Air-lift reverse circulation hole cleaning and drilling process is the dominant hole-washing and cuttings-discharging technology for hydrological wells, large-diameter engineering boreholes and geothermal well construction. It achieves pump-free reverse circulation cuttings removal relying on the density difference of gas-liquid two-phase flow. This process has remarkable operating advantages including high-efficiency discharge of large-size rock cuttings, thorough hole-bottom cleaning and slight disturbance to borehole walls.
 

Ⅰ Working Principle

This technology relies on the communicating vessel principle and the lifting mechanism generated by density difference of gas-liquid two-phase fluid. Compressed air is adopted to adjust the fluid density inside drill pipes so as to form a pressure difference between the inside and outside of drill pipes and drive circulating cuttings removal. No mud pump is required to provide circulating power throughout the whole process, and the specific workflow is as follows:

1.High-pressure compressed air generated by the air compressor is delivered downhole to the mixer via dedicated air supply pipelines.
2.Compressed air is evenly injected into the flushing fluid inside drill pipes through tiny orifices on the mixer, forming numerous dispersed microbubbles. After sufficient gas-liquid mixing, the density of the mixed fluid inside drill pipes drops to 0.4–0.6 g/cm³, far lower than the density of the original flushing fluid in the annulus outside drill pipes (1.0–1.15 g/cm³ for conventional clear water or drilling fluid).
3.A stable hydrostatic pressure difference forms between the interior and exterior of drill pipes, driving the low-density gas-liquid mixture to flow upward along the inner cavity of drill pipes. Meanwhile, the upward aerodynamic thrust of high-pressure compressed air further accelerates the upward flow velocity, and the superposition of two driving forces realizes efficient fluid lifting.
4.The high-speed upward-flowing gas-liquid mixture entrains rock cuttings, silt and other solid impurities at the hole bottom and transports them all the way to the surface cuttings discharge outlet along drill pipes, completing hole-bottom cleaning and well flushing. Flushing fluid in the annulus replenishes the hole bottom simultaneously, forming a fully closed reverse circulation loop.

Compared with forward circulation well flushing, this process creates mild flow field disturbance at the hole bottom, avoids secondary fragmentation of rock cuttings and delivers thorough cleaning. It is especially suitable for large-diameter boreholes, loose formations and deep well flushing operations.

Ⅱ Complete Supporting Equipment

The entire air-pressure reverse circulation well flushing system consists of five core components: air compressor, air-water swivel, mixer, air supply pipeline and downhole cuttings suction port. It has two mainstream configurations, namely the concentric single-wall drill pipe system and the parallel dual-wall drill pipe system, applicable to different borehole depths and diameters.
 

1.Air Compressor

（1）Selection Requirements
As the core power source of the whole system, the air compressor shall maintain stable output air pressure and flow without pressure fluctuation to prevent circulation interruption. For shallow wells (≤50 m), compressors rated at 0.7 MPa meet general engineering well demands; medium-deep wells (50–150 m) adopt 1.0–1.7 MPa models; deep wells (150–300 m) and large-diameter boreholes require high-pressure compressors of 1.7–2.5 MPa.
（2）Model Selection
Screw air compressors have replaced traditional piston compressors for on-site application at present. Featuring compact structure, few moving parts, low operation and maintenance cost, and continuous stable exhaust pressure and flow, they support long-term uninterrupted well flushing. A pulse air supply module (pulse frequency: 0.5–2 Hz) can be integrated; intermittent air supply eliminates pipeline air lock and boosts cuttings removal efficiency by 10%–15%.
（3）Flow Rate Selection
The air flow rate shall match the inner diameter of drill pipes. Insufficient flow leads to weak cuttings carrying capacity, while excessive flow causes violent fluctuation of drilling fluid level and damages borehole wall stability. The reference baseline flow for general engineering wells is 6 m³/min, which shall be increased to 12–20 m³/min for large-diameter boreholes.

2.Air-Water Swivel

The air-water swivel is a critical transition component for suspending drill strings and isolating air supply and cuttings discharge channels, integrating air inlet and discharge pipelines and bearing the full self-weight of drill strings. It is classified into concentric and parallel types matching two sets of drilling tools respectively.

