Geological sequestration of carbon dioxide in deep saline aquifers: coupled flow-mechanical considerations
thesisposted on 03.03.2017 by Rathnaweera, Tharaka Dilanka
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Global warming is an extremely crucial challenge for 21st century researchers and numerous climate change policies and mitigation options have been initiated throughout the world during the last few decades. After much research on these approaches to mitigate global climate change, CO2 sequestration has been identified as the only technology which can reduce CO2 emissions on a significant scale from fossil fuel power plants and other industrial processes like steel, cement and chemical production in an economically and environmentally friendly way. Sequestration involves the long-term storage of captured CO2 in deep subsurface geologic reservoirs such as oil/gas fields, deep saline aquifers, unmineable coal seams and mineral sequestration. Of these methods, deep saline aquifers have been identified as an effective geological formation for sequestering anthropogenic CO2 emissions and isolating them from the earth’s atmosphere. However, long-term interaction between injected CO2 and aquifer brine causes its chemical and mineralogical properties to be significantly changed, resulting in altered hydro-mechanical performance compared to its natural condition. Ultimately, these altered hydro-mechanical properties affect the efficiency and safety of the sequestration process. In relation to the safety of the sequestration process, CO2-induced reservoir performance maximizes the CO2 leakage risk by the fracturing or reopening of pre-existing non-transmissive faults in the reservoir. The efficiency of the storage process may be significantly jeopardized due to the fluctuation of the reservoir permeability induced due to rock mineral alterations. Therefore, it is important to understand the reservoir’s chemical, mineralogical and hydro-mechanical performances induced due to CO2 injection before beginning any sequestration project. In addition, the long-term integrity of a potential aquifer formation should be investigated thoroughly, taking into account a number of operational and reservoir variables. Of these variables, CO2 flow rate, injection pressure and number of injection wells are important operational variables, while depth, salinity, aquifer pH, rock mineralogy and aquifer chemical composition are reservoir variables that should be considered in reservoir performance analysis. Therefore, the main aim of this thesis is to investigate the influence of CO2/aquifer brine/reservoir rock interactions on the chemical, mineralogical and hydro-mechanical behaviours of deep saline reservoir rock masses under deep reservoir conditions. In the present study, a comprehensive literature review of the chemical, mineralogical and hydro-mechanical processes involved in deep saline sequestration was carried out, highlighting some factors (operational and reservoir variables) and theories related to the mechanical and flow behaviours of deep saline formations. Based on identified research gaps, the objectives of this study were defined. This study has two components: experimental which is the major component of the study, followed by some numerical studies. The experimental part is achieved by the comprehensive chemical, mineralogical, micro-structural, mechanical and permeability investigations of CO2 flow-related properties of intact reservoir rock specimens obtained from the Gosford Basin in New South Wales, Australia. The Gosford formation mainly consists of sandstone and it is known as Hawkesbury sandstone. The numerical section reports the results of investigations of the mechanical and flow-behaviour of the Hawkesbury formation under both laboratory- and field-scale conditions. The experimental study started by investigating the mechanical behaviour of natural aquifer formations prior to CO2 injection under both unconfined and confined environment. First, the influence of different salinity conditions on the mechanical properties of reservoir rock in an unconfined stress environment was investigated. Three different salinity conditions (10, 20 and 30% NaCl concentration by weight) were used to saturate the samples and the saturation was done in desiccators under vacuum for one month. After reaching full saturation, a series of unconfined compressive strength tests was performed on brine-saturated reservoir rock samples. In order to identify the influence of salinity on the fracturing process of the reservoir rock, acoustic emission (AE) and ARAMIS technologies were also incorporated in the strength tests. The effect of salinity on the mechanical behaviour of reservoir rock was checked by comparing the results of water-saturated specimens tested under the same conditions. After evaluating the salinity effect in an unconfined environment, experimental testing under confined stress conditions was initiated to understand the effect of confining stress and salinity on rock failure under reservoir conditions. The