Establishment of an assessment method for site-scale suitability of CO2 geological storage

ZHANG Wei

National Geological Library of China (Geoscience Documentation Centre), China Geological Survey, Beijing 100083, China.

Abstract: Geological storage of CO2 (known as geological sequestration) is increasingly seen as a viable strategy to reduce the release of greenhouse gases into the atmosphere. China has become one of the largest emitters of CO2 in the world. Therefore, alongside other emissions reductions measures, the deployment of geological storage projects to capture CO2 in China is essential. This paper focuses on the establishment of qualitative and quantitative assessment methods for site-scale suitability of CO2 geological storage in deep saline formation systems.This is based on numerical modelling prior to the development a specific geological storage project, providing a more accurate selection of preferential sites from a list of potential storage locations. However, the detailed design of specific geological storage projects was not considered.

Keywords: Carbon dioxide; Global warming; Geological sequestration; Suitability assessment;Numerical modelling

Introduction

The Energy Information Administration (EIA)of the US Department of Energy (DOE) pointed out that China is the world's largest emitter of CO2in 2011, accounting for 26.8% of the world's total CO2emissions (approximately 8.7 gig tons (Gt) of CO2) (Energy Information Administration (EIA),2013). The reduction of anthropogenic CO2emissions in China is therefore a very important and urgent issue.

Geological formations suitable for CO2storage should have three characteristics, including storability (capacity or containment), injectivity,and seal security (confinement) (Stefan Bachu,2008). Storability is the potential storage volume/mass of CO2in geological formations (a spatial concept) (Stefan Bachu, 2008). Injectivity means that the injection of CO2at sufficient injection rates does not pose any risk to surrounding geological media (eg. caprocks and potable shallow aquifers) and surface organisms such as humans, animals and plants (Michael A Celia and Jan M Nordbotten, 2009). Injectivity is a timedependent concept in that it is measured as a rate of flow (Stefan Bachu, 2008). Hence injectivity is extremely important to the amount of CO2which can be safely injected into geological formations(Holloway Sam, 2001). Seal security describes the capability of low-permeability caprocks to prevent not only the upward migration and leakage of the free-phase CO2gas, but also the movement of brines in geological formations which contain varying concentrations of dissolved CO2into shallow potable aquifers, the ground surface and also into the atmosphere (a quantitative concept).The movement of gas and brines in this way depends on criterion such as the minimum accepted storage efficiency of the target geological formation.

This paper focuses on the establishment of assessment methods for site-scale suitability for geological storage of CO2in deep saline formation systems, based on numerical modeling prior to the actual development of specific geological storage projects and detailed designs a specific geological storage projects were not considered. Rather the purpose of this paper is to establish assessment methods for CO2geological storage in a specific site through the combination of the numerical simulations. These were established to investigate the possible geochemical reactions and subsurface fluids (gaseous and dissolved CO2, and brines)flow in our studies and combined with geological site characterization methods investigated by other researchers, to provide a more accurate selection of preferential sites from a list of potential storage locations for safe, long-term and CO2geological sequestration.

1 Results and discussion

1.1 Geological site characterization

Site characterization is a substantial task in the selection process for suitable CO2storage, being considered the most time-consuming and costly part of the selection process for potential CO2storage sites (IEA Greenhouse Gas R&D Program(IEA GHG), 2008). In addition to previous geological data such as regional structure, stratigraphy,borehole and seismic activity, in order to provide sufficient geological information for the assessment of a specific site, new geological data should also be measured, collected, and implemented according to the parameters required by the numerical modeling estimation (Stefan Bachu,2002; Christine Doughty et al. 2008; Gibson-Poole et al. 2008; Cutis M Oldenburg, 2008; Grataloup et al. 2009; Members of the CO2Capture Project,2009; Yang et al. 2010).

1.2 Qualitative estimation based on numerical modelling results

Our numerical modeling studies demonstrate the relation between storability, injectivity, and seal security in storage location selection (Fig. 1),which can be qualitatively used as an assessment method for site-scale suitability of CO2geological storage in deep saline formation systems. In our present assessment method, we mainly consider changes in pressure buildup during the injection period (injectivity), and leakage of the free-phase CO2gas and carbolic acid brines into the target geological formation (seal security). For storability,we divide this concept into short-term storability and long-term storability, the former being dependent on storage efficiency (see Fig. 1) during the injection period, and the latter dependent on storage security (see Fig. 1) during the postinjection period. The effect of geo-mechanical problems on the seal security is not currently considered.

