Hongyan Qu

Impact of thermal processes on CO2 injectivity into a coal seam

The objective of this study is to investigate how thermal gradients, caused by CO2 injection, expansion and adsorption, affect the permeability and adsorption capacity of coal during CO2 sequestration. A new permeability model is developed in which the concept of elastic modulus reduction ratio is introduced to partition the effective strain between coal matrix and fracture. This model is implemented into a fully coupled mechanical deformation, gas flow and heat transport finite element simulator. To predict the amount of CO2 sequested, the extended Langmuir sorption model is used, with parameters values taken from the literature. The coupled heat and gas flow equations, are solved in COMSOL using the finite element method. The simulation results for a constant volume reservoir demostrate that thermal strain acts to significantly reduce both CO2 injectivity and adsorption capacity. These impacts need to be considered in the calculation of the optimum injection rate and the total sequestration capacity.

Multiphysics of Coal-Gas Interactions: The Scientific Foundation for CBM Extraction

Coal permeability is perhaps the most important parameter for the implementation of primary methane recovery and CO-ECBM technology. It is well known that sorption-induced strain and thermal gradients have significant influence on permeability change. In this study, a general form of permeability model is developed, which includes the impact from the geomechanical process, coal swelling/shrinkage, gas pressure change, and thermal change. Then it is extended to apply on three different coal mediums: (1) Unfractured coal; (2) Fractured porous coal media; (3) Fractured coal with rigid matrixes. Based on this permeability model, a set of governing equations is built up, which fully couples the geomechanical deformation, gas and heat flow processes. This coupled model is used to evaluate the influence from different media types on permeability evaluation, gas injection and production performance, and the effective stress change around the wellbore. In addition, numerical simulations under both isothermal and non-isothermal conditions are conducted. Significant influence on the permeability evolution, injection and production performance was observed for both cases. Copyright 2010, Society of Petroleum Engineers.

Simulation of coal permeability under non-isothermal CO2 injection

CO2 injection into coal seams is a non-isothermal process, which has significant impact on coal permeability but has not been well studied. In this paper, a non-isothermal model coupled with nonlinear gas flow and matrix deformation was developed. The effects of temperature change on each term of the effective strain during the CO2 injection scenarios, as well as the variations of fluid properties over a range of sub- and supercritical-thermodynamic conditions were investigated. This model involves the balance of thermal energy and the law of heat transfer. Two non-isothermal cases of CO2 injection were studied and compared with the isothermal case. The results show that CO2 injection into coal seams reduces coal permeability for all three cases. The coal matrix expands with temperature increase due to the thermal expansion and shrinks due to the decrease in adsorption amount. However, the final permeability with low-temperature CO2 injection remains lower than that with high-temperature gas injection since the effect of sorption-induced strain on permeability outweighs that of the thermal deformation. The increase in temperature leads to the reduction in coal swelling (with the decrease of CO2 adsorption capacity), resulting in larger cleat aperture and higher coal permeability for the cases studied in this work. [Received: November 29, 2015; Accepted: June 22, 2016]