Gas-well testing in the presence of desorption for coalbed methane and Devonian shale

Very large potential gas reserves are present in both coalbeds and Devonian shales. In both formation types, long-term gas production is the result of gas desorption from the matrix. Experiments on core samples show that the desorption obeys a Langmuir isotherm. An approximate analytic solution is derived for single-phase gas flow when gas is present as both free gas and gas adsorbed on the matrix. The solution was verified against a finite-difference solution; the agreement between the two methods is excellent. The analysis of test data is simplest if the test is conducted at a constant mass flow rate. It is shown that the effect of desorption cannot be detected in production testing; therefore, analysis can be performed to obtain permeability as if the desorption effect is absent. The procedure to obtain desorption parameters is also given. The effect of desorption shows up in an increased compressibility, of the form c/sub g/ + (..cap alpha..c/sub m//phi) + (rho/sub sc/V/sub LrhoL/T/sub z-bar//phirho-bar/sub g/T/sub sc/z/sub sc/(rho/sub L/ + rho-bar/sub g/)/sup 2/), which can be much larger than the gas compressibility. The effect of desorption on total production is substantial. When gases are adsorbed, total production over the life of a well canmore » be an order of magnitude higher.« less