Well flushing is a key process in well completion process. Geothermal wells in engineering construction usually undergo simple or complex well washing process, and adopt a variety of well washing methods. Under special circumstances, a well-washing method is repeatedly used to make the water quantity and temperature of geothermal wells meet the requirements of design or contract indexes, and at the same time to reach the optimum water quantity and temperature as far as possible. The quality of the project is qualified. Finally, the water quantity and quality are tested by pumping test and sampling test.
Compressed air well washing is a common method, which requires air compressor to provide compressed air (positive and reverse circulation), oscillation, suction of internal flushing fluid, dredge cracks and remove sediment.
There are two kinds of air compressor well washing devices, parallel type and concentric type, which can be selected according to the situation. By means of the air pressure of the air compressor, the water level in the hole oscillates, the well pipe is impacted, the mud is destroyed, and the sediment at the bottom of the hole is discharged from the ground. Air compressor well washing is one of the commonly used methods. Under normal conditions, the well washing effect is better.
Pumps and air compressors wash wells. In larger and deeper drilling, this combined well washing method is often used, that is, the drill pipe is down to the mud sediment nearby, the sediment is flushed to a certain height by means of water pump, and the sediment is discharged from the ground by means of air compressor. This method has better effect for boreholes with deeper aquifer and more mud sediment. For example, in the water-deficient areas of the red beds in the Yangtze-Huaihe River, the strata are purple-red clay rocks, and the collapse blocks are serious, so mud drilling is used. Because of the deep hole and long drilling time, the mud pastes the dead wall. This is the way to do it.
The application of compressed air technology in geothermal well flushing is mainly used in geothermal well flushing with compressed air. The high pressure and high displacement air compressor commonly used in oil drilling industry in geothermal well flushing with compressed air has the capacity of 10 cubic meters and 150 kilograms. Backwash method is adopted. Drill pipe is used for air duct. Generally, the depth of down is 500 to 1000 meters. Compressed air flushing produces intermittent jet outflow (continuous water outflow when the water volume is large), and the jet height can reach several meters. The well washer is composed of air duct and water outlet pipe. It is installed in parallel. When the compressed air reaches a certain depth of the outlet pipe through the air duct, it mixes with the liquid in the outlet pipe and forms negative pressure in the outlet pipe, which makes the liquid in the outlet pipe move upward. Then the mud sand and rock debris in the lower part enter the outlet pipe and form a mixture of gas, liquid and solid, which is discharged out of the well together. The mechanism of well washing is similar to that of positive circulation.
In the same well, the free suspension velocity of cuttings with the same diameter is different due to different washing methods (different upstream and downstream passages of flushing fluid). When flushing with the same air compressor, the reverse circulation cuttings discharge ability is stronger than the positive circulation, and the discharged cuttings particles are larger than the positive circulation.
The oscillation range of the water column in the well can reach several hundred meters. The instantaneous release of the water column pressure of several hundred meters helps to dredge the water from the aquifer. According to the ratio of the time duration and the interruption time, the water output capacity of geothermal wells can be estimated. After the completion of compressed air well washing, water pump is usually used for well testing.
A high-pressure air compressor with four-stage pressure, 10 cubic meters of flow per minute and 25 MPa of high pressure can eject a water column more than 2,000 meters deep from the surface and form negative pressure in the well instantaneously, which can make the water in the formation wash out naturally and effectively improve the well washing speed.
Well washing is a relatively flexible use of air compressors, but also an important manifestation of air compressors in construction industry.