A novel approach to precise evaluation of carbon dioxide flow behaviour in siltstone under tri-axial drained conditions
Introduction
As the greatest contributor, CO2 accounts for about 64% of the total greenhouse effect and therefore has had a considerable influence on global warming, particularly since the Industrial Revolution (Bryant, 1997, Xie et al., 2015). Geological storage of CO2 is considered to be an efficient solution to the reduction of anthropogenic CO2 emissions and has been widely tested in the field (Riddiford et al., 2003, Fischer et al., 2013, Ren et al., 2015a). According to current practices, the main potential geological media for sub-surface storage of CO2 are depleted oil and gas reservoirs, deep saline aquifers and unmineable coal beds (Bachu, 2000, Baines and Worden, 2004). The potential to enhance oil and gas recovery from deep unconventional geological formations through CO2 sequestration is an appealing feature of the process (Olsen, 2011, Perera et al., 2012b, Perera et al., 2013, Perera et al., 2015), and an incremental oil recovery of 10% was achieved after CO2 injection in the Jilin tight oilfield (Ren et al., 2015b, Ren et al., 2016). The latest trend in using CO2 in deep geological formations in gas/oil recovery is its use as a fracturing fluid to create hydraulic factures in the formation to enhance production (Conti et al., 2014, He et al., 2014, Middleton et al., 2015, Wanniarachchi et al., 2015, Yang et al., 2015).
The safe storage of the injected CO2 in any potential geological sink is greatly dependent on the CO2’s upward migration through the caprock or low permeable rock layers lying above the formation (Li et al., 2006). In production-enhancement practices such as hydro-fracturing, the risk of CO2 leakage is mainly associated with the permeability characteristics of the reservoir rock (Detournay and Cheng, 1992, Gupta and Bobier, 1998, Ishida et al., 2004, Fjar et al., 2008, Ishida et al., 2012). Reservoir permeability determines the productivity of any unconventional reservoir. In the light of all of these facts, the important role of the permeability of reservoir rock/caprock in deep earth CO2 injection applications is clear.
Since siltstone is a common type of rock in underground reservoirs as shales and their cap-rocks (Clarkson et al., 2012), a precise understanding of CO2 permeability behaviour in such a common rock type is certainly important for both geological CO2 storage and CO2-enhanced unconventional gas/oil recovery-enhancement. However, the complicated thermodynamic properties of CO2 have become a major challenge (Perera et al., 2011a), as they cause the physical properties of CO2 (e.g. density, compressibility) to be highly pressure/temperature-dependent. This is mainly caused by the easy phase transition of CO2 from gaseous, liquid and supercritical states with changing pressure and temperature conditions. This even causes different phases of CO2 to exist from the ground surface to the reservoir rock with changing pressure and temperature conditions with geological depth (Pruess, 2011). In addition, the opposite phase transitions can be expected during any CO2 flow back or leakage into the atmosphere from geological formations. This indicates the importance of identifying the permeability characteristics of CO2 under its various phase conditions in different reservoir rocks.
The existing experimental studies on CO2 permeability have been performed using both steady-state and unsteady-state approaches. Of these, the steady-state condition is achieved by opening the downstream to the atmosphere while offering atmospheric downstream pressure conditions. However, in such situations super-critical or liquid CO2 phase conditions cannot be secured downstream and there is therefore phase variation of CO2 throughout the sample (Perera et al., 2011a). Such issues can be overcome using the unsteady-state approach, in which the downstream is closed, allowing the opportunity for downstream pressure development (Perera et al., 2012a). The calculation of the rock mass permeability in unsteady-state testing is much more complicated than in steady-state testing, and requires many assumptions to be made, adding uncertainly to the permeability calculation (Carles et al., 2007). Compared to the unsteady-state approach, the steady-state approach offers a more reliable pathway for reservoir rock mass permeability calculations (Boulin et al., 2010). However, steady-state permeability calculations may have considerable errors if the possible CO2 phase transition through rock mass is not considered (Ranathunga et al., 2015).
This study therefore proposes a more reliable approach to the calculation of reservoir rock’s apparent permeability for CO2, following the conduct of a comprehensive set of permeability experiments on siltstone incorporating the CO2 phase transition between liquid and gaseous phases. Dry siltstone samples were tested for permeability using a steady-state tri-axial permeability approach at room temperature. Scanning electron microscopy (SEM) analyses were also carried out to clarify the relationship between chemical constitution and pore path channel for CO2 movement in the rock mass, and the X-ray powder diffraction (XRD) analytical technique was used to quantify the crystalline material content of the siltstone samples.
Section snippets
Sample description
Siltstone samples were collected from the Eidsvold formation, Queensland, Australia. This formation is a mixture of sandstone, siltstone and mudstone and formed in three distinct periods (Triassic, Jurassic and Cretaceous) during the Mesozoic era. The total porosity of the dry siltstone measured using the mercury intrusion method was around 19.45% and bulk and skeletal densities were around 2.24 g/ml and 2.78 g/ml, respectively. This siltstone has around 100 nm average pore diameter, and the
Comparison of two different permeability calculation methods
In all the tri-axial experiments, downstream CO2 flow rate reached a steady-state condition within around 10 min and the steady-state flow rates for the tested conditions are shown in Fig. 9. This figure confirms the linear variation between downstream steady-state flow rate and injection pressure. Since this linear relation confirms the existing laminar flow behaviour of CO2 in the sample under the tested condition, Darcy’s equation can be used to calculate CO2 permeability in dry siltstone
Conclusion
A novel, more precise permeability calculation is proposed in this study for use in CO2 drained experiments in siltstone, taking into consideration the phase transition that occurs in CO2 between the upstream and downstream of the sample. Based on the results, the following conclusions can be drawn:
- 1)
The use of the traditional technique (the Darcy equation using downstream flow rate) for CO2 permeability calculation in low permeable rock masses like siltstone in drained permeability tests creates
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