Using geological strength index (GSI) to model uncertainty in rock mass properties of coal for CBM/ECBM reservoir geomechanics

The use of geostatistics has become an important concept to characterize uncertainty in reservoir simulations. Only recently has the concept been applied to geomechanics and coupled reservoir geomechanical simulations. The work presented here describes how the Geological Strength Index (GSI) is used to represent the fractured reservoir properties of a coalseam reservoir, and link changes in flow properties with stress changes as well as to strength and deformation. The approach is demonstrated by using a spherical geostatistical model to create 11 GSI realizations and one constant GSI model. Two years of primary methane production and then 3 months CO2 storage were simulated using each of the 12 models as inputs. Much higher cumulative gas and gas production rate profiles were observed for models with a mean GSI below 80, than the 80GSI model. All models above 80 GSI show much lower cumulative gas and gas production rates than the 80GSI model. The same trend held true for the CO2 injection component of the operation. Stress path profiles for three monitoring points in the reservoir showed large variability, but similar paths, whereas monitoring points in the overlying caprock showed very low variability for each realization. (C) 2012 Elsevier B.V. All rights reserved.