Abstract
In deep geothermal boreholes, an effect of temperature (so-called thermal lift) is observed, which results in the volumetric expansion of the fluid extracted. This process results in increased wellhead pressure values being recorded; in the absence of an appropriate correction, hydraulic properties of the reservoir layer cannot be properly determined. As an example of this effect, the Chochołów PIG-1 (CH PIG-1) geothermal borehole situated in Podhale Basin in Poland was used. Hydrodynamic tests including two pumping phases were carried out in the well in order to establish the basic hydraulic properties related to the determination of its operational resources (maximum water extraction value–capacity) and permissible groundwater level. Particular attention was paid to the thermal lift effect in the borehole. The conductivity, which depends on the pumping level, could be two to three times higher with temperature correction than results without any correction. The goal was to analyse the variability of the observed physiochemical properties of the exploited geothermal waters and to determine the correlation between the properties analysed and the temperature of the geothermal water. For the relationship between temperature and the observed pressure at the wellhead, the value of the correlation coefficient was negative (a negative linear relationship was determined), which means that as the temperature increases, the wellhead pressure decreases. The hydrodynamic tests carried out in the CH PIG-1 borehole and the analysis of variability of selected ions and parameters in exploited water were necessary to assess the possibility of increasing the efficiency (Q) of the CH PIG-1 borehole and to determine the water quality and its natural variability. Such information is crucial for the functioning of the recreational complex based on the use of geothermal water. A study of the phenomena affecting the exploitation of hot water from deep boreholes enables their effective exploitation and the use of resources in accordance with the expectations of investors.
Highlights
The physical process of volumetric expansion of fluid is observed in a borehole’s column used to extract thermal waters from the depths, and this is known as thermal lift
During the exploitation of the thermal borehole, this effect is manifested by an increased water table level or increased wellhead pressure compared to the cold borehole, the bottom pressure is the same in both cases
Since it is often impossible to measure the bottom pressure during a hydrodynamic test, the results obtained from measuring the water table or the wellhead pressure should be adjusted for the thermal effect by calculating the so-called reduced pressure [2,3], allowing the determination of absolute drawdown in the borehole, undisturbed by thermal conditions of the medium
Summary
The physical process of volumetric expansion of fluid is observed in a borehole’s column used to extract thermal waters from the depths, and this is known as thermal lift. Thorough knowledge of this phenomenon and appropriate corrections make it possible to properly determine the hydraulic properties of the reservoir layer The impact of this phenomenon on the correct interpretation of hydrodynamic test results increases with borehole depth and formation temperature [1,2]. During the exploitation of the thermal borehole, this effect is manifested by an increased water table level or increased wellhead pressure compared to the cold borehole, the bottom pressure is the same in both cases This affects the design filtration parameters, mainly hydraulic conductivity. By adjusting the results of hydrodynamic tests for the effect of borehole heating, more accurate values of filtration parameters of the aquifer, including hydraulic conductivity, are obtained [1,2]. The correct determination of aquifer filtration properties only becomes possible after the thermal lift effect has been adjusted for, i.e., the value of the recorded pressure in artesian/subartesian systems has been reduced to the value of pressure under thermal equilibrium with the surrounding formation in the borehole [4]
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