Experimental study of the enhanced stimulation of a deep carbonate thermal reservoir in the Xiong'an New Area

Geothermal energy, as a clean and renewable resource distributed worldwide, has received extensive focus in recent years. With the improvement in drilling and logging technology, the depth of geothermal exploration has gradually increased. Carbonate reservoirs are presently the main layer for geothermal development and use in China that have the characteristics of wide distribution, large reserves, and easy reinjection. The current use is limited to the strong karst development zone, approximately 200 m at the top of the reservoirs. Because of the low permeability and strong heterogeneity, the deep carbonate geothermal reservoirs cannot be commercially developed. This study aims to solve the key technical problems of efficiently developing deep carbonate geothermal reservoirs with extreme thickness. The target section was selected by analyzing comprehensive logging and fracture imaging logging data. An innovative simulation technology combining hydraulic jetting and acid fracturing is developed, which has the characteristics of fixed-point fracturing, effective sealing, strong penetration, and a large stimulation range. A production enhancement test was conducted for carbonate geothermal wells in the following order: comprehensive logging, imaging logging, casing cementing, perforation, pumping test, small pressure test, hydraulic injection acid fracturing, pumping test (after fracturing), and other construction processes. Comprehensive logging is an effective means to interpret the macroscopic pore and permeability properties of a reservoir and can be used to initially select the target geothermal reservoir. Fracture imaging logging can provide a more intuitive understanding of fracture development and distribution characteristics. The results show that the fracture density of geothermal well D22 does not decrease substantially with increasing depth, and the fracture width tends to decrease with depth clearly. The experimental geothermal well D22, which has the largest thickness of carbonate geothermal reservoir exposed in the Xiong'an New Area, was selected to perform a pilot field test of stimulation. The results show that the water inflow of the target section at 3024–3174 m increased from 4.72 m³·h⁻¹ before stimulation to 44.10 m³·h⁻¹ after stimulation, increasing by 8.3-fold. The unit water inflow increased from 0.024 m³·(h·m)⁻¹ before stimulation to 0.745 m³·(h·m)⁻¹ after stimulation, increasing by 30-fold. The reservoir permeability coefficient increased from 4.4×10⁻³ m·d⁻¹ to 146.3×10⁻³ m·d⁻¹. The wellhead water temperature increased from 60.0 °C before stimulation to 66.5 °C after stimulation. Therefore, the development potential of deep and thick carbonate geothermal reservoirs can be substantially improved through the developed stimulation. This research can provide technical support for the large-scale development of geothermal resources in China.