Failure mechanism of gas anchors in coalbed methane horizontal wells

The gravitational gas anchor performs poorly in coalbed methane horizontal wells, with severe continuous gas production through the tubing. To clarify its failure mechanism, a liquid drainage and gas production experimental setup was constructed. It is 11 m high, inclined at 6° to the horizontal, and equipped with a progressing cavity pump (PCP) and a visual gas anchor. A high-speed camera was used to monitor the gas-liquid two-phase flow patterns inside the gas anchor and at the tubing outlet. Continuous gas production through the tubing was taken as the criterion for gas anchor failure. Experiments on gas anchor failure were conducted under different PCP rotational speeds 100~300 r/min, gas flow rates 100~1 100 m³/d, and bottomhole pressures 150~300 kPa. In the horizontal well, the gas anchor separates the annular space of the oil ring and the inlet of the screw pump through internal liquid phase separation to achieve the separation of gas-liquid two-phase in the annular space. The failure of the gas anchor occurs when its liquid discharge rate exceeds its liquid intake rate. At a PCP speed of 200 r/min and a gas flow rate of 400 m³/d, gas enters the anchor only in the form of bubbles. When the PCP speed is increased to 300 r/min, the gas-liquid interface inside the anchor drops below the inlet, allowing direct gas flow from the annulus into the pump and resulting in anchor failure. Reducing the PCP rotational speed, optimizing the shape of the gas anchor inlet, and increasing the depth of the gas anchor can all raise the gas flow rate at which failure occurs. The findings provide guidance for adjusting production strategies and improving gas anchor design in CBM wells.