Application of Acid Fracturing Technology to Improve Water Yield of Deep Carbonate Geothermal Wells

To achieve the “carbon peak” goal, green and low-carbon energy development is the key. The development and utilization of geothermal resources and the development of deep carbonate geothermal wells in China are analyzed. Based on the principle of chemical reaction between hydrochloric acid and the main components of carbonate rock, four rock samples from geothermal wells were dissolved with 15% and 20% hydrochloric acid, respectively. The comparison showed that 15% hydrochloric acid was more economical and effective. The percolation capacity of acid dissolution fractures was tested through percolation capacity experiments, and the optimal amount of acid solution was determined to be 2-8 PV. It was verified that acid dissolution helps improve fracture percolation capacity. Through the on-site testing of acid fracturing technology in the Xiong’an New Area, it has been verified that the on-site implementation of multi-stage acid fracturing has a significant effect on increasing production. Finally, conclusions are drawn. The method of using acid solution dissolution to increase the water yield of deep carbonate geothermal wells is feasible, and the multi-stage acid fracturing technology is suitable for deep wells and can increase the water output of geothermal wells, which has good promotion value.


Introduction
The key period for China to achieve a "carbon peak, carbon neutral" is the "14th Five Year Plan".To meet the demand for energy production and consumption during the "carbon peak" period, green and low-carbon development of energy is the core.As one of the five non-carbon-based energy sources, geothermal energy has the characteristics of sustained and stable supply, efficient recovery, and renewable energy.Making good use of geothermal energy is significant to achieving the dual carbon goal.Hydrothermal geothermal resources are very abundant in China, and carbonate rock thermal reservoirs are the main battlefield for the development of hydrothermal geothermal resources.Improving the water yield and productivity of deep carbonate geothermal wells is a direction for increasing the geothermal energy supply.

Types, characteristics, and uses of geothermal resources
Geothermal energy is "thermal energy that exists in rock, soil, fluids, and magma bodies within the Earth and can be developed and utilized by humans".
Geothermal resources are "geothermal energy, geothermal fluids, and their useful components" [1], which are mainly divided into three categories: shallow geothermal energy, hydrothermal geothermal resources, and dry hot rocks.The temperature and depth characteristics [2] and main uses [3,4]

Reserves and distribution of hydrothermal geothermal resources
There are abundant hydrothermal geothermal resources in China, with a shallow geothermal resource of 4000 meters equivalent to 1.25 standard coal×10 12 t, and an annual recoverable amount equivalent to 18.65 standard coal×10 8 t.The main components are medium and low-temperature geothermal resources, with a resource amount equivalent to 1.23 standard coal×10 12 t.The annual recoverable amount of geothermal resources is equivalent to 18.5 standard coal ×10 8 t, and the power generation potential is 150×10 4 kW.The medium and low-temperature geothermal resources are concentrated in large and medium-sized sedimentary basins, including North China Plain, Hehuai Plain, Subei Plain, Songliao Basin, Fenwei Basin, etc.Those are the regions with the greatest potential for geothermal resource development [3,4].

Geothermal heat storage
A thermal reservoir refers to a stratum, rock mass, or structural zone buried underground with effective porosity and permeability, in which the stored geothermal fluid can be used for development.
Geothermal energy storage stores geothermal energy through convection and enrichment of heat carrying fluids.
Carbonate rock thermal reservoirs are the main battlefield for the development of hydrothermal geothermal resources in China, characterized by wide distribution, large thickness, and easy recharge.The total distribution area of carbonate rocks in China accounts for about one-third of the land area, and carbonate thermal reservoirs buried in the deep are widely distributed in basins such as North China, Guanzhong, Sichuan, Fenwei, Ordos, North Jiangsu, and Southwest Shandong [6].

