Evaluation of deep granitic geothermal resource potential based on discrete fracture network model: Taking 3003.17 m geothermal exploration well in Wendeng granite body in Shandong Province as an example

This paper is the result of geothermal exploration engineering.

Objective A geothermal exploration well of 3003.17 m has been drilled in the granite body of Wendeng, Shandong Province. To guide the next step of reservoir selection and resource development, it is necessary to conduct an assessment of the geothermal resource productivity of the deep granite thermal storage based on the existing data.

Methods We established discrete fracture network model of hydraulic fracturing based on the geological data, the deep well temperature data, core physical and mechanical test results. The potential created heat reservoirs were simulated for target reservoir. According to the fracturing results, a coupled numerical model of seepage and heat transfer in fractured heat reservoir is established using TOUGH2-EOS1-MINC code. The influence of fracture spacing, well pattern and injection rate on productivity is studied.

Results The natural fractured reservoirs with stress shielding should be selected as the target reservoirs in deep geothermal exploitation. Large well spacing should be maintained, and reservoirs with overdeveloped natural fractures should not be selected. For the same heat reservoir, the variation of each performance index of five−well mode and three−well mode is similar, but their dynamic variation of productivity is different. The pressure index is more sensitive to the well pattern than the temperature index. The power generation of five−well mode is slightly higher than that of three−well mode for the same reservoir.

Conclusions When selecting the target reservoir, the high temperature reservoir with moderate natural fractures and high stress shielding should be preferred. Meanwhile, a large amount of fracturing fluid should be injected to drive the reservoir to extend as far horizontally as possible. For the ideal large-scale heat reservoir, the five-well mode should be preferred, but its overall economic benefits should be considered.