A new coal particles cleanout technology in coalbed methane wells in China

Abstract

Abstract Coal particles regularly flow into coalbed methane (CBM) wells with the formation water. Once they settle down in the bottom of CBM wells, the pump will get stuck and the reservoir will be buried. In conventional oilwells, sand cleanout is usually operated by circulating the cleaning fluid into the wellbore to bring sand particles to the surface. However, when applied in CBM wells, this traditional hydraulic particles hoisting technology would leak the working fluid into the formation, destroy the coalbed formation structure and jam the formed channel of gas because the bottomhole pressure (BHP) is low. In this paper, a continuous vacuum cleanout technology will be introduced, and it combines a novel jet pump with a concentric tubing string (CTS) to remove coal particles without placing hydrostatic loads on the reservoir. The novel jet pump has two nozzles and the CTS is assembled by the outer tubing and inner tubing. More specifically, a high pressure working fluid is pumped through the CTS annulus from the surface and then it is divided into two parts. One part of the fluid acts as the sand carrier fluid. The other part acts as the power fluid of the jet pump to create a localized drawdown which vacuums the carrier fluid together with coal particles into the jet pump, and then circulates coal particles to surface through the inner tubing. The detailed structure and the principle of the technology system are described, while a theoretical model is built to design and optimize the system which is based on the coal particles transport experiment data and the jet pump theory. The field applications have proved that the coal particles cleanout technology makes significant improvements in achieving high efficiency and preventing leakage in CBM wells, and more importantly, reduces the skin damage and improves the CBM production.