Ⅰ Working Mechanism

As a hydraulically assisted reverse circulation technology, jet reverse circulation well flushing relies on jet negative pressure to carry cuttings. It features fast startup and stable operation, and can be used independently or combined with suction and air-lift well flushing technologies.

1.Working Process and Operation Key Points

（1）Medium intake: High-pressure clean water is connected through the high-pressure water inlet (Mark 1) and evenly distributed to each group of nozzles along the annular cavity of the ejector.

（2）Jet pressurization: High-pressure water flows through the nozzles (Mark 2) to accelerate and form high-speed jet streams, generating a stable negative pressure zone (Mark 3) in the central channel of the ejector.

（3）Negative pressure suction: Relying on the pressure difference between the hydrostatic column pressure of the wellbore and the central negative pressure zone, the flushing fluid carrying rock cuttings at the bottom of the well is continuously sucked upward along the inner channel of the drill pipe into the central channel of the ejector.

（4）Mixed slag discharge: The high-speed jet stream is fully mixed with the sucked cuttings-laden flushing fluid and discharged to the ground together through the lower outlet (Mark 4), realizing continuous well flushing and efficient slag removal.

2.Key Operation Requirements

Before pump startup, check that nozzles are unblocked and all pipelines are well sealed. Maintain stable pump pressure during operation to ensure the upward flow velocity of cuttings-laden fluid inside drill pipes is no less than 3 m/s, preventing cuttings sedimentation and pipe blockage. The ejector device is prioritized for shallow construction; air-lift or suction reverse circulation shall be combined for medium and deep wells.

Ⅱ System Structure and Layout Forms

1.Core Equipment Composition

The complete system consists of three core units: high-pressure centrifugal water pump, high-pressure water supply pipeline and ejector assembly. The ejector adopts a flange modular assembly structure, with 4–6 groups of wear-resistant nozzles evenly arranged around the cavity circumference. The receiver is a straight tubular component whose inner diameter matches that of the drill pipe, forming a large flow channel to avoid pipeline blockage by large-particle rock cuttings. Wear-resistant materials such as high-chromium cast iron and tungsten carbide can be adopted for vulnerable parts including nozzles and receivers to effectively extend service life.

2. Two Installation Configurations of Ejector

（1）Downhole built-in type: The ejector is installed at the downhole working section. High-pressure water is supplied through the interlayer of double-wall drill pipes or special water delivery pipelines. Negative pressure directly acts on the well bottom to achieve optimal slag removal efficiency, suitable for construction in shallow loose sand and pebble strata.

（2）Surface wellhead type: The ejector is mounted on the outlet pipeline of the drill swivel, featuring convenient disassembly, assembly and maintenance, applicable to batch shallow well construction with frequent site relocation.

3.Composite Efficiency Enhancement Configuration Scheme

Two ejectors can be installed in parallel on a single drilling rig to increase fluid delivery displacement through superposed negative pressure. Meanwhile, the jet system can serve as an auxiliary pressurization unit and form a composite well flushing system with suction reverse circulation and air-lift reverse circulation, making up for insufficient slag removal power of a single process.

Ⅲ Standardized Technical Parameter System

1.Basic Parameters of Water Supply Centrifugal Pump

Centrifugal high-pressure water supply pumps are adopted with a rated head of 60 m and a rated displacement of 120 m³/h. Due to jet pressurization effect, the actual water-lifting displacement of the ejector can reach more than twice the rated flow of the water supply pump.

2.Measured Reference Indexes of Imported Drilling Rigs

Taking the matching system of German B3-A drilling rig as an example: the inner diameter of the supporting drill pipe is 120 mm, the upward flow velocity of flushing fluid inside the drill pipe exceeds 3 m/s, and the comprehensive displacement of the complete jet system is approximately 160 m³/h.

3.Control Standards for Drilling Fluid Flow Velocity

In accordance with current drilling engineering specifications, two benchmark flow velocity control criteria are established:

（1）The upward flow velocity of cuttings-laden fluid inside drill pipes shall be stably controlled at 3–4 m/s, a fixed reference value to guarantee full suspension and transportation of rock cuttings.

