Comparative analysis of ground source heat pump drilling construction techniques under complex geological conditions

: The buried pipe is an important part of the geothermal heat pumps system. One of the main reasons for the high initial installation cost of the ground source heat pump system is that its heavy drilling workload, long construction period and high cost account for the system cost. Especially in some complex geological conditions, the high proportion of drilling costs directly increase the overall cost of the geothermal heat pumps system, which to a certain extent restricts the popularization and application of geothermal heat pumps system. Based on the comparative analysis of different types of geological conditions and different ground source heat pump drilling technologies, this paper starts with the research and analysis of basic geological structure. On the premise of meeting the requirements of the system, this paper aims to improve the drilling efficiency and reduce the cost by rationally designing pipe Wells and optimizing the drilling technology scheme.


Introduction
The research and project implementation of underground pipe Ground source heat pump is the latest of the three forms of Ground source heat pump in China, and its cost and operating costs are relatively higher than those of ground source heat pump and surface water ground source heat pump systems.However, these do not hinder the rapid development of buried pipe ground source heat pumps.Ground pipe ground source heat pumps use circulating water in buried pipe heat exchangers to exchange for temperature difference energy stored in the soil, without worrying about the extraction and pollution of natural water sources.Therefore, they have wider applicability and higher safety and stability, especially in hot summer and cold winter areas as a new type of air conditioning cold and heat source.The tube well of ground source heat pump is an important part of the Ground source heat pump, which is one of the main reasons for the high initial installation cost of Ground source heat pump because of its heavy drilling workload, long construction period (relatively), and high proportion of costs in the system cost (generally 15%~20% for simple soft soil layers, 30%~35% for rock formations, and 45%~50% or more for special complex formations).Especially in some complex geological conditions, the high proportion of tube well cost directly increases the overall cost of Ground source heat pump, which restricts the promotion and application of Ground source heat pump to a certain extent [1][2][3] .
The geological conditions, drilling equipment, technological conditions, and other factors in the construction area directly affect drilling efficiency and cost, and to some extent determine the development prospects of ground source heat pump drilling.

Ground source heat pump
The Ground source heat pump is a heating and air conditioning system consisting of a water source heat pump unit, a Geothermal energy exchange system and a system inside a building, with rock and soil, groundwater or surface water as the low temperature heat source [4][5][6] .
According to the different forms of Geothermal energy exchange and buried pipes, Ground source heat pump are classified as follows Table 1: Ground source heat pumps not only transfer heat or cooling capacity from the underground soil to the desired location, but also utilize the enormous heat and cooling storage capacity of the underground soil.In winter, the ground source transfers heat from the underground soil to the building, and in summer, the underground cooling capacity is transferred to the building.A cooling and heating cycle system is formed annually, achieving energy-saving and emission reduction functions [7] .

Characteristics of Drilling Construction
According to the requirements of Ground source heat pump, generally speaking, its drilling has the following characteristics [8] : 1. Simple structure (1) The depth is shallow, usually around 100m to 120m; (2) The aperture is relatively small, between large and small apertures, so it is generally referred to as drilling or drilling; (3) The well structure is simple and does not require high requirements, usually based on ensuring verticality, meeting the requirements of pipe laying and backfilling.
2. Large workload and concentrated construction areas; 3. Tight construction schedule requirements; 4. The unit price of drilling is constrained by the overall investment of the system; 5.The geological conditions of individual project construction have certainty (1) Pay attention to geological exploration of test hole strata; (2) Pay attention to the design of tube well structures, method selection, construction equipment, and process selection; (3) Predict drilling construction costs.6. Due to geological conditions, engineering construction has multiple repetitions.

