Abstract:

Carbonate thermal reservoirs have the characteristics of rich geothermal resources, good mining conditions and large water inflow of single wells, and represent one type of the main thermal reservoirs for the development and utilization of middle and deep geothermal resources. In the process of long-term geothermal exploitation and tail water reinjection, it is necessary to maintain the hydraulic connection between pumping and irrigation wells to avoid thermal breakthrough, The long-term monitoring of thermal reservoir temperature, analysis, and calculation of heat sources in pumping and irrigation wells are important topics for sustainable development and utilization of geothermal resources. This paper introduces the distributed optical fiber temperature measurement technology of pumping and irrigation wells, monitors the influence of pumping and reinjection of carbonate thermal storage on ground temperature, and analyzes the heat sources and quantity of heat storage temperature recovery using the monitoring data. The results show that (1) the heat storage temperature of the pumping well decreased significantly on the sixth day after heating with an average decrease of 1.6 ℃、 which was affected by the small spacing of pumping and irrigation wells and the dominant channel of carbonate heat reservoir, and the pumping and irrigation well produced a thermal breakthrough； the heat storage temperature of the reinjection well failed to recover to the initial temperature before the second heating season due to a large amount of low-temperature tail water reinjection for a long time, and the temperature of pumping well basically recovered. Analysis reveals that the main source of reservoir heat recovery is geothermal water convection and heat accumulation, followed by high-temperature reservoir and geothermal water conduction and heat accumulation. The results of this study show that under the condition of large-scale reinjection, the temperature of reinjection well decreases and the thermal breakthrough of pumping and irrigation well is an inevitable development trend. In order to promote the sustainable development and utilization of geothermal resources, it is very necessary to study the appropriate well spacing of pumping and irrigation wells to prevent thermal breakthrough in a short timescale.

Key words: carbonate thermal reservoir, distributed temperature sensing, thermal breakthrough, thermal recovery