Survey on Geothermal Resources in Zhangjiakou Area

. The Zhangjiakou area is rich in geothermal resources, and many counties in the region have discovered low-temperature geothermal. Structurally, Huailai County, Zhangjiakou City is located at the intersection of Zhangjiakou-Penglai Fault Zone and Shanxi Fault Basin (or Fenwei Seismic Tectonic Zone), namely the Yanqing-Huailai Basin (Yanhuai Basin). A large amount of geothermal resource investigation and research works has preliminarily defined that the area has good geothermal resource accumulation conditions. From 2019 to 2020, the China Geological Survey has organized several investigation and research in the area for in-depth geothermal resource. As one method of the survey results verification, a parameter well for geothermal resource survey was deployed as ZK02 and completed at 3000m depth. The ZK02 well was completed at a depth of 3006.9m, drilling through the Quaternary, Neogene, and Jixian strata, and entered the Archean gneiss strata. The well successfully explored high-quality artesian heat storage in the Jixian strata, obtained deep geothermal geological data and physical data, revealed the regional stratigraphic sequence and geological structure characteristics, and provided a scientific basis for regional geothermal resource potential evaluation. Based on the engineering practice of ZK02 well, this paper systematically summarizes drilling technology and experience from well structure to drilling equipment, construction technology and key technology, analyses the complex conditions and countermeasures downhole, and provide reference for the follow-up regional geothermal resources investigation, research and development and.


Preface
The Zhangjiakou area is rich in geothermal resources, among which, Chicheng, Huailai, Yangyuan, Yuxian and other counties have discovered low-temperature geothermal, the current development and utilization methods are mainly wellness tourism [1].Structurally, Huailai County, Zhangjiakou City is located at the intersection of Zhangjiakou-Penglai Fault Zone and Shanxi Fault Basin (or Fenwei Seismic Tectonic Zone), namely the Yanqing-Huailai Basin (Yanhuai Basin).A large amount of geothermal resource investigation and research work has preliminarily defined that the area has good geothermal resource accumulation conditions [2][3] [4].In order to further ascertain the accumulation conditions and resource potential of deep geothermal resources in the Yanhuai Basin, from 2019 to 2020, the China Geological Survey organized investigation and research for in-depth geothermal resources in the area, and deployed high-precision gravity and magneto telluric (MT) , Controllable source audio magneto telluric (CSAMT) and seismic surveys and other comprehensive geophysical exploration work [5]; At the same time, in order to verify the survey results, a 3000m deep geothermal resource survey parameter well "ZK02" had been deployed and completed.The ZK02 well was completed at a depth of 3006.9m,drilling through the Quaternary, Neogene, and Jixian strata successively, and entered the Archean gneiss strata.The well successfully explored high-quality artesian heat storage in the Jixian strata, obtained deep geothermal geological data and physical data, revealed the regional stratigraphic sequence and geological structure characteristics, and provided a scientific basis for regional geothermal resources potential evaluation.
This article combines the engineering practice of ZK02 well and systematically summarizes the drilling construction technology and experience, which can provide reference for the follow-up regional geothermal resource investigation, research, development and utilization.

Geological Profile
ZK02 Well is located in the north of Donghuayuan Town, Huailai County, adjacent to Guanting Reservoir in the north, and structurally located in the southwest of Yanfan Basin.As the depth of the peripheral known boreholes is shallow, and there are very few reference materials [6][7], this well is the first mid-deep geothermal parameter well in the survey area.Inferred from regional geological data analysis and geophysical prospecting results, the strata in the Donghuayuan area are mainly composed of Quaternary, Neogene, Jurassic, Jixian, Great Wall and Archean gneiss, as shown in Table 1.During the drilling of this well, the strata encountered were Quaternary, Neogene, Jixian and Archean from shallow to deep, no Jurassic strata were seen.As shown in Table 2.The Quaternary stratigraphic lithology from shallow to deep are: brown fine gravel, mainly composed of quartz and granite; light gray clay, medium plasticity, containing silty fine sand; light gray coarse sand, mainly composed of quartz, sub-circular, sorted Etc., partly with sandy soil and fine gravel.
From shallow to deep, the Neogene stratigraphic lithology are light gray sandy mudstone, gray claystone, light gray brown sandy gravel, sandy mudstone, and sandy gravel.
The Jixian system is composed of Tieling Formation, Hongshuizhuang Formation, Wumishan Formation, Yangzhuang Formation, and Gaozizhuang Formation from shallow to deep.The lithology is dominated by dolomite, and there are well-developed fissures in some well sections.
The lithology of Archean strata is granite gneiss, gray and off-white gneiss-like structure, and the main components are feldspar, quartz, hornblende, mica and other dark minerals, which are recrystallized.