Initial formation of the geothermal energy industry in China
In China, geothermal energy is mainly used directly, among them, hydrothermal type is the main force of the geothermal energy industry.Since the 21st century, the development scale and total utilization of geothermal resources have reached the first in the world.By the end of 2020, the total heating and cooling area of geothermal energy in China has reached 13.9×10 8 m 2 , firmly ranking first in the world.Among them, hydrothermal geothermal energy heating 5.8×10 8 m 2 , shallow geothermal energy heating and cooling 8.1×10 8 m 2 , can replace 4100 standard coal per year ×10 4 t, CO 2 emission reduction 1.08×10 8 t.
In recent years, the exploration, development, and utilization technologies of geothermal energy have been continuously innovated in China, and the geothermal energy equipment level has been continuously improved [2].In the Beijing Tianjin Hebei region, the annual exploitable heat of geothermal fluid under the consideration of reinjection conditions for hydrothermal geothermal resources is 2.3×10 18 J, equivalent to 0.78 standard coal×10 8 t, indicating the capacity and demand for large-scale clean heating in the region.In Xiong'an New Area, which strives to build a national model of carbon peak and carbon neutrality city, geothermal energy and other clean energy heating areas exceed 700×10 4 m 2 , giving birth to China's first geothermal heating "smokeless city", and the "Xiongxian model" of geothermal heating has promoted the development of geothermal industry during the 13th Five Year Plan period in China.

Current Situation of Exploration and Development of Deep Carbonate Geothermal Wells in China
Since 2016, China Geological Survey (Hereinafter referred to as CGS) has organized and implemented the "National Geothermal Resources Survey, Evaluation, and Exploration Demonstration" project (renamed as the "Beijing Tianjin Hebei Geothermal Resources Survey, Evaluation, and Comprehensive Development Demonstration Project" in 2018).Through the implementation of the project, the distribution, potential, development, and utilization status of geothermal resources in key areas, such as Beijing, Tianjin, and Hebei, have been basically identified, creating a new situation for the exploration, development, and utilization of deep geothermal energy in China [13].
CGS has detected high-productivity geothermal wells in several places in Beijing, Tianjin, and Hebei.For the first time, a new reservoir in the second member of the Wumishan Formation was drilled in Donglihu Lake, Tianjin.The bottom hole temperature after drilling was 105℃, the wellhead outlet water temperature was 100℃, and the daily water inflow of a single well reached 3120 m 3 , confirming the existence of high-productivity new reservoirs under the main production layers in the Tianjin area [11].Two geothermal wells, D35 and D34, have been deployed in the north and south of Yanling Buried Hill.The final hole depth of D34 is 4507.43m, the highest water temperature at the wellhead is 123.4℃, and the unit water inflow is 0.651 m 3 /hꞏm.Effective utilization of thermal resources is 3.65×10 7 kJ/a, equivalent to about 3867 t/a of standard coal, which is the highest temperature geothermal well in the North China Basin so far.The final hole depth of Well D35 is 3853 m, the maximum water temperature at the wellhead is 109.2℃,and the unit water inflow is 3.320 m 3 /hꞏm.Effective utilization of thermal resources is 5.70×10 7 kJ/a, equivalent to about 5606 t/a of standard coal, which is the largest geothermal well with the largest production capacity in the North China Basin so far [9].
Xiong'an New Area has the best conditions for the development and utilization of geothermal resources in the central and eastern regions of China, and carbonate thermal reservoirs are the main geothermal development and utilization layers in the Xiong'an New Area [6].Since 2017, in order to support and serve the development and utilization of geothermal clean energy in the Xiong'an New Area, CGS has made new progress in the exploration of deep bedrock thermal reservoir and has detected the geothermal well with the highest temperature in North China Basin in the deep carbonate rock in the northeast of Gaoyang geothermal field.CGS has concluded from the practice of optimizing the location of deep high-temperature geothermal wells in the Xiong'an New Area that an appropriate burial depth of bedrock is conducive to the formation of high-temperature thermal reservoirs if economic and technical conditions permit.Taking the North China Basin as an example, bedrock aquifers buried at depths of 3000-5000 m with developed fractures such as carbonate rocks may host geothermal resources of 100-150℃ or even higher temperatures [9].

Principles of Chemical Reaction between Hydrochloric Acid and Carbonate Rock
The main mineral components of carbonate formation are calcite CaCO 3 and dolomite CaMg (CO 3 ) 2 .Hydrochloric acid reacts with calcite and dolomite chemically, as shown in Formulas ( 1 ① We weigh out about 1 g of core powder; ② We place it in the prepared acid solution for 30~60 min; ③ We rinse it with deionized water after filtration; ④ We weigh the quality of filtered core powder; ⑤ We calculate the acid solubility of the core based on the quality difference.15% and 20% hydrochloric acid are prepared for dissolution reactions on the cores (mainly composed of dolomite) taken from geothermal wells D22 and JZ03.The experimental results are shown in Table 2.
Table 2.The test results of the hydrochloric acid dissolution rate show that the acid dissolution rate of carbonate rock cores dominated by dolomite is above 78%, which is very conducive to the formation of acid dissolution fractures with high conductivity.The dissolution rates of 15% hydrochloric acid and 20% hydrochloric acid are not significantly different.Considering the economy and cost, it is recommended to use hydrochloric acid with a concentration of 15%.