Fig. 1 Relations between storability, injectivity and seal security as an assessment method for site-scale suitability of CO2 geological storage in deep saline formation systems

1.3 Quantitative estimation based on qualitative estimation

Stefan Bachu (2003) proposed an assessment method for obtaining quantitative evaluations in terms of a basin's suitability for the geological storage of CO2. Bachu's method is considered as a set of 15 criteria based on a series of parametric normalization procedures, from which a single total score for each basin can be calculated. The total scores for these basins can then be compared and ranked to determine the most suitable basin or region for CO2storage. We note that the criteria(potential influence factors for a given basin),classes (distribution of scores based on different functions (eg. linear, geometric, and exponential)for a given criterion) and weights (the relative importance of various criteria) can be changed according to the specific conditions and research process.

Curtis M Oldenburg (2008) developed a spreadsheet-based screening and ranking framework (SRF) for evaluating multiple sites on the basis of their potential for health, safety and environmental (HSE) risk due to CO2leakage and seepage. The qualitative and independent assessment for HSE risk is performed through three basic characteristics of a CO2geological storage site as follows: (1) potential of the target formation for long-term containment of CO2; (2) potential for secondary containment if the primary target site leaks; and (3) potential of the site to attenuate and/or disperse leaking of CO2if the primary formation leaks and secondary containment fails.Through user input of property values, the scores for each characteristic (eg. primary containment,secondary containment, and attenuation potential)of possible storage locations can be calculated,thereby allowing for relative ranking and consequent selection of most promising storage sites.

Based on previous assessment methods and concepts (eg. Stefan Bachu, 2003; Curtis M Oldenburg, 2008), we established a quantitative evaluation method for CO2geological storage(injectivity, seal security and storability) in a specific location.

Taking the estimation of the suitability of injectivity for a potential storage site for example(for detail modeling results on the injectivity), for each criterion i in Table 1, a value Fiis assigned,which can determine the importance of a specific storage site k in the potential storage sites for the criterion. For the criterion in the positive parameters listed in Table 1, the lowest (min (Fik))and highest (max (Fik)) values characterize the worst and best suitable injectivity for the criterion i.However, for the criterion in the negative parameters, the lowest (min (Fik)) and highest(max (Fik)) values characterize the best and worst suitable injectivity for the criterion i. Because the value Fihas different ranges for each criterion,making comparisons and manipulations difficult,the Fikfor each potential storage site k is normalized according to:

For the criterion in the positive parameters, Pi= 0 for the least favorable value (i. e. Fik= min(Fi))and Pi= 1 (i. e. Fik= max(Fi)) for the most favorable value for all the criteria. For the criterion in the negative parameters, in order to make the meaning of its normalized value consistent with that for the positive parameters, we will calculate the Pikfor the negative parameters based on Eq. (2),given by:

Note that the parameterization and normalization can transform various influence factors into dimensionless variables that vary between 0 and 1. And if max (Fik) is equal to min(Fik), the Pikfor all sites is equal to 1. The individual normalized values can be added to produce a general score Rk(Ikfor the injectivity, Skfor the seal security, and Ckfor the storability(capacity or containment)) as follows:

Where wiare weighting factors, which are normalized by:

The weights wiassigned in this study to different criteria are shown in Table 1. Note that the criteria, values, normalization functions and weights can be changed according to the specific conditions and research process. The general scores represent the relative ranking of the potential storage sites without indicating their absolute performance (eg. physical and chemical behaviors) for different characteristics such as injectivity, seal security, and storability.

Table 1 Values and weights assigned to the criteria for assessing and ranking potential CO2 storage sitesin terms of their suitability in injectivity

1.4 Quantitative estimation based on specific numerical modelling

The Certification Framework (CF) method proposed by Curtis M Oldenburg(2009) is a standardized way for project proponents, regulators, and the public to analyze, understand and assess risks and uncertainties of CO2geological storage in a simple, transparent and accepted way,.This is based on the concept of effective trapping,which allows for potential and possible leakage of CO2and/or brines below agreed-upon thresholds.This method considered the occurrence of conduits such as wells and faults, impacts occurring to compartments which are a vulnerable entity or a collection of vulnerable entities (eg. potable groundwater aquifer or aquifers) (Curtis M Oldenburg, 2009), the likelihood of impact and risk, and plume migration.

