Interaction analysis for CO2 geological storage and underground coal mining in Ordos Basin, China
Introduction
The Ordos basin is widely known as the “coal sea” in northern China. Coal-related industries dominate the local economic structure and provide the foundation for the local economy of the basin (Chen et al., 2013, Wu, 2013). The coal is primarily used for electricity generation or the coal chemical industry, both producing massive CO2 emissions (OECD/IEA., 2013, Vishal et al., 2013a). To mitigate the environmental impact of the emission, huge amounts of anthropogenic CO2 must be reduced, and CO2 geological utilization and/or storage is one possible solution (IPCC, 2014, Xie et al., 2013, Li et al., 2015).
The CO2 storage mode can be divided into the following four categories according to variations in geological sites and the morphology of the sequestration: 1) Geological storage, which injects CO2 into deep geological bodies, such as the saline water layer, depleted oil and gas reservoirs and unminable coal seams (Ranjith et al., 2012); 2) marine storage, which traps the injected CO2 by the pressure of the sea water and various biochemical effects (Li et al., 2009b); 3) mineral carbonation sequestration, in which carbon dioxide reacts with alkaline minerals to generate carbonate minerals (Xu et al., 2004, Pang et al., 2012); and 4) direct utilization, in which carbon dioxide can be directly used in the industry, such as for CO2 enhanced oil recovery and CO2 enhanced geothermal system (ACCA21, 2014, Xu et al., 2015, Yang et al., 2014, Yang et al., 2012). Only the first three categories are suitable for large-scale commercial exploitation, while the fourth category is not a long-term sequestration method because the amount of carbon dioxide that can be processed is minimal. Since the early 1990s, a series of important investigations by IPCC (Metz et al., 2005) have confirmed three geological formations that are suitable for the sequestration of carbon dioxide: deep saline formations, depleted oil and gas fields and coal seams. Of these three formations, the storage in deep saline aquifers is the most promising for large industrial-scale CO2 geological storage (IPCC, 2014). Suitable saline aquifers for CO2 sequestration exist in the Ordos Basin but with widely distributed superior quality coal overlaying the formation (Li et al., 2013). The formation is the target for CO2 geological storage, but it is expected that the coal will continue to be mined even after the CO2 sequestration (Li et al., 2014b).
In coal mining, the strata above and below the coal seam gradually lose the support with the progress of mining. Because the original state of equilibrium is disturbed, roof caving and/or floor heaving may occur, and both phenomena will cause stress re-distributions in the affected strata. Because the coal seam in the basin is primarily horizontal, the mining activity will inevitably affect the stress distributions and stability in the caprock of the CO2 storage when the mined-out area reaches a significant size (Vishal et al., 2013b). This effect will increase the risk of leakage of the sequestered CO2 (Liu et al., 2014). However, a large-scale CO2 injection lasts for a long time and will cause changes of stress and deformation in the rock formations above the reservoir (Wang et al., 2015), which in turn may also affect the mining activity. It is therefore important to understand how the state of rocks changes under the combination of coal mining and CO2 storage and how the safety issues are affected under the combined activity, which remains a very challenging research topic (Fei, 2014). In other words, the combined effect of CO2 sequestration in saline aquifers and coal mining in the overlaying strata is a problem worthy of detailed investigation.
Laboratory study is of limited value in this case because the problem involves highly complicated couplings among hydrogeology, geochemistry, thermodynamics, and rock mechanics. Studies using direct engineering implementation are not possible either because of significant difficulties and budget constraints. Numerical simulations, however, can provide a tool to address the problem. However, there is no single software package that can be used to solve this complex coupled problem. In this paper, we report a tailored thermo-hydro-mechano-chemical (THMC) coupling platform that is customized to handle the problem named “AEEA Coupler”, which was developed by linking Simulia ABAQUS (stress analysis) and Schlumberger ECLIPSE (reservoir simulation) using Python 2.7 (Fei, 2014, Fei et al., 2014). The applicability and accuracy of the developed coupler were tested by benchmark studies (Fei, 2014), and they will be reported in the coming international publication.
The simulations are conducted based on the formation data of Ordos Basin and information of Shenhua 0.1 Mtpa and 1.0 Mtpa CCS demonstration projects (Kuang et al., 2014). This paper presents the analyses addressing the following five areas:(1) Changes in the reservoir pore pressure and displacement; (2) Analysis of the boundary coal pillar stabilities; (3) Stress and displacement analysis of the overlapped region between the coal mining and CO2 injection activities; (4) Range of mining coal floor analysis; and (5) Surface subsidence. This paper also explores the countermeasures to be investigated that use CO2 sequestration and coal mining projects at the same venue.
Section snippets
Coupled mechanism of the AEEA coupler
In the AEEA Coupler, each software package runs separately but exchanges data at every simulation step (Li et al., 2006). The coupled equations that are used consider the change in the effective stress within the rock and its effects on rock porosity and permeability. The effects of fluid pressure and temperature on the deformation of rocks are also considered in the coupling process (Alonso et al., 2012, Fall et al., 2014, Li et al., 2009a, Rutqvist et al., 2010). As shown in Fig. 1, first, a
Coupled analysis for coal mining and CO2 geological storage
An analysis of the lithologic features (Fig. 3) shows that the mudstone in the Liujiagou group has good physical properties for sealing (Li et al., 2013, Liu and Li, 2014) and therefore this group is adopted as the reservoir-caprock system for this study.
In this study, simulations are conducted for the duration of the three-year injection with the injection rates of 0.1 Mt/year and 1 Mt/year. The mining excavation is assumed to advance at a speed of 15 m/day. During the simulation of excavation,
Variation in the reservoir
According to the results of our previous work (Fei et al., 2014, Li et al., 2014b), the stress in the casing reaches its designed capacity when coal mining occurs within 90 m of the wellbore. Fig. 7 shows the pore pressures at the top of reservoir for every year during the CO2 injection and when the casing reaches its designed strength. The pore pressures at the injection well increase in the first 3 years, and the values are 21.97 MPa, 22.36 MPa and 22.61 MPa in the 1st year, the 2nd year and the
Conclusions and suggestions
Based on the characters of stratigraphic data in the Ordos Basin and Shenhua CCS demonstration project, with a CO2 injection rate of 0.1 Mtpa, a coupled multiphysical analysis with AEEA Coupler was conducted to investigate the interaction of CO2 storage in a deep saline aquifer and overlying coal exploitation. Considering the prospects for the future commercialization of CCS, the multiphysical analysis was completed with injection rates of 0.1 Mtpa and 1.0 Mtpa. The results are listed as follows:
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Acknowledgments
WBF thanks the China Clean Development Mechanism (CDM) Fund (Grant No. 2012087) for support. QL thanks the NSFC (Grant No. 41274111) and the Hundred Talent Program of the Chinese Academy of Sciences (Grant No. O931061C01). The Shengli Oilfield Company is also acknowledged. We thank Dr. Chaoshui Xu (University of Adelaide) for providing helpful suggestions for the manuscript. The interaction proposed in this manuscript was first presented in the ARMS8 (the 8th Asian Rock Mechanics Symposium),
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