Geological conditions of common construction areas
Geological conditions are the basis for establishing the structure of the pipe well, selecting construction methods, construction equipment, and processes; One of the important factors affecting drilling costs [9][10] .
Due to the drilling depth generally ranging from 100m to 120m, which is located in the upper part of the rock and soil layer, the geotechnical engineering geological conditions are complex.Common geological layers include backfill layers, gravel layers, karst layers, as well as water swelling layers and structural fracture zones, all of which can have an impact on the drilling process.
The drilling technology of ground source heat pump under different geological conditions is completely different from the general geological exploration or other engineering drilling purposes, so the construction considerations are also different, and the methods used are relatively more [11][12][13] .
1. Sedimentary strata formed during the recent geological history period The sedimentary strata formed during the recent geological history period generally have a relatively simple structure and less difficult construction, generally including: (1) The Quaternary (Q) and Neogene (N) are commonly referred to as overburden layers, which are loose, non cemented, or semi consolidated loose formations.They are easy to drill and have high drilling efficiency, low energy consumption, and relatively low drilling costs [14][15] .
(2) The density and strength of rocks in Eogene (E) and previous strata have increased with the deepening of strata and the advance of Geologic time scale.The difficulty of breaking rocks (or drilling into holes) increases relatively, resulting in reduced drilling efficiency and high energy consumption, leading to a natural increase in drilling costs; Drilling under geological conditions with large dip angles often leads to hole inclination, and anti inclination measures must be taken.
2. Geology of Metamorphic rock and magmatic rock Under the influence of geological movement and Orogeny in various periods, the formation of complex, dense and hard rock mass is affected by folding, fracture, distortion, metasomatism and other actions of rock strata (even multiple periods).Moreover, the bedrock in such areas has generally undergone natural factors such as longterm weathering, erosion, erosion, and metamorphism in geological history.Geological phenomena such as ancient weathered crust and structural fracture zones are relatively developed, and the rock structure is uneven, which will increase the difficulty of drilling construction [16] .

Mountain front and valley strata
The slope and flood deposits in such areas are relatively developed, and due to the short distance of sediment transport, the sorting and rounding are poor.Generally, they are mainly composed of gravel layers and coarse particle deposits.Its main characteristics are: (1) The particle size, thickness, and even composition of the accumulation vary; (2) Loose structure, no bonding or poor bonding degree, no filling; (3) When the void ratio is large and located below the regional groundwater level or with a supply source, it is often a rich aquifer.
It is difficult to drill a well in this type of formation (with a large thickness), and the following problems are often encountered during the drilling process: (1) The drilling circulating medium (liquid or gas) is lost and unable to form effective circulation; (2) Difficulty in effectively crushing gravel and extremely low drilling efficiency; (3) Grinding tools consume a lot and are mostly damaged by abnormal wear and tear; (4) The frequency of hole collapse and block falling is high, which can easily cause drilling accidents; (5) Even if the hole is formed, in the case of bare holes, the hole wall is irregular and smooth, making it difficult to lower the pipe (the hole wall is unstable); At present, the most effective method for construction in this type of formation is the air down-hole hammer and pipe drilling technology.However, there are still limitations to this technology in China, mainly including: (1) The depth is limited, and the single level depth generally does not exceed 30m; (2) Groundwater (especially rich water layers) has a significant impact on air drilling; (3) Due to the use of high pressure air compressors, power consumption is high; (4) The complete equipment and supporting tools are expensive.

Various backfill layers
Due to the disorder and diversity of backfill materials, it can have varying degrees of impact on drilling, but the main impact is also on large-scale, deep thickness, and blocky backfill.Its characteristics are similar to those of the mountain front and valley strata [17] .

5
Common Drilling Construction Techniques

Ordinary rotary drilling rig
The rotary drilling rig is driven by a power device to rotate the rotary device of the drilling rig, thereby driving the drill rod with a drill bit to rotate, and the drill bit is used to cut soil.The rotary drilling rig is used for grouting piles with mud protection walls, and the drilling method is rotary drilling.
Classification of drilling rigs [9] According to different mud circulation methods, it can be divided into forward circulation rotary drilling machines and reverse circulation rotary drilling machines.
(1) Positive circulation drilling operation, driven by the rotary device of the drilling rig, the drill rod and drill bit rotate to cut and break the rock and soil.During drilling, mud is used to protect the wall and discharge slag; After the mud is pumped into the inner cavity of the drill pipe, it is shot out through the mud outlet of the drill bit, driving the drilling slag to rise along the annular space between the drill pipe and the hole wall, overflow into the sedimentation tank, and return to the mud tank for purification.
(2) Reverse circulation drilling operation involves mud flowing from the hole mouth through the gap between the drill rod, drill bit, and hole wall into the bottom of the hole, and then carrying drilling slag back to the ground mud settling tank from the drill bit and drill rod [11] .