Well Structure
Before the ZK02 well, the stratification and burial depth in the deep part of the region were inferred based on the existing geological data and geophysical prospecting results.Therefore, in order to ensure the smooth implementation of the drilling project to achieve the geological purpose, an extra section is reserved in the well structure design.According to the drilling engineering design, the well structure has three sections with the completion diameter is Φ215.9mmbare hole.The Φ152.4mm diameter is reserved as an alternative plan for downhole complex drilling situation.Shown in Table 3.In the actual drilling process, the bottom depth of the Neogene was 1327.9m, and no Jurassic stratum was seen.The Jixian system as the main reservoir has well-developed fissures and relatively fragmented partial stratum, which will cause risk of well wall collapse during the pumping test.According to the engineering design, the well structure and casing depth have been optimized and adjusted: First is increasing depth of the first and second section casing to the top of the Jixian formation; Second is insert Φ139.7mm casing + screen pipe when the third section completed, and arranged the screen pipe according to the distribution of the hot water layer.Shown in Table 4.The well structure design is very important to the exploratory well drilling project especially in unknown or more complex formations [8][9], and the adjustment of well structure during the drilling process is more common [10] [11].A common practice in well structure designing in unknown or more complex formations is to reserve an extra section, or to develop an alternative well structure which can effectively prevent the implementation of the drilling project from being too passive or forced to stop due to the complexity situation downhole [12] [13].In the engineering design, The ZK02 well has reserved an extra section which provides ample room for the adjustment of the well structure, during the implementation of the project, reasonable adjustments were made to the well structure according to the stratum conditions, which provided a strong guarantee for the smooth implementation of the project.

Drilling equipment selection and matching
The selection of drilling engineering equipment must considering about geological purpose of drilling, regional environment, construction technology, completion depth, completion diameter and other factors [14][15] [16].In recent years, with the continuous acceleration of the exploration and development of geothermal (hot dry rock) resources, the workload of geothermal drilling has increased sharply.Existing large-diameter geothermal drilling equipment mainly includes water well drilling rigs and oil drilling rigs.The advantages of water well drilling rigs are that the equipment is relatively compact, easy to relocate, and there are few construction crews; its disadvantages are mainly that the drilling depth is limited by the lifting ability, and the installation conditions of the well control equipment and solid phase control equipment are limited by the height of the first-floor platform.The installation conditions of logging sensors are poor.The oil drilling rigs are relatively mature, with a complete range of drilling depth capabilities.The structure and design of the drilling rig meet most needs of well control equipment and solids control equipment, also the ability to handle complex accidents is relatively strong.In the process of drilling equipment selection, especially exploratory wells, new types of drilling rigs with maturity and recent age should be selected first, which can provide an effective guarantee for the smooth implementation of the project.
The ZK02 well is the first exploratory well in the area.There is no reference to the well-informed well.It only has a rough estimate of the strata encountered downhole, and there is little information available for prediction of downhole conditions, and based on the principle of ensuring the safety of drilling, large capacity redundancy needs to be considered when selecting drilling rigs.Therefore, standard petroleum ZJ-30 electric drilling rigs are used, and all supporting equipment are electric, which is more friendly to the surrounding environment of the construction, and the diesel generator set is only used as an emergency backup.The main equipment list is shown in Table 5.
First, based on the drilling depth requirements, the selection of standard petroleum ZJ30 equipment can effectively ensure provide the lifting force required to drill to the designed well depth; Secondly, the supporting system according to petroleum standards can effectively ensure the compatibility of equipment, operation safety, construction efficiency and fulfil other requirements; Third, the use of all-electric equipment can greatly improve work efficiency, reduce environmental pollution, and save drilling costs.At the same time, in order to improve drilling complexity and risk response capabilities, a full set of well control equipment and blowout prevention tools are provided on site to cope with blowout risks, and diesel units are provided to cope with complex situations such as equipment failures and emergency power outages.

The second section drilling
The strata encountered in the second section drilling are mainly Tertiary sandy gravel, claystone, Jixian sandy gravel, mudstone, and dolomite.The drill tool combination used is Φ311.2mmdrill bit + Φ203.2mm drill collar + Φ177.8mm drill collar + Φ127 drill rod + Kelly bar.The mud specific gravity is 1.02~1.08g/cm3,the weight on bit is 20-120kN, and the displacement is 22-30L/s.For drilling in this section, the following points should be paid attention to: (1) Adopt pendulum drilling tool combination to prevent deflection; (2) In High steepness well sections with easy deviating conditions should intensify inclination measurement, master the data and take technical measures timely, and use the MWD inclinometer to prevent inclination and maintain straightness according to the actual situation; (3) Make a record of the drilling fluid, discover formation leakage and water inrush in time, and adjust the performance of the drilling fluid and drilling parameters accordingly; (4) Adjust the performance of the drilling fluid in time, balance the formation pressure, prevent block dropping and collapse, connect a single root to reduce the number of strokes, minimize the number of trips, and prevent the drill bit from colliding with the well wall; (5) Ensure that the drilling equipment is operating well; (6) The use of screw + cone bit compound drilling can effectively increase the mechanical drilling speed.However, attention should be paid to the formation conditions, and conventional drilling tool combinations should be used as far as possible in the leak-prone formations to make preparations for leaking at any time.

