1. Drilling Structure and Drilling Sequence
 

For each large-diameter drilling project, the drilling structure shall be designed in combination with engineering requirements, geological conditions and hydrogeological data. As a core component, the drilling sequence shall be incorporated into the design simultaneously. The drilling structure serves as the fundamental basis for drilling construction, and its rationality directly determines the success or failure of the drilling project, hence requiring high priority.
 

In the process of drilling structure design, key factors such as drilling process requirements, performance of available equipment and construction capacity shall be comprehensively considered. The design procedure shall follow the principle of deriving upward from the final well diameter. Reasonable diameter-variable design shall be adopted in response to special requirements such as lithological changes of formations and casing running.
 

The design shall strictly comply with the following principles:
 

(1) Fully meet the core engineering and geological requirements for drilling, with strict control over key indicators including well deviation and borehole diameter accuracy, to ensure that the drilling results satisfy subsequent operational needs.

(2)Provide sufficient and reliable technical support measures, with reasonable margins reserved in borehole diameter design. For instance, where conditions permit, the final well diameter may be enlarged by one grade from top to bottom to avoid borehole shrinkage caused by mid-construction formation changes or abnormal hydrogeological conditions, ensuring continuous drilling construction.

(3)Accurately grasp geological data and formation variation rules, and strictly implement preliminary survey requirements. Once the large-diameter well location is determined, small-diameter exploratory holes shall be drilled first to precisely identify the lithology, structure and hydrogeological characteristics of the formation at the well location, providing a solid basis for scientific design of the drilling structure.

(4) Optimize economic benefits while satisfying the above design conditions by simplifying the drilling structure as much as possible. An overlarge borehole diameter increases rock-breaking workload, reducing construction efficiency and raising tool wear costs. If mud wall support is sufficient to maintain borehole stability, this method shall be prioritized to reduce casing running, save material costs and lower overall drilling expenses.

2. Demonstration of Drilling Structure

Figure 1 shows a schematic diagram of a standard large-diameter drilling structure under common formation conditions. The well site features a thick overburden layer, weathered surface bedrock, and fractured shallow rock strata prone to water loss. The designed final well diameter is 960 mm.

The specific structural design scheme is as follows:

After excavating 3–5 m downward from the surface, a wellhead casing is installed and fixed by layered tamping with clay around the casing bottom. Drilling is performed with a 1300 mm diameter bit; after passing through the overburden layer and strongly weathered bedrock using mud wall support, a 1240 mm steel casing is run, and the casing bottom is sealed with underwater cement slurry. Subsequently, drilling through the fractured formation is conducted with 1160 mm drilling tools, followed by running a 1130 mm steel casing, whose bottom is also sealed with underwater cement mortar.

Although direct drilling to the designed depth with final-diameter tools is feasible under such conditions, considering the relatively large drilling depth for construction safety and borehole stability, a 1060 mm bit is first used to drill a certain section. Upon verification of stable formation conditions, the bit is replaced with a 960 mm one to complete drilling to the final depth.

This relatively complex drilling structure is mainly applicable to scenarios where personnel must enter the well for geological description and field tests after completion. For projects with lower requirements on drilling accuracy and structure, such as water wells, the drilling structure can be significantly simplified. Even after passing through loose overburden, single-diameter drilling can be achieved using mud wall support.

Figure 1 Standard Drilling Structure for Common Formations

3. Drilling Sequence

Drilling sequence refers to the rational construction flow determined under complex formation conditions with thick overburden layers. Its core is directly related to the borehole protection method and construction economy. For example, mud used as a flushing fluid during overburden drilling effectively stabilizes the borehole and prevents collapse. After penetrating the overburden into intact bedrock, clean water drilling can be adopted where no additional wall support is needed. This process not only saves mud material costs but also maintains a clean bottom hole through clean water reverse circulation well flushing, thereby improving drilling efficiency, extending tool service life and reducing drilling costs. The optimal drilling method shall be selected through economic comparison of the two processes.

Under normal circumstances, segmental drilling with casing running for wall protection is preferred. Compared with one-step well completion, this method effectively avoids borehole collapse risks. Blindly adopting one-step completion would not only increase mud consumption and reduce efficiency but also introduce safety hazards due to unstable upper formations. One-step completion is only permissible when the upper formations are confirmed to be collapse-free and construction conditions are controllable.

