A series of mathematical and numerical models to simulate the simultaneous flow of methane and water in a coal seam are presented. Transportation of methane is described using multi-mechanistic flow concept whereas desorption of methane from coal grain surfaces is formulated by a quasi-steady state kinetic model.
The simulator employs a two-dimensional, rectangular coordinate, point-distributed grid system and has options to accommodate a number of hydraulically fractured or unstimulated wells. In order to describe the flow in hydraulic fractures, a secondary one-dimensional grid is superimposed over the primary grid system. Flow equations in the matrix and finiteconductivity fracture have been solved simultaneously. The coupling between fracture and matrix flow equations is established via a "fluid loss" term.
The model is also capable of handling a number of horizontal boreholes. Multiple boreholes can originate from the bottom of a common shaft or from different shafts and they are allowed to intersect each other. Furthermore, options exist to include a number of impermeable geological barriers within the system.
An assortment of boundary conditions can be imposed on the system at its external boundaries as well as at its internal boundaries. Along these lines a special algorithm is devised and implemented successfully in order to be able to specify methane drainage rate from a virgin coal seam which has macropores that are originally 100% saturated with formation water.
The simulator has been tested under stringent conditions and found to be stable. Finally, the validity of the developed model has been verified against some existing field data.