Remote Sensing and CO2 Assesment

 

The most significant and advantageous characteristic of coal seam reservoirs with respect to CO2 storage is the storage mechanism involved: CO2 is held by very strong forces adsorbed on the micro pore surface area, which makes the storage mechanism permanent relative to other storage mechanisms such as EOR, EGR and Aquifer storage.

 

The first objective of the project was to establish a better understanding of coal matrix pore structure, the kinetic process of diffusion for CH4 and CO2 and correlation of these with that obtained from the RECOPOL site for further upgrading of the institutional ECBM simulators.

The second objective of the project was to establish an understanding of the complex geomechanical and fluid flow processes occurring around an UCG (Underground Coal Gasification) cavity to be able to elaborate optimal CO2 storage and gas recovery techniques using simulation.

 

The characteristics of a number of coal samples tested in the project indicated that the coals exhibit a multimodal pore size distribution and contain negligible to significant amounts of connate water. As the gas sorption/storage occurs mainly in the micropore region, this data is intended to provide an insight into the coal microstructure and the sorption capacities of coal. The CO2 extraction results obtained during the chemical adsorption experiments showed that other processes were also occurring and may affect permeability. As hydrocarbons are removed by the CO2, the coal will not return to its initial state after pressure release, indicating an irreversible process.

 

A bidisperse pore diffusion model was developed and implemented in the in-house CBM/ECBM simulators of the partners. A coalbed permeability model based on the mechanical response of coalbeds to primary and enhanced methane production was also developed. Extensive numerical simulation work revealed that almost all production effects are the result of permeability changes resulting from either swelling-shrinkages processes or some yet to be explained phenomena. Possible explanations for some of these could be changes in adsorption characteristics or changes in effective adsorption/absorption surfaces.

 

In this project, the partners proposed that, in a manner similar to the permeability enhancement observed around underground longwall operations, the combustion chamber/cavity and consequent stress relief resulting from Underground Coal Gasification (UCG) operations would create a highly fractured and enhanced permeability zone around the UCG cavity. Such permeability enhancement would be particularly effective in the overlying coal seams. CO2 ECBM simulations were carried out for three cases: base case primary recovery, strong roof strata and weak roof strata. Primary recovery was simulated for initial 10 years and enhanced coalbed methane recovery due to UCG stimulation was simulated from year 11 to year 30. The simulation results indicated that for relatively strong overburden rock, CBM production and CO2 storage would be effective up to 140 m above the UCG reactor. In the case of weak overburden strata, the methane recovery and CO2 storage would be significantly higher and extend up to 160 meters above the gasification chamber.