Structural composition
The rotary drilling rig mainly consists of a flat frame, belt drive wheel, gearbox, winch, rotary drilling disc, universal joint drive shaft, gantry frame, crown wheel, drill pipe, and drill bit.
(1) Flat frame: The chassis of a drilling rig, which stabilizes the drilling frame and fixes the winch power device; (2) Belt drive wheel: a transmission device that transmits power from one drive wheel to another through a belt; (3) Gearbox: bears the force of transmission machinery, and its main function is to convert the power generated by the belt conveyor and transmit it to the rotary drilling plate through the universal joint transmission shaft, fixed on the flat frame; (4) Winch: power device for lifting drill pipe and drill bit, including motor and lifting wire rope; (5) Rotary drilling disc: used to apply drilling pressure to the drill bit, improve the stress condition of the drill pipe string, and fixed on a flat frame; (6) Universal joint transmission shaft: a transmission device that transmits the power generated by the belt drive wheel to the rotary drilling disc and is fixed on a flat frame; (7) Gantry frame: Steel structure column tower.It is used to lift the drill bit, drill pipe and cast-in-place pile reinforcement cage.The gantry is connected with a diagonal brace to form a stable structure.The beam between the columns is provided with a guide pulley anchor support; (8) Heavenly wheel: a turning device for lifting the steel wire rope at the tail of the pulley block, used to lift the active drilling rod; (9) Drill pipe: The rod connecting the drill bit and drill collar to ensure the verticality of the hole; (10) Drill bit: The bottommost part connecting the drill pipe.Cut the rock and soil at the bottom of the hole to form the hole.
Applicable Geology (1) Suitable for filling layers, silt layers, cohesive soil layers, silt layers, sandy soil layers, and other soil layers; (2) Suitable for geological conditions such as sand and gravel layers with a content of no more than 15% and a particle size of less than 10mm, as well as soft bedrock; (3) Suitable for bored piles with a pile length of less than 200m and a pile diameter of 0.8-1.2m.

Hydraulic Downhole Hammer
Hydraulic down-hole hammer (also known as hydraulic hammer or hydraulic hammer) is a device that generates impact load in hydraulic impact rotary drilling.It uses the energy provided by the mud pump during the drilling process to directly drive the hammer inside the hydraulic hammer to form reciprocating motion, and continuously applies a certain frequency of impact load to the lower drilling tool, thereby achieving impact rotary drilling.
Hydraulic down-hole hammer drilling is a significant reform of conventional rotary drilling and a new drilling method following modern diamond drilling and air drilling.It effectively utilizes the weakness of hard rock with high brittleness and low shear strength, which is not resistant to impact.It is an effective drilling technology to solve the low drilling efficiency and poor drilling quality of hard rock layers and some complex rock layers.
Characteristics Compared with conventional rotary drilling, hydraulic down-hole hammer drilling has the following advantages: (1) Fully utilize conventional on-site supporting equipment without changing existing operating procedures; (2) Due to the high pump pressure of the mud pump, the hydraulic hammer is suitable for larger hole depths (up to 5118m); (3) High drilling efficiency in hard rock formations.(4) Broken strata are not easy to block the core, and the footage of each round is long; (5) Effectively extending the lifespan of drill bits; (6) Reduce the strength of hole inclination to a certain extent; (7) Low energy consumption and less environmental pollution; (8) Not afraid of aquifers, with strong wall protection ability; (9) Easy to operate and maintain; Classification (1) Positive action type: The return movement of the valve and hammer relies on their respective springs, while the stroke relies on water hammer to drive the hammer downward and generate impact energy.
(2) Reaction type: Using the pressure increase of the flushing fluid to push the hammer upward and compress the working spring to store energy.When the valve is opened, the liquid flow is unobstructed, and the pressure in the working chamber decreases.The working spring releases the stored energy to drive the hammer downward and produce an impact.
(3) Double acting: The return stroke and stroke motion of the hammer are driven by the pressure of the flushing fluid, and when the hammer descends, impact energy is generated.
(4) Compound action formula: A product developed by combining any two of the three action forms mentioned above.
(5) Hollow through type: developed in conjunction with gas lift and hydraulic center reverse circulation drilling, it differs from ordinary hydraulic hammers in that there is a through cylindrical hole along its axis from top to bottom, forming a reverse circulation continuous transportation channel for rock cores (samples) during drilling.Therefore, all parts of the heart are connected, such as the check valve, cylinder seat, piston, and drill bit, which are cylindrical.
(6) Derived products that combine with other technologies (such as rope core, liquid gas dual use, etc.).