same salinity conditions were used for the confined stress series. The results of these brine-saturated sandstone strength tests revealed some important characteristics of their failure strength and fracturing mechanisms under natural aquifer conditions. In general, it was observed that brine saturation can considerably increase the reservoir rock strength compared to water saturation, and the reason for this was confirmed by the results of scanning electron microscopy (SEM) analysis, where a significant number of NaCl crystals were observed in the brine-saturated reservoir rock pore space. After evaluating the mechanical behaviour of natural aquifers, experimental testing under CO2-reacted conditions was started to evaluate the influence of CO2/brine/rock interaction on the mechanical properties of reservoir rock. The prepared samples were first saturated with brine and then reacted with CO2 using a specially-designed reaction rig. Finally, CO2-reacted samples were tested under both unconfined and confined stress conditions. A series of unconfined compressive strength tests coupled with AE and ARAMIS was initially conducted to understand the influence of CO2 on the mechanical properties of reservoir rock. SEM, X-ray diffraction (XRD) and X-ray fluorene (XRF) analyses were carried out to evaluate the rock mineral alterations induced due to CO2 injection. A comprehensive mechanical investigation was carried out by performing a series of tri-axial strength tests on these CO2-reacted samples for a range of injection (0.25-16MPa) and confining (2.5-20MPa) pressures at a constant temperature of 35°C. According to the results, CO2/brine/rock interaction causes reservoir rock failure strength to be considerably reduced compared to its natural condition. In addition, the tri-axial strength test results revealed the influence of injection and confining pressures on the mechanical properties of reservoir rock during sequestration. After the mechanical tests, the flow behaviour of reservoir rock was investigated by performing undrained and drained high-pressure permeability tests for both brine-saturated and CO2/brine-reacted samples. The brine-saturated samples were tested in order to identify the effect of salinity on the flow behaviour of reservoir rock. In addition, the influence of CO2-phase change, injection and confining pressures on the permeability characteristics of reservoir rock were evaluated. The tests on brine-saturated samples showed that salinity has a significant influence on the permeability of reservoir rock. According to the results, high salinity conditions tend to reduce the aquifer permeability by NaCl crystallisation. The effective stress coefficient for CO2 permeability also showed a significant correlation to salinity. The experimental testing performed on CO2/brine-reacted samples indicated the influence of CO2/brine interaction on the overall flow performance of the reservoir rock. The inductive-coupled plasma mass spectroscopy (ICP-MS) and inductive-coupled plasma atomic emission spectroscopy (ICP-AES) analyses confirmed the dissolution/precipitation of rock minerals upon exposure to CO2 and brine. In the case of different sandstone formations, the hydro-mechanical behaviour was observed to be governed mainly by the rock minerology. Two types of sandstone: carbonate- and silicate-cemented were therefore chemically, mineralogically and mechanically tested in order to understand the influence of rock mineralogy on CO2 sequestration. In addition, it is important to identify CO2 storage enhancement techniques in deep saline sequestration. Therefore, the present study proposes a potential CO2 storage enhancement technique: enhancement of CO2 storage through the co-injection of CO2 and brine into saline aquifers. Mechanical and permeability experimental results were used to develop laboratory- and field-scale models using the COMSOL Multiphysics tool. First, a triaxial strength model at laboratory scale was developed using experimental data. After the model was validated, it was further extended to study the failure behaviour of brine-saturated reservoir rock under high confining pressure conditions. According to the results, the numerical model developed considering the stiffness degradation mechanism of reservoir rock can accurately simulate its salinity-dependent stress-strain behaviour under laboratory condition. Interestingly, the predicted results under high confining pressure conditions revealed that this rate of strength increment reduces as the confining pressure increases. Importantly, the model clearly showed a reduction of the pore fluid salinity on reservoir rock strength characteristics with increasing reservoir depth or confinement, mostly due to the more significant effective stress at such extreme depths. This indicates an important insight on CO2 sequestration in deep saline aquifers: the salinity-dependent strength alterations are not as important for extremely deep aquifers as for shallow aquifers. (...)re structure to significantly change during CO₂ sequestration.