To date, many coal bed methane (CBM) wells have been de-watered using sucker rod or progressive cavity pumps. These methods have shown to be an effective method of reducing wellbore water levels, but they are not without limitation or operational problems. Past experience has shown an effective installation is one that all allows annular production of the methane gas, and the production of water up a center tubing.
This paper will describe high-volume artificial lift technology using specifically-designed gas lift method s to de-water CBM wells in the Black Warrior Basin. Also provided is a discussion of the principal surface and subsurface components employed in the system and the results of an actual field test.
The principal technical contribution of the gas lift de-watering method is improved well maintenance and production optimization made possible by the use of conventional wireline service methods. Since the water production tubing string provides a through-conduit to the bottom of the well, well cleaning operations to remove coal fines or reservoir debris are easily accomplished by conventional bailing or reeled tubing methods. Cost reductions due to simplicity of the installation and elimination of wear associated with mechanically driven pumps arealso considered.
Since 1971, Coalbed Methane (CBM) wells have been de-watered using conventional means of artificial lift. To date, wells in the Black Warrior Basin have typically been produced using either sucker rod or progressive cavity pumps, with problems ranging from stuck rod strings to damaged pump barrels. Wells are generally drilled from 1000 to 2500 feet, with some new wells reaching depths up to 6000 feet. Water has been produced inside 2 7/8 inch tubing while the methane gas has been produced up the annulus. Initial production ranges from 50 to 1000 BWPD (averaging 300 to 350 BWPD). The wells generally require 3 to 12 months to de-water.
According to local operators, the current requirement of a Black Warrior Basin completion is that the Flowing Bottomhole Pressure (FBHP) across the coal seams (the desorption pressure) is perceived to be approximately 5 to 10 psi. Therefore, the most effective installation is one that achieves this objective at an acceptable coast.
Many wells in the basin are initially brought on line by utilizing single point injection of air down the production string. This practice allows the wall to (l) be cleaned out of any remaining sand and/or coal fines and (2) reach a lower fluid production rate in a shorter period of time. Upon reaching this target flow rate the well can then be placed on artificial lift.
A large number of wells are scheduled for completion over the next few years, with many to be drilled to greater depths (3500 to 4500 feet). Artificial lift technology commonly used in conventional oil and gas completions can now be applied to coalbed methane wells, in order to maximize both fluid production and operational efficiency while minimizing costs.
Over the past few years, gas lift has been introduced to the coalbed methane industry in Alabama as an alternative method to de-water wells.