Pneumatic Downhole Hammer
Pneumatic down-hole hammer air drilling refers to a new drilling technique in which compressed air or a gas-liquid mixture containing compressed air is used as the flushing medium, or compressed air is used as both the power of the rock breaking machine and the flushing medium.Air drilling can be either positive circulation drilling or reverse circulation drilling.
Compressed air is used as both a flushing medium and aerodynamic energy.A pneumatic down-hole hammer is connected between the drill bit and the drill pipe to convert the energy of compressed air input through the drill pipe into mechanical impact energy, and a piston is used to impact the drill bit to achieve rock fragmentation at the bottom of the hole.Its rotation is still driven by the drill pipe, which is also driven by the drill pipe,.The characteristics are large impact function, low rotation speed, low axial drill bit pressure, high rock crushing efficiency, and long service life of the drill bit.It is suitable for drilling medium hard and above rock layers.Compared with general rotary drilling, it can increase the drilling speed by 5-10 times.A ball tooth DTH hammer drill bit can reach 2000m of diamond limestone.Mainly used for drilling bedrock water wells and related engineering holes, it can also achieve good technical and economic results in areas with widely distributed years of thick frozen layers, as well as anhydrous deserts and high mountain areas.
This method is suitable for drilling hard rock layers, and the drilling is not prone to deviation, making it more advantageous for use in water scarce areas.Using it to drill hard rock and pebble layers can improve drilling efficiency by several to ten times compared to conventional rotary drilling and mechanical impact drilling, and has been promoted as an advanced technology.Its application field is constantly expanding, mainly used for drilling blasting holes in mines in the past.Currently, it has been applied in fields such as hydropower, railways, highways, water wells, building foundation piles, geological exploration, etc.
Classification According to the structural characteristics of the gas distribution system, down-hole hammers are divided into two types: valved and valveless.
(1) Valved down-hole hammer.The airflow is controlled by the valve plate and can be divided into two types according to the exhaust method: side exhaust and center exhaust.The side exhaust valve down-hole hammer was the earliest used, and its exhaust gas is discharged from the side of the hammer head.The exhaust gas from the downhole hammer with a central exhaust valve is discharged through the central hole of the hammer head.In comparison, using a central exhaust method to remove rock debris has a good effect, a long service life of the hammer head, and high drilling efficiency.
(2) Valveless down-hole hammer.There is no valve plate, and its gas distribution system is composed of a set of holes arranged on the cylinder wall, which automatically distribute gas during piston movement.In the total stroke of the piston movement, there is a section that relies on compressed air expansion for work.Under the same pressure conditions, the compressed air consumption is only 2/3 of that of a valved DTH hammer.

