New strategies for in-situ characterization of coal

Coalbed methane has gained recognition as an unconventional natural gas resource. Methane gas that was once vented to atmosphere through a mine ventilation system is now being collected from vertical boreholes drilled prior to the underground mining operation and is being marketed profitably to natural gas pipeline companies. One serious limitation to the economic production of coalbed methane has been the lack of reservoir characterization technology. The dual-porosity nature of the coal seam together with the sorption phenomena makes the implementation of the classical pressure transient analysis temperature unfeasible to estimate in situ reservoir properties. This program identifies the need for and introduces novel pressure transient analysis strategies which are specifically developed for coalbed gas reservoirs. Mathematical and numerical developments for single-phase (gas) and two-phase (gas and water) flow dynamics which articulate the pressure transient behavior of the coalbed methane degasification wells are described. For the single-phase flow conditions a one-dimensional mathematical model is developed in radial-cylindrical coordinates which describes unsteady-state sorption and diffusion in coal matrix, coupling of the matrix and fracture systems and the laminar transport of gas in the fracture network. In order to obtain the analytical solution to the mathematical model, the transport of methane inmore » the coal matrix is described using spherical elements. The mathematical models which account for unsteady-state and pseudosteady-state matrix/fissure methane transfer are incorporated to the transport equation. The validity of the proposed solutions was checked against a numerical model and very good agreements are obtained.« less