（2）The downward flow velocity of flushing fluid in the wellbore annulus shall be controlled at 0.03–0.04 m/s, about 1% of the upward flow velocity inside drill pipes. Low downward flow velocity reduces scouring disturbance of fluid to wellbore soil and maintains structural stability of loose strata.

4.Matching Relationship for Large-Diameter Drilling

For large-diameter engineering drilling, the recommended ratio of wellbore inner diameter to drill pipe inner diameter is 10:1, which balances fluid delivery flow rate and wellbore stability and serves as a general matching standard for engineering design.

Ⅳ Applicable Boundaries and Operation Characteristic Analysis of the Process

1.Effective Operation Depth Limitation

Jet negative pressure is driven by the hydrostatic pressure difference of the wellbore and restricted by fluid pressure attenuation. The effective operation depth range of this process is 50–70 m. When the well depth exceeds this range, the slag removal efficiency of independent jet reverse circulation decreases significantly, requiring combined air-lift and suction composite circulation processes.

2.Performance Comparison with Mainstream Reverse Circulation Processes

（1）Compared with suction reverse circulation: It avoids vacuum failure and equipment shutdown caused by pipeline air leakage, delivering stronger continuous operation stability.

（2）Compared with air-lift reverse circulation: The supporting equipment has lower installed power and faster startup response, and effective circulation can be formed immediately after pump startup.

（3）Inherent limitations of the process: Restricted effective operation depth, low factory matching rate of domestic drilling rigs, and high construction cost for field modification.

3.Applicable Scenarios of Composite Well Flushing Processes

（1）Alternating forward and reverse circulation well flushing: Mostly applied in loose silty soil and sand strata to reduce risks of wellbore collapse and sand outflow induced by continuous pumping.

（2）Air-water coupled jet well flushing: Suitable for dry drilling conditions with extremely low groundwater level or no underground water. Air is switched as the circulation medium, and jet pressurization is adopted to solve the difficulty of slag removal in dry boreholes.

（3）Specification for circulation medium selection: Clean water is prioritized as the well flushing medium for conventional strata with low cost and zero water and soil pollution. Low-solid mud is added temporarily for wall protection only in special strata with leakage and collapse tendency, and thorough well cleaning shall be conducted after construction to eliminate residual mud.

Ⅴ Comprehensive Evaluation of Various Reverse Circulation Well Flushing Processes

1.General reverse circulation well flushing process: High slag removal efficiency and excellent comprehensive performance, serving as the preferred slag removal and purification process matching full-face drilling.

2.Air-lift reverse circulation process: Wide stratum adaptability, simple equipment structure and high operation reliability. When combined with suction and jet processes, it can cover drilling operations of shallow, medium and full depths.

3.Medium selection criterion: Clean water acts as a green and economical general medium, while mud is only an emergency wall protection measure for special strata.

4.Technical development direction: The industry is currently focusing on research on dry air jet well flushing and foam mixed liquid jet well flushing technologies, which are applied to special engineering scenarios such as arid and water-deficient areas, karst broken strata and contaminated site remediation, expanding the application boundary of jet reverse circulation technology.

Ⅵ  Conclusions

Jet reverse circulation well flushing achieves efficient slag removal based on the jet negative pressure mechanism, featuring fast equipment startup, stable operation and low installed energy consumption, with remarkable application advantages in shallow large-diameter drilling of 50–70 m. This process can form a composite system with suction and air-lift well flushing to make up for the depth limitation of a single process. During construction, the upward flow velocity inside drill pipes and downward flow velocity in the annulus must be strictly controlled, the matching ratio of wellbore and drill pipe inner diameters shall be optimized, and circulation media including clean water, air and mud shall be flexibly selected according to stratum and hydrological conditions. The future improvement of air-foam fluid coupled jet well flushing technology will further broaden the application scope of this process under complex working conditions.