Determination of construction plans for different geological structures
Studying the formation composition within the construction depth range is beneficial for systematically formulating drilling plans, determining drilling techniques or composite drilling techniques, and optimizing drilling to improve drilling efficiency and reasonable drilling costs.According to the characteristics of ground source heat pump tube wells, the rock and soil layers exposed by drilling can be divided into: 1. Simple strata, drilling reveals a single type of strata, and the construction method is simple.
(1) In general, loose soft soil and sandy soil formations, ordinary rotary drilling machines and simple mud wall protection can be used, and the process is simple; (2) In general rock formations, the formation is stable, the structure is simple, the rock hardness is low, and the drillability is good.A rotary drilling rig with simple mud protection can also be used for construction.If conditions permit, using pneumatic down-the-hole hammering is more conducive to improving efficiency; (3) When the rock hardness is high, it is easy to use pneumatic down-hole hammers for drilling.
2. Composite formation, drilling through two or more types of formations, requires more than one process or drilling method or combination of drilling tools to complete the drilling.
(1) The bedrock is covered with loose clay and loess layers: when the thickness is not large and the soft layer wellbore is relatively stable, pneumatic down-hole hammers can also be used for drilling; (2) When the overlying loose layer is sandy soil, sand layer, or gravel layer, isolation measures must be taken; When the formation is rich in water, mud drilling must be used first, isolated, and then converted into pneumatic down-hole hammer drilling; (3) When the thickness of the overlying loose layer is large and isolation from the casing cannot be achieved, while traditional casing isolation is cumbersome, careful selection should be made between mud drilling and air drilling after casing isolation.
3. Complex formations, which are difficult to drill and complete under ordinary technical conditions or are prone to complex situations during the drilling process.
(1) A pebble layer with a larger diameter and thickness; (2) Formation with developed fractures; (3) Formation fracture and fracture zone; (4) Karst developed strata, etc.Generally, it is a difficult point in drilling construction, and traditional drilling methods are often unable to achieve it.It is also difficult to use pneumatic down-hole hammers and pipe drilling.

7
Calculation method for drilling quantity

Underground heat exchange
The underground heat exchange can be calculated by the following formula: Q1'--Heat emitted into the soil in summer, kW; Q1--Summer design total cool load, kW; Q2'--Heat absorbed from soil in winter, kW; Q2--Winter design total heat load, kW; COP1--The refrigeration coefficient of the water source heat pump unit under design conditions is generally taken as 2.5~5; COP2--The heating coefficient of the water source heat pump unit under design conditions is generally taken as 2.5~5.

Calculation of drilling rig quantity
The length of vertical buried pipes can be calculated by the following formula: The number of drilling rigs can be calculated by the following formula: n--Ground source heat pump well depth, m; N--Number of drilling wells,

