The influence of high-yield-water characteristics on productivity of CBM wells and expulsion and production method carried out in Yanchuannan block of the Ordos basin, China

Abstract

In order to further study the influence of high-yield-water on the productivity of CBM (coalbed methane) wells and the expulsion and production method carried out in CBM wells, by means of analyzing and researching production characteristics and geologic condition of the CBM wells with high water yield in Yanchuannan block located at the eastern margin of Ordos basin, the mechanism of high water yield decreasing the productivity of CBM well was discussed, and the expulsion and production method for this type of CBM well was proposed. The results show that high water yield would decrease the productivity of CBM wells, and the mechanism is: first, in some circumstances, high water yield could reflect that there was dissipation during the process of coalbed methane reservoir forming, which would lower the gas saturation of coal gas reservoir and reduce the productivity of CBM well; second, a large quantity of coalbed methane dissipated in the form of solution gas, causing the practical reservoir pressure when gas appeared in casing to be lower than critical desorption pressure of the coal bed; finally, the CBM well with high water yield would have higher requirements of discharge and mining installation, system and continuity, and any link with problems would have a great impact on the well’s productivity and would increase the difficulty of discharge and mining. In the case of wells with high water yield, the key is to select applicable discharge and mining installation, which should be able to make the bottom hole flowing pressure decline smoothly and fast, and make the wells produce gas as quickly as possible but able to slow down the rate of discharge and mining properly when gas has appeared. In addition, in view of the CBM wells with high water yield, an installation lectotype method based on Darcy’s law was proposed, which was found with good accuracy and practicability through field application.