Experiment on Improving Percolation Capacity by Reaction of Hydrochloric Acid with Carbonate Rock
The SY/T 5358-2002 reservoir sensitivity flow experimental evaluation method is used, and the experimental steps are as follows: ① We take the core from Well D15 (3015.09-3016.75m) to create artificial fractures; ② We place a core flow meter and measure the initial permeability by positive displacement of standard saline water; ③ We forward displacement of acid solution (15% HCl+3% corrosion inhibitor) 2PV (2 times the amount of pore volume acid, about 15 ml of acid), displacement of 0.5 ml/min; ④ We forward displacement of standard brine until the permeability stabilizes; ⑤ We repeat Steps ②, ③, and ④.Table 3 and Figure 1 show the experimental results of the evaluation of acid dissolution fracture seepage capacity.The results in Table 3 and Figure 1 show that with the multi-stage injection of acid, the core permeability first increases and then decreases, with the optimal amount of acid added 2-8 PV.The multistage acid injection has a significant effect on improving reservoir percolation ability, which is helpful to improve fracture percolation ability.

Application of acid fracturing technology in the Xiong'an New Area
The DRR well is located in Santai Town, Baoding City, at the core of the Rongcheng Protrusion, where the carbonate thermal storage depth is generally around 3000 m [14].The depth of the target layer for DRR well transformation is 3024-3174 m, and experimental results show that the transformation target layer is mainly composed of dolomite, accounting for over 80% [14].Through testing the dissolution rate of rock core acid, it is shown that the dissolution rate of rock core acid reaches over 78%, which is conducive to the formation of acid corrosion fractures with high conductivity.The difference in dissolution rate was not significant when using 15% hydrochloric acid and 20% hydrochloric acid tests [14].
In May 2021, a multi-stage acid fracturing operation was performed on the site for the DRR well.The pumping fracturing fluid displacement was 3.5-4.1 m 3 /min, the pumping fracturing fluid pressure was 39-57 MPa, and the pumping fracturing fluid volume was 396.76 m 3 .The pump injects acid with a displacement of 0.6-3.0m 3 /min, a pressure of 7-34 MPa, and a volume of 95.89 m 3 .The construction curve [14] is shown in Figure 2. The multi-stage acid fracturing has significant stimulation effects.The unit water inflow has increased by 30 times, from 0.024 m 3 /hꞏm to 0.745 m 3 /hꞏm.The water inflow has increased by 10 times, from 4.72 m 3 /h to 44.10 m 3 /h.The water temperature at the wellhead has increased by 6.5℃, from 60.0℃ to 66.5℃ [14].The comparisons of pumping results before and after the stimulation transformation are shown in Table 4.

Conclusion
1.The method of using acid solution dissolution to increase the water yield of deep carbonate geothermal wells is feasible.In addition to optimizing the use concentration and amount of hydrochloric acid, it is also necessary to comprehensively consider factors such as reservoir thickness, reservoir temperature, natural fracture development, and hydrochloric acid action distance [14].
2. The multi-stage acid fracturing technology has been applied for the first time to DDR wells in deep carbonate thermal reservoirs in Xiong'an.This technology can be widely applied to deep wells.It can not only effectively communicate with the thermal reservoir aquifer in carbonate rocks, but also improve the length of acid corrosion fractures, increase the conductivity of the thermal reservoir, and ultimately increase the water output of geothermal wells [14].The on-site application of this technology has achieved a significant increase in production, indicating its good promotion value [14].

Figure 2 .
Figure 2. Multi-stage acid fracturing construction curve of DDR well.

Table 1 .
are shown in Table 1.Characteristics of main types of geothermal resources in China.
According to the SY/T 5336-2006 Core Analysis Method Standard, the acid solubility test was conducted using finely processed core powder (grinding and drying carbonate rock cores, sieving 80 meshes).The experimental steps are as follows: Table of experimental results of acid solubility of coring from different wells.

Table 3 .
Test Results of Acid Dissolution Fracture Seepage Capacity.Figure 1. Test Results of Acid Dissolution Fracture Seepage Capacity.

Table 4 .
Pumping results before and after increasing production of DDR well.