Author: Marija Macenić, mag.ing.min.
In the Republic of Croatia hydrocarbon production began back in the end of the 19th century. However, modern oil and gas production began in the mid-20th century and is still continuing today. According to data from INA d.d. group, there are around 4500 exploratory, production and development wells in Croatia (INA d.d., 2018). By the end of the 20th century production decline, higher water cut, and pressure decrease in reservoirs were noticed. This results in abandoning or reassigning production wells in exploratory or monitoring wells. It can be assumed that the number of reassigned wells will increase in the future in Croatia and over the world.
As an alternative to the expensive process of abandoning wells, temporary abandoned wells can be reassigned to exploit available geothermal energy with various methods. One method proposes to reassign such wells into enhanced geothermal systems (EGS). In this case, colder fluid is injected into reservoir rock (via injection well), circulates through already existing fractures (Cheng et al., 2016) or through fractures caused by hydraulic fracturing (Caulk i Tomac, 2017), where it is heated. The heated fluid is then produced through production well. The other method is the so-called exceptional enhanced geothermal system (EEGS) which proposes the use of abandoned wells to inject air which enables the process of oxidation in the reservoir (Zhang et al., 2008). Water is injected before air injection, which would heat during oxidation. Such heated water can then be produced, along with certain amount of residual oil. The third method proposes an indirect circulation of a working fluid through deep borehole heat exchanger (Morita et al., 1985). Research showed that in the case of deep wells/borehole heat exchangers (BHE) coaxial system is favourable considering larger heat transfer surface area of the pipes and favourable hydraulic properties, when compared to classic 1U and 2U heat exchangers (Kujawa et al. 2006). The revitalization of (temporary) abandoned oil and gas wells is set in few steps. The process begins with packer being installed at the well bottom or directly above perforated intervals. With this, the well is closed and isolated from surrounding rock. Any remaining fluids are then removed, and casings are cleaned and inspected. Afterwards, tubing is installed which, even though closed at the bottom, is perforated and enables fluid flow. The casing pipes in this case are then outer, and tubing is inner pipe of deep coaxial heat exchanger, through which water, as a working fluid, is circulating (Figure 1).
Figure 1. An example of a well (Pčelić-1) after installing deep coaxial heat exchanger (Macenić, 2020).
One of the goals was to determine available heating energy by using deep coaxial heat exchanger in the continental Croatia, given its higher geothermal gradient. Within the thesis, archival data for 154 wells were collected, and with the collected and processed data, a new geothermal gradient map of continental Croatia was obtained (Figure 2).
Figure 2. Novel geothermal gradient mapo of continental Croatia (Macenić, 2020).
Determining available heating energy was done for selected depths from 1500 up to 4000 m (with the step of 500 m), geothermal gradients from 0,034 up to 0,050 °C/m (with the step of 0,004 °C/m) and fluid flow from 10 up to 30 l/s (with the step of 5 l/s), with assumed system life period of 20 years. Also, available heating energy was done for two cases, depending on user needs – constant and variable heating load. Constant heating load presumes the load is constant throughout the year and life cycle of the system. Application of such loads are found in industry, for example in drying industry, aquaculture, balneology, greenhouses etc. Such application is rare, when compared to variable heating loads. Variable heating loads is dependent on climate conditions seen throughout the year, i.e. the system is used during heating season. Variable loads are usually used in space heating, most commonly housing or office space heating, and in some industry processes, such as greenhouses. Analysis was done for lower and higher temperature boundary, in regard with thermo-technical system used. With higher temperature boundary thermo-technical system directly uses available heating energy, while with lower temperature boundary use of heating pump is needed. Figure 3. shows results for constant heating loads, with respect to lower and higher temperature boundary, with the lowest and highest chosen geothermal gradient. It is seen that for lower temperature boundary available heating loads are higher, considering the same conditions of fluid flow and depth, compared to higher temperature boundary.
Figure 3. The results of constant heating loads in the case of higher (left) and lower (right) temperature boundary, with the geothermal gradient of 0,034 and 0,050 °C/m at different depths and fluid flow (Macenić, 2020).
Figure 4. show results for variable heating loads, for higher and lower temperature boundary, at chosen geothermal gradients at 0,034 and 0,050 °C/m. As with constant heating loads, it is seen that lower temperature boundary allows for higher available heating loads (at full load hours at 1500 h), with the same conditions of depth and fluid flow.
Figure 4. The results of variable heating loads in the case of higher (left) and lower (right) temperature boundary, with the geothermal gradient of 0,034 and 0,050 °C/m at different depths and fluid flow (Macenić, 2020
The research determined available heating loads by using reassigned abandoned oil and gas wells in the continental Croatia, via indirect circulation of the working fluid through deep coaxial borehole heat exchanger. By revitalizing (temporary) abandoned oil and gas wells it is possible to reduce costs of abandoning for well owners. Also, it is possible to use these assets, in relatively fast and simple process, to exploit available geothermal energy and to meet EU demands of increasing renewable energy in total energy balance of the country.
Caulk, R. A. & Tomac, I. (2017). Reuse of abandoned oil and gas wells for geothermal energy production. Renewable Energy, 112, 388-397.
Cheng, W., Liu, J., Nian, Y. & Wang, C. (2016). Enhancing geothermal power generation from abandoned oil wells with thermal reservoirs. Energy, 109, 537-545.
Kujawa, T., Nowak, W. & Stachel, A. A. (2006). Utilization of existing deep geological wells for acquisitions of geothermal energy. Energy, 31, 650-664.
Macenić, M. (2020). A conceptual model of exploitation of geothermal energy by revitalization of abandoned oil and gas wells in the continental part of the Republic of Croatia. PhD thesis, University of Zagreb, Zagreb.
Morita, K., Matsubayashi, O., & Kusunoki, K. (1985). Down-hole coaxial heat exchanger using insulated inner pipe for maximum heat extraction. Geothermal Resources Council Trans., 9(1), pp. 45-50.
Zhang, L., Yuan, J., Liang, H. & Li, K. (2008). Energy from abandoned oil and gas reservoirs. SPE Asia Pacific Oil &Gas Conference and Exhibition 2008, 20-22. 10. 2008., Perth, Australia.
Marija Macenić is a research assistant at the Department of petroleum and gas engineering and energy, at Faculty of Mining, Geology and Petroleum engineering, University of Zagreb. She got her PhD title in May 2020, with thesis A conceptual model of exploitation of geothermal energy by revitalization of abandoned oil and gas wells in the continental part of the Republic of Croatia.