Reservoir and geomechanical coupled simulation of CO2 sequestration and enhanced coalbed methane recovery

Abstract

Coalbeds are an extremely complicated porous medium with characteristics of heterogeneity, dual porosity and stress sensitivity. In the past decades great achievements have been made to the simulation models of pressure depletion coalbed methane (CBM) recovery process and CO2 sequestration and enhanced coalbed methane (ECBM) recovery process. However, some important mechanisms are still not or not properly included. Among them, the influence of geomechanics is probably the most important one. Because of its influence coalbed permeability, the key parameter for the success of recovery processes, changes drastically with alterations of in situ stresses and strains during these processes. In present reservoir simulators, the change of coalbed permeability is estimated with analytical models. Due to the assumptions and over simplifications analytical models have limitations or problems in application. In this research to properly estimate the changes of permeability and porosity in the simulation of CO2 sequestration and ECBM recovery process, comprehensive permeability and porosity models have been developed with minimum assumptions and simulation methods established. Firstly, a set of continuum medium porosity and permeability coupling models is built up and a simulation procedure to apply these models in reservoir and geomechanical coupled simulations proposed. Using the models and simulation procedure a sensitivity study, mainly on the parameters related to coalbed permeability change and deformation, has been made for the CBM recovery process. Then based on the understanding, a set of discontinuum medium porosity and permeability coupling models is developed and a procedure to apply these models in reservoir and geomechanical coupled simulations presented. The new models are more comprehensive and adaptable, and can accommodate a wide range of coalbeds and in situ conditions. The proposed equivalent continuum deformation model for coal mass is validated by simulating a set of lab tests including a uniaxial compression test in vacuum and a CO2 swelling test under axial constraint in the longitudinal (vertical) direction. At last the discontinuum medium porosity and permeability coupling models and the simulation procedure are successfully applied to simulate part of a series of micro-pilot tests of ECBM and CO2 sequestration at Fenn Big Valley of Alberta, Canada.