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Geopolymer as well cement for geological sequestration of carbon dioxide
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posted on 28.02.2017by Mohamed Careem, Mohamed Nasvi
Carbon capture and storage (CCS) is one of the feasible solutions to reduce carbon dioxide (CO2) emission levels without affecting the usage of fossil fuels. Wellbore integrity needs to be maintained for leak-free storage and well cement plays a major role in wellbore integrity as it provides the necessary zonal isolation. To date, ordinary Portland cement (OPC)-based sealant has been used in injection wells and it has been found that it experiences cement degradation and is unstable under CO2-rich down-hole conditions. Therefore, current research has focused on geopolymer, an alkali-activated inorganic binder, as an alternative well sealant material to OPC. The main objective of the research is to study the flow and mechanical behaviour of geopolymers under CO2 sequestration conditions using experimental studies and numerical modelling techniques. Comparisons were made with the outcomes of traditional OPC-based well cement materials. Uniaxial compression and triaxial flow experiments were conducted as part of the experimental program and the COMSOL mulitiphysics numerical package was used to conduct numerical analysis.
A series of uniaxial compressive strength tests were performed to study the mechanical behaviour of the wellbore materials geopolymer (G), formation rock sandstone (S) and G-S composite materials under different in-situ conditions. The mechanical behaviour of three different materials in water and brine with two different NaCl concentrations (5% and 15%) was experimentally studied. As expected, peak compressive strength of all three materials reduces in water and brine water, and lowest reduction rates were observed for geopolymer saturated in 15% NaCl brine. In addition, the strength reduction rates of geopolymer are less than that of traditional OPC-based well cement materials. The lowest reduction in compressive strength of geopolymer is due to the increased resistance to alkali leaching of geopolymer in brine water compared to water. The compressive strength of sandstone saturated in brine water does not vary significantly compared to that of water, and this is due to the higher quartz content and lower NaCl concentration in brine water. Previous researchers have also noticed that higher quartz content sandstone is not sensitive to microstructural changes in brine water.
As a typical wellbore experiences a range of temperatures with injection depth, the mechanical behaviour of geopolymer and class G cement was studied at different curing temperatures (23-80 ºC). The optimum curing temperature of both geopolymer and class G cement leading to higher mechanical strength is between 55-60 ºC. Geopolymer shows higher compressive strength and Young’s modulus values at elevated temperatures (> 36 ºC), whereas G cement possesses higher values at lower curing temperatures (< 36 ºC). The failure of geopolymer is shear at lower temperatures, while splitting failure is observed at elevated temperatures. On the other hand, class G cement fails in shear manner regardless of the curing temperature. Therefore, geopolymers can be employed at deeper depths of the well, where the temperature is higher, while G cement is suitable at shallow depths. A numerical study was performed to predict the mechanical behaviour of geopolymer at different temperatures. First, the model was calibrated with the experimental results and then a parametric study was performed to predict the mechanical behaviour under higher confining pressures. The structural mechanics module of COMSOL multiphysics was used, and a confining pressure range of 5-25 MPa was applied to geopolymer at different curing temperatures (23-80 ºC). As the temperature is increased from 23-60 ºC, the resulting failure strength increases by 205-320 % at different confining pressures (5-25 MPa), and a reduction of 9-10 % was observed from 60 ºC to 80 ºC. Increase in confinement and temperature increases the ductility and mechanical integrity of geopolymers due to plastic flow.
When geopolymer is used as well cement in CCS wells, it is exposed to CO2 rich environment. Therefore, an experimental program was conducted to study the mechanical behaviour of geopolymer saturated in CO2. Based on the experimental findings, geopolymers show excellent mechanical integrity in CO2 as there was no sign of either strength reduction or cement degradation in geopolymer up to 6 months in CO2. In addition, geopolymer could not be fractured in hydraulic fracturing experiments, which employed different water injection pressures, confining pressure and tube lengths. Under any stress exposure conditions, the primary objective of well cement is to provide zonal isolation in the well. Since geopolymer could not be fractured under extreme stress exposures, it can provide good mechanical integrity and zonal isolation under severe loading conditions.
Permeability tests were conducted with the high pressure triaxial rig available in the laboratory to investigate the permeability of wellbore materials under different in-situ stress conditions. A set of drained experiments was performed on geopolymer samples, and it was noticed that the apparent CO2 permeability of geopolymers (2×10-21 m2 to 6×10-20 m2) is lower than that of traditional American Petroleum Industry (API) class well cements (10-20 to 10-11 m2). In addition, increases in injection and confining pressures reduce the permeability of geopolymers. Another set of undrained flow experiments was conducted to study the sub- and super-critical CO2 permeability of wellbore materials (geopolymer (G), sandstone (S) and G-S composite). The permeability of sandstone and G-S composite materials is approximately thousands of times higher than that of geopolymer. Both the permeability and percentage permeability reduction (per 1 MPa increase in downstream pressure) reduce significantly from sub-critical to super-critical CO2 pressure conditions, implying the importance of super-critical CO2 phase conditions for effective and leak-free underground storage. An attempt was made to model the CO2 flow through geopolymer under laboratory triaxial stress conditions. The CO2 flow through geopolymer could be modelled using COMSOL multiphysics. The permeability values obtained from the model were consistent with the experimental outcomes. In addition, CO2 pressure and concentration distributions in geopolymer were also studied under various injection and confining pressures.
The permeability of geopolymer depends on many factors, one being the mix composition of the geopolymer. Therefore, a series of undrained flow experiments was performed to study the effects of three different types of geopolymer on CO2 permeability, and existing class G cement was also tested for comparison purposes. Different types of geopolymers were made by adding 0, 8 and 15% of slag to fly ash (by mass). It was noted that the CO2 permeability values of geopolymers were approximately 100-1000 times lower than class G cement. In addition, the permeability of 15% slag added geopolymer was approximately 10 times lower than fly ash-based geopolymer, and this is explained by the reduction in pore diameter and increase in pore area of geopolymer with the addition of slag. Finally, the effect of temperature on the permeability of fly ash-based geopolymer was experimentally studied for a range of temperatures. The apparent CO2 permeability of geopolymer increases with increases in temperature from 23-70 ºC, however the permeability values (0.0004-0.04 μD) are well below the API recommended limits of 200 μD. An empirical formulation was developed to predict the permeability of geopolymer at different temperatures under various injection and confining pressure conditions.