermal conductivity of cement sheath is decreased or the cementing section is extended, the wellhead water temperature can be increased by 0.23-1.3 ℃, but its cost is high and its implementation is diffcult. If the steel pumping pipe is extended and the insulating coating is painted, the wellhead water temperature can be increased by 0.6-1 ℃, and this process can be implemented easily with lower cost. Finally, the functional equation between the wellhead water temperature and the daily water pumping capacity was ftted. It is shown that by virtue of this functional equation, the wellhead water temperature of geothermal wells with the similar string structure in porous sandstone at different fow rates can be predicted quickly. The research results provide the theoretical basis and guide for the prediction of wellhead water temperature of geothermal wells in porous sandstones and the selection of thermal effciency improvement process on site.