Flow assurance application to unconventional gas wells: down-hole separation efficiency calculations with computational fluid dynamics

Abstract

Coal bed methane (CBM) operations reduce the reservoir pressure to desorb gas from the coal matrix. Internationally, this is often achieved by drawing down water through pumping from the reservoir, by surface or down-hole pump. Pumping water lowers the bottom-hole pressure and also provides a first pass separation of water and gas in the well annulus. Set-point pressure pumps are typically used to pump water up the inner tubing, whilst gas is produced in the outer annulus. In many operating CBM applications this separation is adequate, removing a large percentage of bulk and droplet-based water from gas, negating the need for secondary surface separation. Key to this separation is the ability of the water to drain downwards to the bottom of the well without being carried over with the upwards moving gas. This is a counter-current flow regime with large superficial gas velocities (as high as 30 m/s) and small superficial liquid velocities (less than 1 m/s). The annular/droplet regime is prevalent and there is potential for droplet carry-over with gas, affecting the need for further separation. Computational fluid dynamics (CFD) was used to model the process of gas and water separation down hole in the perforation zone to calculate the fate of water droplets as they pass downwards counter-current to upwards moving gas. A flow envelope was developed to calculate the carry-over liquid flow rate for a range of gas flow rates. This work assists the design requirement for wellhead surface separation or otherwise for Australian CBM applications.