An investigation of the effect of carbon dioxide sequestration on the behaviour of brown coal

Geological sequestration has the potential to store massive amounts of CO2, which otherwise would contribute to the global warming. Sequestration in unminable coal seams has been identified as one of the promising geological storage options currently available. The most noteworthy difference in this method of CO2 storage from the other geological storage options is its ability to generate revenue by the production of methane gas, which can offset partly the cost of sequestration. In addition, coal’s ability to adsorb CO2 when injected in to a coal seam, and the available large surface area for adsorption, makes coal an extremely competitive option for the CO2 storage. Extensive research work that has been executed to better understand CO2 sequestration via pilot projects, commercial projects, laboratory studies, numerical work and analytical studies have contributed to create a positive attitude on people’s minds to some extent. However, still there are unresolved areas in CO2 sequestration in coal that needs to be investigated further. Important coal properties such as permeability and behaviour related to mechanical properties, when CO2 is injected into coal need to be further researched and comprehended clearly before attempting commercially at large scale. The current research study investigates the effect of carbon dioxide (CO2) on the behaviour of mechanical properties of coal. The research program consists of an extensive experimental study and a numerical model. Experimental work carried out during the current research study can be mainly divided in to three segments. They are, • Development of reconstituted coal specimens • Investigation of coal permeability and effect of exposure to CO2 on permeability • Behaviour of coal mechanical properties on exposure to CO2. Highly heterogeneous nature of coal sometimes makes it difficult to interpret the laboratory derived results. Therefore, a homogeneous sample with reproducible properties would be highly advantageous in performing laboratory experiments to investigate the behaviour of mechanical properties of coal in coal CO2 sequestration studies. An extensive experimental study was performed to develop homogeneous reconstituted coal specimens with reproducible properties. Investigation on the development of reconstituted coal was started with black coal samples readily available in the laboratory. Portland cement was used as a binding agent. Mechanical properties of reconstituted black coal were investigated and the observed results were used to develop a correlation between Uniaxial Compressive Strength (UCS) and point load index (Is50). An extensive laboratory work was performed to develop a reconstituted brown coal specimen using brown coal taken from Latrobe Valley (Hazelwood mine) in Victoria. Reconstituted specimens were developed with and without binders. Portland cement was used for the specimens with binders. However, the binderless reconstituted specimens made with moist coal produced a better reconstituted material with mechanical properties close to that of natural coal. Furthermore, gas sorption experiments performed on reconstituted coal specimens with and without cement appeared to have similar gas sorption properties with reference to the observed gas sorption data. Permeability tests were performed on natural coal and reconstituted coal, with an intension of understanding the permeability behaviour at different confining pressures and gas injection pressures. Tests were performed using alternative injection patterns with CO2 and nitrogen (N2) in order to understand the effect of CO2 on coal permeability. Permeability observed during N2 injection is significantly greater than that of CO2. Permeability loss due to CO2 injection was temporarily recovered when N2 was flooded through coal. However, recovered permeability was soon dropped further, when more CO2 was flooded through coal. Furthermore, observed results revealed the permeability dependency on effective stress, gas injection pressure, coal swelling and gas type. These experimental results have been later confirmed by performing a numerical model developed using COMET3 software. Investigation of mechanical properties tests have been started with black coal specimens readily available in the laboratory. Uniaxial compression tests were performed on black coal specimens saturated with CO2. The observed results were compared with specimens with no CO2. Acoustic Emission (AE) measurements were taken simultaneously with the uniaxial compression tests. Important observations have been made on the effect of carbon dioxide saturation implications on crack thresholds. The uniaxial compressive strength and elastic modulus behaviour when specimens have been saturated with CO2 has been consistent with the previous findings. Testing of Latrobe Valley brown coal was started with uniaxial compression testing. CO2 saturation caused an observable reduction in coal strength and elastic modulus. Furthermore, it showed an increasing effect with increasing saturation time. Triaxial tests carried out on brown coal with gas saturations such as CO2 and N2 did not confirm any variation of mechanical properties. Simultaneously with the triaxial tests, AE measurement was carried out. Similar to the results observed with black coal under uniaxial compression testing, effect of CO2 saturation implications on crack thresholds were observed