In addition, multi-stage reaming completion involves higher construction difficulty, with close attention required to upper borehole stability at each stage, which may lead to borehole collapse, pipe sticking and other issues. Therefore, in conventional large-diameter drilling construction, the sequence of first passing through the overburden, installing protective casing, and then drilling the bedrock section is generally adopted to ensure construction safety and quality.

4. Well Site

In addition to completing drilling design, formulating technical support measures and inspecting and maintaining construction machinery, the core preparatory work before drilling is the planning and layout of the well site, providing a safe and compliant operating environment for subsequent drilling.

4.1 Well Site Layout

Well site layout shall be scientifically planned according to drilling diameter, depth, adopted process and mechanical equipment performance to ensure smooth and safe construction.

(1) Layout for Core Drilling

Core drilling is normally designed with a diameter no greater than 1.5 m and a depth not exceeding 50 m. Full-face drilling with similar construction parameters may adopt the same well site layout, as detailed in Figure 2.

Figure 2 Layout of Core Drilling Site

(2) Layout for Reverse Circulation Full-Face Drilling
Reverse circulation drilling requires more equipment and auxiliary facilities, with heavy drilling tools and large borehole diameter and depth, resulting in a larger well site area and more complex layout. Where mud is used as flushing fluid, additional mud purification equipment shall be installed to ensure mud recycling and compliance of the construction environment.

(3) Layout for Trench Drilling
In trench drilling or large-area interconnected sleeve drilling, the drilling rig requires frequent movement. Meanwhile, steel cage installation and underwater concrete pouring are required after trench formation, leading to a more complex well site layout. Figure 3 shows a schematic layout for diaphragm wall drilling.

Figure 3 Layout of Trench Drilling Site

4.2 Wellhead Protection

During drilling, the wellhead is an area of concentrated heavy load, where the derrick feet and large-diameter drilling tools are suspended during lifting. Additionally, large volumes of flushing fluid are injected into or discharged from the wellhead in both direct and reverse circulation well flushing. For unstable formations such as leaky strata and quicksand layers, borehole stability is maintained by raising the flushing fluid level inside the well to create back pressure. Thus, wellhead protection is a critical operation in drilling construction.

For well locations with loose overburden, a 3–5 m deep pit shall be manually excavated, and a steel wellhead casing shall be installed. High-quality clay shall be filled and compacted in layers at the lower part of the casing, and the outer side shall also be backfilled and tamped. For permanent wells, reinforced concrete wellhead casings may be constructed in conjunction with surface construction to enhance stability and durability.

For bedrock well locations or those with shallow overburden, a 3–5 m deep pit shall be excavated manually or by drilling and blasting before running the wellhead casing. Apart from ensuring wellhead stability and flushing fluid circulation, this provides operating space for large-diameter drilling tools (typically about 4–5 m in length) to guarantee normal drilling.

4.3 Well Site Foundation

Large-diameter drilling involves numerous and heavy construction equipment, requiring a solid and stable well site foundation to prevent settlement and collapse during construction. If the well site is narrow and requires filling, piers or pile foundations shall be installed as supporting structures to ensure bearing capacity. The well site shall generally cover no less than 50 ㎡, with reserved access for heavy vehicles and supporting water supply and drainage pipelines and ditches to meet construction water needs.

Foundation treatment focuses on the area around the wellhead:
For large-diameter and deep wells, a concrete foundation platform shall be poured at the wellhead to fix the derrick feet and frame evenly. For small-diameter and shallow wells, wooden or concrete sleepers shall be pre-buried for support. Both concrete platforms and embedded sleepers must be horizontally leveled to avoid equipment inclination, which would affect drilling accuracy and construction safety.

4.4 Equipment Installation

Correct installation of mechanical equipment is a prerequisite for normal operation and a key factor in ensuring drilling quality and construction safety, requiring strict compliance with the following requirements:

The drilling rig frame shall be bolted to the foundation platform and adjusted to be horizontal. Meanwhile, the centers of the crown block, hook and rotary table shall be precisely aligned with the drilling center, and the derrick shall be vertically installed to prevent deviation during drilling.

Components such as auxiliary winches, derrick guy lines, and dead ropes for drill pipe handling tongs shall be securely connected to embedded ground anchors. Derrick guy lines shall be properly tensioned to maintain stability and avoid swaying or toppling during construction.

Upon completion of equipment installation, a comprehensive test run shall be conducted to inspect the operation of all components. Defects shall be adjusted promptly until the equipment runs normally and all parameters meet standards, after which formal drilling may commence.

Supplement: Drilling Methods