Design and test of the self-propelled dynamic balance confining pressure water jet drilling

Aiming at the requirements of the test on high-pressure water jet drilling in the research of increasing pro- ductivity for the equipment with minimum sidetracking in CBM wells,in order to simulate the status of the self-pro- pelled dynamic balance downhole confining pressure water jet to approach more closely the situation of the actual water jet drilling test,the test device for the self-propelled dynamic balance confining pressure drilling was developed. The device includes three parts:dynamic balance force measurement,counter weight and confining pressure tank. It can conduct water jet drilling test for self-propelled force,drilling rate and different combination of nozzles on coal samples with 200 mm in diameter and 300 mm in length in submerged state and within 10 MPa. Then the calibration test of the test device was carried out by using the method of counter weight and interior sealing resistance balance during uni- form motion,the relationship between the resistance of the sealing element and the required counter weights under dif- ferent confining pressure was found. Finally the water jet drilling test was conducted under non-submerged,submerged and confining pressure status by using the simulating test device,the relationship among the jet flow,angle and drilling rate,hole-forming diameter and pressure of drilling with the self-propelled rotary nozzle was obtained. The test results indicated that under the same conditions and the sub-merged status,the drilling rate and the self-propelled force were reduced at certain extent. Under confining pressure status,the self-propelled force,the drilling rate and the hole-form- ing diameter decreased with the increase of confining pressure,the self-propelled force and the drilling rate decreased sharply after the confining pressure reached 2 MPa. Through the comparison of the test results of three kinds of noz- zles,when the rear injection angle was small,the self-propelled force was bigger,when the rotary torque was smaller,it was easy to be clamped,therefore it was suggested to use the scheme with the rear injection angle 40° and increase the rotary torque. At the same time,the difference between the measured flow and the theoretically calculated flow was more than 30% ,which indicates throttling occurs in the nozzle. It is suggested to increase the nozzle overflow diame- ter.