1 Core Functions of Drilling Well Flushing

The circulation of well flushing media can simultaneously achieve the three functions of cooling the drill bit, removing cuttings and cleaning the bottom, and protecting the wellbore, which serves as the fundamental guarantee for efficient drilling.

1.1 Cooling Rock Breaking Tools

When the drill bit and rock breaking tools crush rock, a large amount of heat is generated due to the destruction of rock molecular cohesion and the friction between the tools and the rock formation. Field measurement data show that the tool temperature can reach 500~1500℃ under sandstone crushing conditions. High temperatures cause tool annealing, reduced wear resistance, deformation, and even fracture, significantly shortening service life. The well flushing medium continuously removes heat to cool the tools.

1.2 Cuttings Removal and Bottom Cleaning

Drilling continuously produces rock cuttings and debris. If not discharged in a timely manner, they will clog the well bottom, hinder drill bit operation, and in severe cases, cause drill burial accidents. Well flushing carries cuttings through medium flow, in two forms: suspension conveyance and velocity conveyance. Suspension conveyance is suitable for fine-grained cuttings, mostly used with mud media and percussion drilling cuttings barrels. Velocity conveyance transports large-grained cuttings at high flow rates and is the mainstream cuttings removal method. Effective cuttings removal keeps the well bottom clean and improves drilling efficiency.

1.3 Wellbore Support

For loose and collapsible formations, when mud is used as the well flushing medium, it forms a dense mud cake on the wellbore and reinforces the wellbore via mud hydrostatic pressure, effectively preventing caving and collapse, ensuring normal well completion and drilling.

2 Selection and Application of Well Flushing Media

Different construction scenarios and formation conditions require matching well flushing media. Commonly used media in the industry include air, clean water, mud, and foam. Selection shall follow general requirements and be made rationally based on medium characteristics.

2.1 General Technical Requirements for Well Flushing Media

(1) Excellent suspension and transportation capacity for rock cuttings.

(2) Qualified wellbore stabilizing performance to stabilize the wellbore.

(3) Easy control and purification treatment.

(4) Low cost and minimal wear on drilling equipment.

(5) Non-corrosive to the wellbore rock formation, adaptable to various special formation constructions.

2.2 Characteristics and Applicable Scenarios of Various Media

(1) Air: Easily obtained with the lowest cost, suitable for drilling in water-deficient areas. No wellbore stabilizing ability; compressed air preparation requires additional power consumption.

(2) Clean water: Cost-effective, easy to purify, and allows rapid cuttings sedimentation. In compact and clay formations, wellbore stability can be assisted by raising the water level in the well, supporting cyclic reuse.

(3) Mud: Outstanding wellbore stabilizing performance, currently the most widely used well flushing medium. Disadvantages include relatively high procurement and purification costs; special purification equipment has been deployed in the industry to improve treatment efficiency.

(4) Foam: Extremely low specific gravity, with better cuttings suspension capacity than air, mostly used in waterless construction areas. Often mixed with water, air, and mud for complex working conditions.

3 Mainstream Well Flushing Construction Methods

According to the flow direction of flushing fluid, well flushing processes are divided into direct circulation well flushing and reverse circulation well flushing. Reverse circulation is further subdivided into air-lift, pump-suction, and jet processes. At present, reverse circulation is preferred for large-diameter drilling.
 

3.1 Direct Circulation Well Flushing

(1) Working principle: Flushing fluid is delivered to the well bottom via a water pump and drill pipes. After cooling and carrying cuttings, it returns to the surface along the annular gap between the drilling tools and the wellbore. When the flow velocity is insufficient in a large-diameter wellbore, coarse-grained cuttings will remain at the well bottom and be removed collectively after operation.

(2) Application scope: Used with mud for large-diameter loose fine-grained formation drilling; clean water medium is suitable for bedrock coring drilling and can also be used with percussion drills.

(3) Process shortcomings: Large-diameter wellbores require extremely high flow rates to meet cuttings-carrying velocity demands, resulting in high energy consumption and poor economy, making it unsuitable for large-diameter engineering drilling.

3.2 Reverse Circulation Well Flushing

(1) Working principle: Flushing fluid is supplied from the wellhead. Equipment such as air compressors, sand pumps, and injectors create a high flow velocity of over 3 m/s inside the drill pipes, directly sucking the mixture of cuttings and medium from the drill pipes to the surface. Field measurements show that at a flow velocity of 2.5~3.4 m/s, large-grained cuttings of 50~100 mm can be stably transported, achieving excellent cuttings removal efficiency.

(2) Process advantages

1) High flow velocity in pipes, less cuttings accumulation at the well bottom, and effective bottom cleaning.

2) Large drill pipe inner diameter enables transportation of large-grained cuttings, improving drilling efficiency.

3) Wellhead water replenishment raises the water level, enhancing wellbore stability.

4) Groundwater can be directly pumped for construction in groundwater-rich areas, increasing well yield.

5) Well leakage conditions can be addressed by increasing pump capacity and replacing well flushing media.

(3) Sub-processes: Including air-lift reverse circulation, pump-suction reverse circulation, and jet reverse circulation, with consistent core technical parameters.

(4) Key technical parameter control

1) Upward flow velocity in drill pipes: Determined by cuttings grain size, rock specific gravity, well flushing process, and medium. Insufficient velocity fails to carry cuttings, while excessive velocity increases pipe wall friction and energy consumption. The optimal velocity shall satisfy both rapid cuttings removal and annular return velocity without scouring the wellbore.

2) Flushing fluid flow rate: Comprehensively calculated based on drill pipe inner diameter, designed upward flow velocity, and well flushing medium.

3) Drill pipe inner diameter: Refer to two industrial empirical standards: under conventional well diameters, the drill pipe inner diameter is 1/10 of the drilling diameter (no further increase when well diameter > 5 m); the cross-sectional area of the drill pipe inner diameter is 1% of the total drilling cross-sectional area, with drill pipe structural strength verified simultaneously.

4) Annular return flow velocity: Uniformly controlled at 0.03~0.04 m/s to prevent wellbore damage caused by high-velocity water scouring.