WRI’s multi-component coalbed methane reservoir simulator enables modeling of methane production by conventional pressure depletion and by enhanced coalbed methane (ECBM) recovery through injection of an inert gas. The simulator accounts for an unlimited number of gases in the coalbed reservoir. It can also model CO2 adsorption by coal (CO2 sequestration) and the combined stimulation–sequestration effect of inert gas injection.
The simulator (WRICBM) can be used for a variety of coalbed applications to:
- Address one-, two-, or three-dimensional problems.
- Work with variable-length grid spacing, which can be either cell centered or grid centered.
- Specify either no-flow (closed boundaries) or constant pressure (open) reservoir boundaries.
- Specify vertical or horizontal well orientations with radial or linear flow schemes.
- Describe multiple coal types with different sorptive capacities.
- Configure a reservoir with both coal (sorptive) and sandstone (non-sorptive) layers.
- Model multiple relative permeability relationships.
- Model the effects of stress-dependent permeability (SDP) within coal.
- Describe multiple components in a gas.
- Model enhanced coalbed methane (ECBM) recovery processes.
- Model CO2 sequestration.
WRICBM uses a dual-porosity description that represents the structure of a coal bed and the physics of gas sorption with respect to coal. In the model, blocks of coal matrix are isolated by a fracture system (cleats). These blocks communicate with the fracture system, not with one another. The coal bed’s porosity and permeability are accounted for in the fracture system, while the coal is assumed to be impermeable to gas and water.
Free gas movement occurs between the surface of the coal and the fracture system, with the rate of gas transfer determined by the concentration of gas in the fracture, the fracture pressure, and the concentration of gas at the surface of the coal. Surface production and injection is modeled using conventional well calculations. The wells communicate only with the coal fractures. The simulator solves a material balance equation for each gas component and for the water component. The quantity of each gas component adsorbed onto the coal is determined, as is the rate of adsorption or desorption for each gas component.
WRICBM provides the ability to describe gas sorption using three different methods. The equilibrium method assumes that the pressure of a local coal element is the same as that of the fracture system. The pseudo-steady state method assumes that there is a delay (diffusion limitation) in the gas desorbing from or adsorbing onto the coal from the coal’s fracture system.
The fully steady state method assumes a time and concentration delay as above, but accounts for the concentration gradient (as a function of distance into the coal) within the coal element. The time-dependent methods are generally used for coal beds with significant diffusion rate limitations (weeks or months). No other commercially available simulator known can account for all three methods of methane adsorption.
Coal sorption can be calculated using the WRICBM by:
- Equilibrium (dependent only on pressure).
- Pseudo steady state (dependent on pressure, time and average gas concentration).
- Fully unsteady state (dependent on pressure, time and distribution of gas concentration within the coal).
WRICBM has been verified against Amoco’s GCOMP coalbed simulator and a commercial black oil model modified to predict coalbed methane recovery. The model has been used to predict recoveries in the Powder River Basin of Wyoming. The WRICBM simulator provides the answers to address an array of coalbed methane concerns. Licensing opportunities are available.