（1）Concentric Air-Water Swivel
Matched with concentric mixers, the air supply pipe runs through the center of drill pipe inner cavity. Ordinary hydrological drilling swivels can be directly used by drilling a through-hole on the top gland to connect the air supply pipe, with no customized accessories required and strong versatility.
（2）Parallel Dual-Channel Special Air-Water Swivel
Designed for parallel air supply systems with dual-wall drill pipes, compressed air is conveyed through the annular interlayer of dual-wall drill pipes or side-mounted air pipes. The air supply channel is independent of the cuttings discharge cavity of drill pipes, so special side-air-inlet swivels must be adopted to prevent cross-flow between air and cuttings passages.

3.Mixer
Also known as air chamber, the mixer is the key component for uniform mixing of air and drilling fluid, divided into concentric and parallel structures with distinct structural parameters, applicable conditions, advantages and disadvantages.

（1）General Structural Parameters
①Injection Orifice Specification: Orifice diameter 3–5 mm, inclined upward at 45° to ensure air injection consistent with fluid flow direction.
②Total Orifice Area: No less than 1.5–3 times the cross-sectional area of the air supply pipe to realize uniform gas dispersion and centralized air jet avoidance. 
③ Orifice Layout: Dense at the bottom and sparse at the top with a fully sealed bottom. Dense lower holes guarantee sufficient air supply in deep water sections, while sparse upper holes prevent air flow short-circuit.
（2）Concentric Mixer
Fabricated from a 2 m seamless steel pipe, with the air supply pipe arranged at the center of drill pipe inner cavity for air injection and mixing. 
①Advantages: Simple structure, low manufacturing cost, compatible with standard swivels and convenient on-site assembly and disassembly. 
②Disadvantages: The central air supply pipe occupies the cuttings discharge space inside drill pipes and limits the maximum allowable cuttings size; oversized rock fragments tend to jam between the air pipe and drill pipe inner wall and block the discharge passage. 
③Optimization Measure: Install graded grids welded at the downhole cuttings suction port to intercept oversized cuttings. Large fragments are secondarily crushed by the drill bit at the hole bottom before entering drill pipes for discharge, which achieves favorable on-site performance.
（3）Parallel Mixer
Adopted as a short air chamber integrated with dual-wall drill pipes. Compressed air is supplied via the annular interlayer of dual-wall drill pipes, and the entire inner cavity of drill pipes serves as the cuttings discharge channel without internal components occupation. 
①Advantages: Large cuttings passage allowing transportation of larger rock fragments; cuttings removal efficiency approximately 20% higher than concentric mixers. 
②Disadvantages: Restricted by hydrostatic liquid column pressure. When borehole depth exceeds 65 m, the lower liquid column pressure balances the air supply pressure, leading to sharp attenuation of lifting force and dramatic decline of cuttings removal efficiency. 
③Conventional Solutions: Install an extra mixer every 50 m of drilling footage or trip out drill strings to adjust the submerged depth of mixers, which generates heavy auxiliary workload. 
④Upgraded New Technology: Domestically localized self-adaptive segmented mixers imported from Germany are widely used currently. Equipped with built-in one-way check valves and pressure self-adaptive nozzles, they eliminate drill tripping for position adjustment and automatically adapt to pressures at various depths, completely solving problems of upward air leakage and failed cuttings removal in deep wells.

4.Air Supply Pipe

（1）Concentric System A smaller air supply pipe leaves a larger cuttings passage inside drill pipes yet increases flow resistance simultaneously. Field measured data shows the optimal outer diameter of air pipes under the baseline flow of 6 m³/min is 31.75 mm. Pipes smaller than this value suffer sharp rise of frictional resistance and over 15% attenuation of effective air supply, impairing lifting efficiency.
（2）Parallel System Air is supplied through the sufficient annular interlayer cross-section of dual-wall drill pipes, so independent calculation of air pipe diameter is unnecessary and full-range air flow demands can be satisfied.

5.Cuttings Suction Port
The sole inlet for downhole rock fragments and drilling fluid entering drill pipes, directly determining initial cuttings carrying efficiency. Standard installation parameters are specified as follows:
（1）Installation Position: Arranged at the center of the roller cutter bit cutter head, avoiding bit cutting force zones to prevent collision and deformation of the suction port.
（2）Installation Height: The vertical distance from the bottom of the suction port to the hole bottom equals half the height of rock-breaking roller cutters, which avoids direct blockage by bottom sediment while enabling timely suction of rock cuttings.
（3）Flow Cross-Section: The total flow area of the suction port is 1.5 times the through-hole area of drill pipe inner cavities to reduce inlet fluid resistance and guarantee smooth transportation of cuttings slurry into drill pipes.

