t 1 CO 2 / coal interaction Frank van Bergen, Sander Hol, Chris Spiers, Colin Peach Coal-seq 2005

t 2 Acknowledgements This project was made possible by funds from the CATO project, Shell International and TNO. The technical staff members of the Laboratory for High Pressure and Temperature Research of Utrecht University, especially Peter van Krieken and Gert Kastelein, are thanked for their valuable support and suggestions in the experiments and interpretation. Harry Veld and Kathrin Reimer of TNO are acknowledged for their support and suggestions in the coal characterization and gas analyses. The Central Mining Institute and the Brzeszcze mine in Poland and Delft University of Technology are acknowledged for providing the coal samples

t 3 Introduction CO 2 sequestration in coal while producing (enhanced) coal bed methane (ECBM-CO 2 ) considered to be a niche option for CO 2 sequestration for those areas with large industrial sources and few sequestration alternatives Critical factors Permeability (swelling) Exchange ratio at reservoir conditions Field experiment ongoing to test the feasibility Laboratory experiments required to understand the fundamental processes This project aims at the integration of field and laboratory results!

t 4 Injection and production in in-situ coal Injection Production Cleat system: determines permeability Matrix blocks: determines Diffusion Micro pore system: Adsorption/desorption processes from coal surface

t 5 Coal-gas interaction CO 2 interacts with the in-situ coal, causing e.g. swelling Classical idea: Physical process Recent idea: Physical process + chemical process Low P High P Adsorbed phase Low P High P Adsorbed phase

t 6 Characteristics pure processes Physisorption Effects are largely reversible, partly irreversible Coal structure can change mechanically Effects are strongly P dependent Absolute effects are P dependent (i.e. higher P, more swelling), e.g. by a Langmuir relation (Negative) relation between P and reaction time Chemisorption Effects are largely irreversible, partly reversible Coal structure changes chemically Effects are concentration dependent, for a gas thus P dependent Strong (negative) relation between concentration (P) and reaction time Absolute effects are P independent once the sample looses its reactivity, it becomes inert

t 7 vitrinite liptinite inertinite pyrite Organic material - aliphatic groups - aromatic groups - etc. – C – C – – C = C – – C – H – C – O – C – Chemisorption will take place at the molecular level Chemisorption is assumed to affect mostly non-carbon-carbon bonds (oxygen functional groups, Goodman 2005) - relation with C content (rank)

t 8 Focused on Registration of exerted stress after introduction of CO 2 Determination of volumetric expansion of coal under the influence of CO 2 Reversibility / irreversibility of expansion, possible indication of chemical processes Laboratory Experiments Approach

t 9

t 10 Laboratory Experiments Approach Sample Activated Carbon (reference material) Low volatile bituminous coal (Germany) High volatile bituminous coal (Poland) Sample treatment Dried and physically ‘homogenised’ (µm), approx. 10 (gram) Pre-compaction under vacuum at 65 (MPa) He porosity circa 15-20% Gas CO 2 Helium (Reference gas) Nitrogen (data still under evaluation)

t 11 Step 0: compaction in vacuum at constant load of 65 MPa HVB coal & LVB coal become pellets, AC remains powder sample piston Porous plate Sealing o-ring Approach Piston in contact with porous plate, experiences stress of 65 MPa

t 12 sample piston Porous plate Sealing o-ring Approach Step 1: constant volume of sample (piston fixed) Piston just in contact with porous plate, experiences stress of circa 0 MPa

t 13 Step 2: constant volume of sample (piston fixed) Introduction of gas In case of swelling, excess stress is measured excess stress = observed stress – gas stress Approach sample piston Porous plate Sealing o-ring Piston just in contact with porous plate, experiences stress executed by gas

t 14 Low excess stress with HVB & He Decrease in excess stress with carbon content Results LVB

t 15 Applying higher P does not result in much additional excess stress Results

t 16 Results - interpretation Exerted force by the sample after introduction of CO 2 is significant P relationship indicates possibility of chemical reactions However, similar behavior could be expected from physisorption

