Analysis of cuttings carrying capacity in gas drilling

With many advantages like fast drilling speed, low cost and little damage to reservoir stratum, gas drilling has been quickly developed in recent years. During the debris carrying with gas as the recirculated medium in the well shaft, a large number of debris will fail to be discharged in time if the gas injection volume is too small, leading to the decrease of drilling speed and well deviation. However, if the gas injection volume is too large, the wellbore will be eroded under huge annular friction resistance pressure drop, which will easily cause hole wall collapse and even annular blockage will occur. The key of effectively playing advantages of gas drilling technology is the determining of gas injection volume. Based on the research of debris particles´ motion law, this article provides a theoretic basis for reasonable design of gas injection volume by researching carrying flow and establishing a calculation model for carrying capacity of gas drilling. The case shows that: the flow speed of rock fragment increases gradually from well bottom to well mouth, at the connection of drill collar and drill rod because of the increase of annular cross section the flow speed suddenly decreases; the flow speed of debris particles increases along with the injection of gas.