Ⅲ Core Process Parameters

1.Submergence Ratio

（1）Definition 
The ratio of the submerged depth of the mixer below the dynamic water level to the total vertical distance from the mixer to the surface cuttings outlet. As the core index determining whether air-lift circulation can be established, it directly governs the liquid column pressure difference.
（2）Calculation Formula

Where: 
a — Submergence ratio, dimensionless; 
h1 — Submerged depth of mixer below borehole dynamic water level, m;
h2— Vertical height from borehole dynamic water level to swivel discharge outlet on surface, m.
（3）Operational Threshold Requirements
① Critical Working Limit: Stable reverse circulation can be established when submergence ratio ≥0.5; circulation becomes unstable with intermittent cuttings discharge when the ratio ranges from 0.4 to 0.5; no pressure difference and unavailable reverse circulation when submergence ratio ＜0.4. 
②Applicable Depth Boundary: Air-pressure reverse circulation is invalid for boreholes shallower than 7 m due to failure to meet the minimum submergence ratio. Stable construction generally starts at depth ≥10 m, and the optimal submergence ratio of 0.55–0.65 is recommended for deep well operations.

2.Air Supply Pressure

（1）Calculation Formula

Where:
P — Minimum air supply pressure required for system startup, MPa; 
H — Submerged depth of mixer below dynamic water level, m; 
r — Relative density of borehole flushing fluid (1.0 for clear water, 1.1–1.2 for mud); 
ΔP — Total loss of pipeline frictional resistance and local resistance, taken as 0.03–0.05 MPa for general wells and 0.05–0.07 MPa for deep wells with long pipelines.
(2)Applicable Pressure Ranges 
①General large-diameter engineering wells within 150 m depth: 0.7 MPa air compressors fully satisfy construction requirements. 
②Deep wells (150–230 m): High-pressure compressors of 1.7–2.5 MPa are required to match high submergence ratio conditions. 
③Field Measurement Case: For a borehole with diameter 1 m, depth 130 m, air pipe diameter 31 mm and drill pipe inner diameter 120 m, the actual operating pressure only needs 0.3 MPa with air flow slightly lower than 6 m³/min, achieving zero bottom sediment, timely rock cuttings discharge and no secondary fragmentation.

3.Air Flow Rate
Air supply flow is strongly correlated with drill pipe inner diameter and upward flow velocity of rock cuttings. Rated flow shall be selected to ensure the minimum upward cuttings velocity ≥1.2 m/s. Insufficient flow causes cuttings settlement and re-deposition, while excessive flow increases energy consumption and scours borehole walls leading to stability loss.

Ⅳ Common On-Site Operational Problems & Disposal Solutions

1.Poor Cuttings Discharge & Pipeline Blockage After Startup
Fault Cause: Accumulated rock cuttings and coalesced large air bubbles form air plugs, interrupting upward fluid flow. 
Disposal Method: Adopt intermittent air supply pulse disturbance (short air shutoff followed by re-supply). Pressure shock waves break air plugs and disperse accumulated cuttings for rapid pipeline dredging. Long-time high-pressure forced air supply is prohibited to avoid pipeline and mixer damage from pressure buildup.

2.Installation Spacing Specification for Mixers
The vertical distance between the mixer and drill string reducer joint shall be no less than 1 m to prevent abrupt flow field change at the reducer and impaired gas-liquid mixing effect. For boreholes deeper than 50 m, blind extension of downhole air supply pipelines is forbidden. The submerged depth of mixers shall always match the air supply pressure to avoid circulation interruption from insufficient pressure.

3.Cuttings Discharge Pipeline Layout & Cuttings Content Control
（1）This technology only applies to vertical downhole lifting and shall not be used for long-distance horizontal surface transportation. Minimize the length of surface discharge pipes connected to swivels to reduce pipeline pressure loss and improve cuttings removal efficiency.
（2）The optimal cuttings content for balanced circulation stability, energy consumption and discharge efficiency is 20%. The system can tolerate extreme conditions with cuttings content up to 50%. If clear water is adopted as flushing medium, the cuttings content shall be appropriately reduced to prevent cuttings settlement caused by insufficient fluid viscosity.
 

Ⅴ Diagnosis and Elimination of Drilling Tool Faults