Case Analysis
The total heating/cooling area of a certain park is approximately 9848m2.According to the recommended values for heating heat indicators, air conditioning heat indicators, and cooling indicators in the "Design Specification for Urban Heat Pipe Networks" (CJJ34-2010), the comprehensive heating indicator is converted into 80W/m2, and the cooling indicator is taken as 100W/m2.After calculation, the design heating load is 780kW, the design cooling load is about 980kW, and the daily average amount of domestic hot water is 8m3.The groundwater resources in this area lack exploration data, and there are also difficulties in recharge and risks of polluting groundwater bodies.The horizontal buried pipes occupy too large an area, the vertical buried pipes are arranged compactly, and the construction operation surface is small, which is the most common system form of the buried pipe Ground source heat pump.
In conclusion, it is proposed to use vertical ground source heat pump as the basic form of this Ground source heat pump.
The buried heat exchanger is buried in the borehole and backfilled with backfill material.As the rock and soil temperature is relatively stable all the year round, the Ground source heat pump can provide heating and cooling for buildings more efficiently.When the system is used for building refrigeration, the waste heat inside the building is transferred to the rock and soil through buried pipe heat exchangers; Under heating conditions, the heat inside the rock and soil is carried into the building.
This project adopts a single U-shaped vertical buried pipe method, with a hole diameter of 150mm.The heat exchange per linear meter of the ground source heat pump is estimated to be 70W/m under summer conditions with an inlet and outlet temperature of 35 ℃/30 ℃ (inlet and out-let water temperature difference of 5 ℃), and 35W/m under winter conditions with an inlet and outlet temperature of 5 ℃/10 ℃ (inlet and outlet water temperature difference of 5 ℃).The effective burial depth of the pipe is tentatively set at 120 meters, and the drilling depth is not less than 122m.
According to geological conditions survey, the drilling area of this project is 0-60m thick with gravel clay layer; 60-120m is the rock layer.The heat exchange indicators for the linear meter of drilling in this project are shown in the table 2 below Due to the complex geological conditions in this area, a composite drilling method is adopted, which involves the construction process of ordinary drilling rig drill-ing+down-the-hole hammer following the pipe, namely: (1) Use a regular drilling rig to drill the well at 0-60m, and use mud for wall protection construction; (2) 60-120m shall be constructed using an air compressor in conjunction with a down-hole hammer and a pipe.
The process flow is construction preparation → setting out → erecting the drilling rig → connecting water and electricity → installing the drill bit → debugging the drilling rig → drilling → through-hole → evacuating the drilling rig → positioning the down-hole hammer on the well position → installing the drill bit and following the pipe to lower the pipe → drilling and blowing the well → lifting the drill rod → lowering the pipe of the underground heat exchanger → preliminary backfilling → pulling the following pipe → backfilling.
Based on the selection results of the ground source heat pump host, calculate the number of wells required for the buried pipe system to meet the peak heat extraction in winter and the peak heat release in summer, and consider the number of wells arranged under the most unfavorable working conditions.The final design of the number of heat exchange wells is based on winter operating conditions, considering a 15% safety margin.A total of 145 heat exchange wells need to be constructed for the outdoor buried pipe system.The distance between buried heat exchange pipes on the ground source side is 4m.Considering various practical issues during the use of ground source heat pumps, a portion of the area is reserved for later drilling.Construct 1-2 designed ground source heat source monitoring wells based on the scale of buried pipes in each project, to monitor real-time soil temperature and serve as a reference factor for determining the heat supply of trough mirror fields during excessive seasons.
The vertical buried pipe in this design is planned to use high-density polyethylene (HDPE) with a diameter of De32 × 3.0, nominal pressure 1.6MPa, material PE100/SDR11/PN1.6MPa.The horizontal buried pipe adopts high-density polyethylene (HDPE) with a nominal pressure of 1.0MPa.The direction and elevation of the horizontal buried pipe of the ground source heat pump need to be determined during the construction drawing stage based on the terrain and geological conditions of the site.The preliminary design of the water center elevation of the horizontal buried pipe return pipe is 1.7 meters below the outdoor surface, and the preliminary design of the horizontal buried pipe water supply pipe is 2.0 meters below the outdoor surface; The water supply and return pipe room is isolated by polystyrene foam plastic plate with a design thickness of 30mm.The excavation depth of the horizontal buried pipe trench is 200mm below the bottom of the horizontal buried pipe.The excavation position of the horizontal pipe trench is adjusted according to the site conditions, and the width is adjusted according to the site conditions.However, it is necessary to ensure the minimum distance between the horizontal buried pipes (which should not be installed tightly) and the minimum distance required for safe construction.The backfill material for buried holes is a mixture of drilling slurry and no less than 40% bentonite as backfill material.

Outlook
Ground source heat pump technology has broad market prospects and requires strong support and support from relevant technologies.From the perspective of drilling, the following opinions are specifically discussed:

