Coal Failure With Continued Depletion of Coalbed Methane Reservoirs and Resulting Jump in Permeability

Abstract

This paper summarizes the findings of a study carried out to explain sudden increases in the permeability of coal, commonly encountered in the northern San Juan basin, with continued production of methane. Change in permeability of coal was first measured in the laboratory using coal from San Juan basin and replicating ideal field conditions, that is, uniaxial strain. The results showed that the permeability increased slowly and continuously for depletion from 1500 to ~600 psi, at which time there was a sudden and large increase in its value. After that, the permeability continued to increase until the pressure dropped to ~50 psi. The total increase in permeability was in excess of 400 times. The initial increase in permeability, and again after the uptick at ~600 psi, is explained by the sorption-associated matrix shrinkage, an accepted phenomenon for San Juan coal. The matrix shrinkage was measured in a separate set of experiments and modeled as a function of the amount of gas desorbed. Finally, in order to explain the sudden uptick in permeability, the failure envelope for the coal type was established. The envelope was plotted using the stress invariants, I1 and J2. The stress path followed during depletion of methane was superimposed on the failure envelope. The results clearly showed that coal failed at approximately 600 psi pore pressure during depletion, matching the pressure at which the sudden uptick in permeability occurred. Prior work carried out replicating field conditions did not exhibit the failure phenomenon since the depth replicated during experimental work was between 2000 and 2800 feet. The failure phenomenon plays a role only for coals at a depth of greater than 3000 feet. The behavior is attributed to a significant decrease in horizontal stress as a result of coal relaxation associated with matrix shrinkage. The loss in horizontal stress results in a large stress differential between the effective vertical and horizontal stresses with continued depletion which, in turn, causes coal failure resulting in a sudden increase in permeability. The finding has significant practical implications in CBM operations since coal failure results in creation of new fractures/cracks providing additional flow paths for gas. However, in the field, it is often accompanied by production of fines and coal rubble, requiring time consuming and expensive well cleanouts as well as temporary drop in permeability.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015