t 17 Step 3: piston removed from sample Approach sample piston Porous plate Sealing o-ring Piston not in contact with porous plate, experiences stress executed by gas

t 18 Step 3: piston removed from sample volume changes of sample allowed Approach sample piston Porous plate Sealing o-ring Piston not in contact with porous plate, experiences stress executed by gas

t 19 Step 3: piston put back on sample, determination of new sample volume Strain = Approach sample piston Porous plate Sealing o-ring Piston just in contact with porous plate, experiences stress executed by gas (measured sample volume – initial sample volume) initial sample volume

t 20 Activated carbon Strain data unreliable because of powder form HVB coal (1) Apparent irreversible strain (swelling) of circa Apparent reversible strain (swelling) of circa 0.01 Results

t 21 HVB coal (2), first introduction of CO 2 Apparent irreversible strain (swelling) of circa Apparent reversible strain (swelling) of circa HVB coal (2), repeat introduction of CO 2 Apparent reversible strain (swelling) of circa 0.01 – Results

t 22 LVB coal, first introduction of CO 2 Apparent irreversible strain (swelling) of circa 0.04 Apparent reversible strain (swelling) of circa 0.01 LVB coal, repeat introduction of CO 2 Apparent reversible strain (swelling) of circa 0.01 – Results

t 23 Results - interpretation Volumetric expansion is significant Strong indications for irreversible chemical reactions, in addition to expansion due to physical sorption

t 24 Step 4: constant volume of sample (piston fixed) release of gas analysis of released gas by GC-MS Approach sample piston Porous plate Sealing o-ring Piston just in contact with porous plate GC-MS Gas

t 25 N.B.: corrected for sulphur coompounds, that could be attibuted to rubber

t 26 N.B.: corrected for sulphur coompounds, that could be attibuted to rubber

t 27 N.B.: corrected for sulphur coompounds, that could be attibuted to rubber

t 28 N.B.: corrected for sulphur coompounds, that could be attibuted to rubber LVB coal

t 29 Results - interpretation Chemical reactions proven by release of higher alkanes (at least up to pentane)

t 30 Results – interpreation Evaluation of “reaction time”, i.e. time at which half of the extrapolated maximum excess stress is exerted Expectation: shorter reaction time at higher P Indications that the coal becomes more chemically inert (“loss of reactivity”) after first introduction of CO 2

t 31 After CO 2 -introduction in coal, chemical reactions are likely to occur next to physisorption Results in permanent coal expansion Force executed by coal expansion is significant Observed effects probably dependent on coal characteristics Rank, composition, etc. Numerical models usually relate adsorption and swelling to P alone, while coal characteristics seem to play an important role Coal becomes chemically “inert” after being in contact with CO 2 2-step modelling? First stage “chemical” modelling and second stage “physical modelling? Observed (chemical) expansion highly relevant to field applications RECOPOL results showed a decrease in injectivity, attributed to swelling, which was irreversible Returning to a similar injection P after build-up and fall-off did not result in similar injectivity Conclusion and implications

t 32 Swelling or shrinkage ? Preliminary experiments under constant load show shrinkage and swelling, depending on stress CO2

t 33 Swelling or shrinkage ? Preliminary experiments under constant load show shrinkage and swelling, depending on stress CO2

t 34 Workshop observations (Frank & Saikat) Other possibilities besides bi-modal pore structure to explain two different diffusion times (Andreas & Nikolai). Dirk was able to explain the sorption CH 4 by the a bi-modal pore distribution but had difficulties with CO 2. Several groups did observe differences in response to multiple cycles of CO 2 exposure More effort should go behind looking into the right way of doing diffusion experiments (e.g polycyclic stress conditions)

t 35 Workshop observations (Frank & Saikat) More dynamic void volume corrections to sorption data (using swelling coeff.) Nikolai: First pressure steps fast (order of hrs.) and subsequent steps slow (order of >3 days) Swelling cannot be explained simply by the volume of the adsorbed phase (Nikolai)