We proposed a quantitative estimation method based on specific numerical modeling, which is classified into four groups including the short-term(injection period) storability, long-term(post-injection period) storability, injectivity(injection period), and seal security (injection period), for CO2geological storage locations.These simulations are also useful for the injection design such as the injection pattern, rate and time.Fig. 2 shows the work flow of this modeling estimation approach.

During the present numerical modeling studies,a series of most perfect and realistic modeling should be included as follows:

(1) A complex 3D geological model that can depict in detail the geological structure and physical characterizations of the specific storage system was required;

(2) A wide range of subsurface thermomechanical-hydraulic-chemical (TMHC) process such as heat transfer, variations in stresses,multiphase fluids flow, and mineral alteration needed to be considered, in order to quantify the storability, injectivity and seal security during long-term CO2geological storage process.

However, the increasing complexity of the geological model coinciding with greater consideration of CO2geological storage process and mechanisms, leads to more challenges in the coupling and improvement of TMHC modeling capability, and the enhancement of computational capability.

2 Conclusions and suggestions

As mentioned previously, the numerical modeling method in our studies points to the needs for consideration of a wide range of subsurface TMHC processes in order to arrive at a holistic site-scale estimation method. However, more challenges are required to perform, such as the coupling and improvement of TMHC modeling capability, and the enhancement of computational capability. Suggestions for future improvements are as follows:

(1) Consideration of heterogeneity in geological media. In real conditions, the potential geological formations that can be used to store CO2should have different ranges of heterogeneous porous media such as continuous very highpermeability sands, intermingled sands and shales with a large high-permeability sand component,and predominantly low-permeability discontinuous shale lenses interspersed with moderatepermeability sand (Doughty et al. 2004).

Fig.2Flow chart of quantitative estimation based on specific numerical modelling

(2) Implementation of geomechanical modeling.Geomechancial modeling requires investigating and identifying the pressure and stress gradients induced by the large injection rate and volume(mass) of CO2, which could reactivate existing fractures and faults, drive new fractures through the caprock, and cause ground surface heave due to a reduction in effective stress in the geological media (Jonny Rutqvist and Chin-Fu Tsang, 2002;Morris et al. 2009; Jonny Rutqvist and Chin-Fu Tsang, 2010). The maximum pressure that will not lead to unwanted potential leakages and damaging effects is called the, aximum sustainable injection pressure (Birkholzer J et al. 2007). Using a combination of leakage and geomechanical modeling a figure for the maximum sustainable injection pressure can be obtained andused for the determination of maximum injection mass and/or rate (Birkholzer J et al. 2007). The sustainable pressure buildup should be assessed based on a specific storage site, considering initial stress fields and geomechanical properties of the rock units at the selected sites (Zhou Quan-lin et al. 2008).

(3) Investigation of leakage possibility of CO2gas through direct high-permeability conduits between the deep saline formations and the shallow aquifers. The existence of high-permeable pathways such as faults, fractures and abandoned wells, and the opening of pre-existing fractures,rock fracturing and fault activation caused by the CO2injection, might accelerate and enhance the migration of CO2gas, dissolved CO2and brines into shallow aquifers and even to ground surface.During the selection of preferential CO2geological storage sites, the sites which have significant leakage pathways should be eliminated from the potential storage sites through previous screening.In addition, geomechanical modeling in which a maximum sustainable injection pressure will be assessed and determined is essential to mitigating the possibility of damage to the intact caprock. It is recommended that future research is conducted to better understand the THMC changes, and self-enhancing and self-limiting actions during the rapid-leakage process.

(4) Investigation of near-surface environments.The purpose of our investigation is to establish a site-scale evaluation method for CO2geological storage in deep saline formation systems (formations and caprocks at the deeper depth). Effects of the injection of CO2gas and the leakage of gas,dissolved CO2, and brines on the near-surface environment such as the pollution (water quality)of shallow potable aquifers (an environmental problem) and changes (eg. water table, and discharge and recharge zones) of regional groundwater systems (a hydrogeological problem), should be assessed in the future.

Acknowledgements

This work was supported by China Geological Survey Project (Grant No. 12120114049401).