Actively introducing new technologies
From the current drilling construction situation of ground source heat pump engineering, the drilling technology and management are not suitable for the market promotion requirements of ground source heat pumps.Compared to other drilling industries: (1) Most of the equipment is relatively outdated and inefficient; (2) The team is uneven and the technical quality is generally not high; (3) The construction technology means are relatively single; (4) Lack of ability to cope with complex geological construction.
These all have an impact on drilling efficiency, quality, and cost, and to some extent even restrict the promotion and application of ground source heat pump technology.
In terms of management and technical force allocation, only emphasis is placed on refrigeration and installation, while neglecting drilling construction; However, the drilling and pipe burying sub projects are all concealed projects and important links in the heat pump system.Therefore, it is recommended that: (1) Strengthen technical management and guidance in the drilling process; (2) Strengthen research on drilling equipment, processes, and methods; (3) Developing towards professionalization of drilling teams and standardization of construction management.
Pay attention to drawing on, introducing, and promoting new drilling technologies.
In recent years, with the development of mineral resources, geological exploration and drilling technology have made significant progress, and replacement equipment, processes, and technologies have been continuously promoted and applied; In terms of ground source heat pump drilling, there are relatively few.Increasing the introduction of drilling technology, through high efficiency, high quality, and cost reduction, is the only way to break through the bottleneck restricting the development of ground source heat pumps.
(1) Hydraulic rock fragmentation and jet drilling technology.In the construction of loose or soft rock layers, the drilling efficiency of rotary drilling with conventional mud can be improved by 3-5 times or even more.
(2) Rotary impact drilling technology can significantly improve efficiency by changing the mechanism of rock fragmentation.The air down-hole hammer drilling technology effectively solves the construction efficiency problem in hard rock formations, which can be improved by 8-10 times compared to ordinary rotary drilling; In addition, hydraulic impact drilling can effectively improve drilling efficiency by 2-3 times even with low power consumption.
(3) The casing drilling technology and synchronous pipe drilling technology can effectively solve the conversion problem of drilling technology in composite formations; It is also the main means to solve the problem of drilling in complex formations.
(4) Sonic drilling technology, electromagnetic vibration pulse drilling, flexible pipe drilling technology, etc. have elevated the level of drilling technology to a higher level.
Drawing on new drilling technology and combining with the requirements of ground source heat pump drilling, by introducing, digesting, and absorbing it to better serve the ground source heat pump technology, it is bound to play a good promoting role.
The development of the market is closely related to the technological progress of related industries; Market demand drives technological progress, and technological progress supports market development.

Reasonable Design of Ground Source Heat Pump Tube Well
Drilling is an important part of the application of ground source heat pump air conditioning technology, so the design of the ground source heat pump tube well must meet the requirements of the effectiveness and economy of the heat pump system.At the same time, the heat pump system should also consider the current technical status of drilling when designing the tube well.When designing and constructing specific projects, comprehensive consideration is needed.
1. Try to avoid constructing ground source heat pumps under complex geological conditions, even if necessary, by increasing the depth of a single well and reducing the workload of crossing complex layers.
2. When designing tube wells in composite formations, the depth or quantity of wells can be adjusted to convert tube wells that require multiple processes into simple or single processes.
3. In areas with abundant groundwater and strong cold heat balance capacity, tube wells can be properly densified, which is conducive to simplifying the subsequent surface workload.
4. The drilling diameter can be determined based on the comprehensive consideration of the cross-sectional size of the U-shaped pipe and the geological conditions of the formation.Generally, "single U" can reduce drilling workload by appropriately reducing the wellbore diameter compared to "double U", especially when the rock hardness is high.In addition, when the diameter of the drill bit is fixed, the oversize coefficient of drilling in loose or soft formations is large, while the oversize coefficient of drilling in dense and high hardness rocks is small.
5. Modern drilling technology can achieve various types of three-dimensional spatial trajectory drilling, and for ground source heat pump tube wells that require such requirements, this technology process can be used for reference.

Conclusion
At present, there are few follow-up casing drilling projects with a depth of more than 70m in the construction of ground source heat pump buried holes.This project is aimed at special and complex geological conditions.By selecting a reasonable drilling process of leading holes and following pipes, and strengthening the control of key processes, the installation of underground heat exchangers can be better ensured.
To sum up, drilling construction is the basic link of the Ground source heat pump and the most elastic part of the cost composition of the entire Ground source heat pump.Starting from the research and analysis of basic geology, under the premise of meeting system requirements, reasonable design of pipe wells, optimization of drilling technology schemes, and strengthening construction organization management are effective ways to improve drilling efficiency, reduce costs, and further expand the market.
The required length of vertical buried pipes in summer, m; QL1--Heat exchange of vertical buried pipes per meter in summer, W/m; L2--The required length of vertical buried pipes in winter, m; QL2--Heat exchange of vertical buried pipes per meter in winter, W/m; L--The required length of vertical buried pipes for the project, m;

Table 1
Classification of Ground source heat pump

Table 2
Table of Heat Exchange Index for Borehole Linear Meters