Environmentally Friendly Drilling Fluid
The selection of drilling fluid formula should consider about all the aspects of pertinence, suitability, temperature resistance, security and environmental protection.
The stratum encountered in the first drilling section was the Quaternary and Neogene.The lithology is mainly light gray clay, light gray coarse sand, sandy gravel, sandy mudstone, etc.The stratum is relatively loose and broken.The higher viscosity bentonite drilling fluid is used to quickly drill through and place the casing.Using bentonite drilling fluid, the formula is as follows: Water+6%~8% Bentonite+0.1%~0.2%Caustic Soda+0.2%~0.4% Tackifier (GZN).
The stratum encountered in the third section is granite gneiss which is stable.A solid phase free polymer drilling fluid system is adopted, and polymers are added to improve the carrying capacity of the mud and ensure the cleanness of the wellbore.If the well wall is unstable, use the mud positive circulation construction instead.When the temperature exceeds 80℃, use the temperature-resistant solid phase free polymer drilling fluid system.The formula is as follows: water + 0.1%~0.2%caustic soda + 0.3%~0.6%thickener (GZN) + 1%~2% plugging agent (GFD-1) + 1%~1.5% fluid loss agent (GPNH) +0.1%~0.2%coating agent (GBBJ).
6 Pumping test

Overview of the pumping test
The ZK02 well used air compressors to clean the wells before the pumping test from January 23 to 24, 2021 in order to remove the blockages such as mud and rock cuttings in the hole especially in the thermal reservoir section to achieve clean water without sand.Combining with the geothermal resources in this area, it is defined that the pumped water is confined water, and the pumping test adopts the three-time depressurization and steady flow test method in a single well.The pumping test started at 20 o'clock on January 25, 2021 and ended at 0 o'clock on January 30, 2021.It lasted 100 hours.the duration and stabilization time of each pumping phase shown in Table 6.The change range of the pumping test when the flow rate is stable is shown in Fig. 2, where q is the fluctuation range of the water inflow relative to the average value, and S is the drop distance in the same period.The data is obtained by field measurement.It can be seen from Fig. 2 that as the drop distance increases, the water inflow gradually stabilizes.

Fig. 2. q-S curve
The entire change process of the pumping test can be intuitively understood Through Fig. 3.The vertical axis is marked with a one-way dual axis Q (pumping volume) and S (pumping depth), the horizontal axis is t (pumping time).It can be seen from Fig. 3 that when the pump is stopped after pumping, the water level can be quickly restored.
The entire change process of the pumping test can be intuitively understood by Fig. 3.The vertical axis is marked with a one-way dual axis Q (pumping volume) and S (pumping depth), the horizontal axis is t (pumping time).It can be seen from Fig. 3 that if the pump is stopped after pumping, the water level can be quickly restored.According to the results of the pumping test of the ZK02 well, the hydrogeological parameters of the well are shown in Table 8.
The geothermal productivity test of The ZK02 well include three pumping tests with depressurization and steady flow in single well.Comprehensive analysis is carried out based on the results of this pumping test.The Q=f(s) relationship curve of this well is a power curve (exponential curve) type, and the pumping duration is analyzed at the same time.It can be seen from the curve that the water level of this well quickly stabilizes during pumping, and can recover quickly after stopping the pump, indicating that the well's thermal reservoir has good permeability and medium water richness, which is basically consistent with the hydrogeological characteristics of this area.It also proves that the pumping test is successful and the hydrogeological parameters obtained are true and reliable.

Conclusion
The ZK02 Well is an exploratory well deployed in Huailai County for deep geothermal resource exploration in Zhangjiakou area.By carefully designing the well structure, optimizing drilling equipment and technology, and controlling key parameters, the mission of 3006 meters footage was successfully completed.The geothermal productivity test through pumping test proved that the thermal reservoir of this well has good permeability and medium water richness, laying the foundation for the future development of geothermal resources in this area.Through the practice of the construction process, the systematic summary of drilling technology and experience can provide reference and support for the follow-up regional geothermal resource investigation, research, development and utilization.

Table 1 .
Prediction of drilling formation in ZK02 well

Table 2 .
Summary of drilled formations in ZK02 well

Table 3 .
Structure design of ZK02 well

Table 4 .
Completion structure of ZK02 well

Table 5 .
List of main equipment Table for Each Pumping Stage

Table 8 .
Calculation table of hydrogeological parameters