PLUNGER-ENHANCED CHAMBER LIFT (PECL) IS THE BEST LIFT METHOD FOR CONTINUOUS DEWATERING OF TIGHT CBM RESERVOIR WITH VERY LOW WATER PRODUCTION AND BOTTOM HOLE PRESSURE

摘要

Dewatering process is in the first step in CBM operations so that methane initially adsorbed on the coal matrix can be produced. For this to happen, formation water must be produced from coal seam in order to decrease reservoir pressure and methane adsorption capacity. Dewatering can last for short period of time (months) or can be as long as the life of a CBM well. In ideal dewatering process, it is important to keep the well flowing bottom hole pressure as low as possible and avoid pressure build up so that the reservoir pressure can be decreased efficiently and significant amounts of initially adsorbed methane can be produced. Problems arise when it comes to producing CBM well from a tight (very low permeability) coal formation. Water production from sand face can be as low as 5 BWPD without high enough bottom hole pressure to produce it to the surface. This requires artificial lift to lift formation water to the surface. Problematically, there is no pump with small enough minimum volumetric capacity to lift 5 BWPD out from the well without pump off condition. The pump off condition will cause increase in liquid column in the well bore and finally the buildup of bottom hole pressure which cause inefficient dewatering process. The reservoir pressure within well drainage area will increase to initial reservoir pressure during the pump off period and methane will stop desorption from coal matrix if increase in reservoir pressure exceeded coal desorption pressure.