Feasibility of Monitoring Techniques for Substances Mobilised by CO2 Storage in Geological Formations

Energy Procedia 23 ( 2012 ) 439 – 448

1876-6102 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SINTEF Energi ASdoi: 10.1016/j.egypro.2012.06.039

TCCS-6

Feasibility of monitoring techniques for substances mobilised by CO2 storage in geological formations

Linda Stalkera,b*, Ryan Nobleb, Bobby Pejcicb, Matthew Leybournec, Allison Hortlea,b, Karsten Michaela,b Tim Dixond and Ludmilla Basava-Reddid

aCO2CRC, Ground Floor, NFF House, 14-16 Brisbane Ave., Barton, ACT 2600, Australia. bCSIRO Earth Science and Resource Engineering, PO Box 1130, Bentley, WA, 6102, Australia.

cGNS, 1 Fairway Drive, Avalon 5010, New Zealand. dIEA GHG, The Orchard Business Centre, Cheltenham, Glos, UK, GL52 7RZ.

Abstract

When a large volume of CO2 is injected into a geological formation this can lead to the mobilisation of substances of a chemical and physical nature. The purpose of this IEA GHG study[1] was to identify typical substances that could be mobilised during geosequestration and to evaluate potential tools for monitoring these substances. This project reviewed the scientific literature patent applications and industry publications relevant to the current monitoring of chemical and physical processes due to CO2-formation water-rock interactions from the deep subsurface through to the soil/water interface. Four major areas were identified for review: 1– physical effects, including pressure effects or displacement of fluids; 2 – geochemical effects, including dissolution of reservoir and seal rocks, as well as the potential for mobilisation of heavy metals; 3 – shallow/surface effects, potential nutrients/toxic compounds affecting soils and microbial communities, as well as groundwater quality; 4 –capture contaminant effects from coal fired plants and other point source emitters. The various processes have different degrees of impact in the three general monitoring domains: a) injection horizon (depleted hydrocarbon reservoir, saline formation), b) above-zone interval (zone directly overlying the seal of the storage interval), and c) shallow subsurface (potable groundwater aquifers, soil). Understanding these processes and mapping their distribution aids in the identification of potential monitoring tools and facilitates an assessment of their utility in a particular monitoring domain.

Some tools already commonly deployed in other industries are highly applicable to the carbon storage industry; for example, downhole pressure gauges from the oil industry and water level loggers from the groundwater industry. In general, geophysical tools were found to be quite a mature method for identifying the presence of gas (hydrocarbons or CO2), but less so for observing mobilised substances and changes in salinity. Tools for measuring trace amounts of hydrocarbons in marine settings are able to be modified in order to be used for monitoring mobilised hydrocarbons entrained in capture emissions, from CO2/source rock interactions or ienhanced oil recovery processes, though many of the tools are not compound specific as yet. The aim of the project is to provide an understanding of the availability of conventional and novel tools for monitoring and verification (M&V) during CO2 injection. Some of these tools

* Corresponding author. Tel.: +61-8-6436-8909; fax: +61-8-6436-8555. E-mail address: [email protected].

Available online at www.sciencedirect.com

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have been successfully employed in current carbon capture and storage (CCS) projects or in alternative applications, such as mineral exploration and ecological studies.

© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer]

Keywords: carbon storage; tools; sensors; geophysics; geochemistry; hydrocarbons; biological.

1. Introduction

This study has been conducted to identify suitable analytes and monitoring tools for substances mobilised by CO2 as proxy indicators of CO2 movement or release (IEA GHG Report)[1]. These indicators were divided into four main categories; (1) physical effects (e.g. changes in pressure, temperature, density, flow); (2) geochemical effects (e.g. diagenetic or dissolution effects); (3) shallow/surface effects (e.g. changes to microbial or ecological communities), and (4) capture gas contaminants that would be entrained in the CO2 plume but could act as chemical tracers.

These categories allowed the identification of typical “analytes” that could be monitored, and necessarily included consideration of the anticipated concentration levels of each of these (Table 1a). The dynamic range of the analyte concentrations can limit the deployment of certain tools where ranges might exceed the measurement capacity of the tool, for example, CO2 concentrations (Table 1a). Furthermore, the conditions of deployment, for example temperature, had to be characterised in order for the tools to be suitably evaluated as fit for purpose in a range of likely monitoring and verification (M&V) settings (Table 1b).

Table 1 (a) Analytes and detection levels of substances that could be mobilized by CO2 and (b) the conditions, limitations and ranges of deployment of such tools.

(a) Analytes Levels (b) Considerations Approximate Ranges CO2 ppb to percent Depth Soil surface to +3km depth pH Relative change Temperature 4°C to ~ 150°C Hydrocarbons ppb to percent Aqueous environment YesAnions and cations mMol Power 240v maximum Tracers or contaminants ppb to percent Data transmission Wire or wireless Pressure/temperature kPa/°C Lifetime Short to long-term Geophysical properties Varies with methods employed Self-calibration Drift rates Biological properties Varies with methods employed Redundant/robust Environmental challenges Relative cost Indicative costs

The sedimentary succession between the injection point and the ground surface can be subdivided into three monitoring domains (Fig. 1): a) injection horizon (saline formation or depleted hydrocarbon reservoir), b) above-zone monitoring interval (directly overlying the reservoir seal), overlapping with c) shallow subsurface zone (potable groundwater aquifers, soil zone). The injection interval is likely to be at high pressure and temperature and there may be significant concentrations of hydrocarbons, either naturally occurring or from the injection stream. These conditions are often detrimental to the longevity and stability of many monitoring tools and a monitoring strategy would minimise the number of wells penetrating this horizon. The zone immediately above the injection horizon would likely have lower pressure and temperature relative to the injection horizon and retrievable monitoring tools are an option. Due to the relatively short distance to the injection interval, there would be a high likelihood to detect the effects of any formation water or CO2 leaving the storage container (such as pressure or geochemical

© 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SINTEF Energi AS

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changes. Those monitoring zones in the shallow sub-surface include monitoring groundwater in open bores and soil sampling. In the shallow subsurface environment, temperature and pressure are much less extreme and there exists a wide range of monitoring tools in groundwater wells and for soil sampling. However, the likelihood of detectable impacts of CO2 storage reaching these environments is generally low and monitoring in these environments may be perceived to be primarily about assurance monitoring.

Figure 1. Schematic of different monitoring domains and applicable monitoring tools for CO2 storage sites.

The project builds on an earlier study carried out for the CO2CRC[2] (Cooperative Research Centre for Greenhouse Gas Technologies) which reviewed the available sensor technologies for the detection and quantification of CO2 and some proxy indicators. That report concluded that optical sensors were the most likely technology to be employed for direct measurement of CO2. However, at the time of writing, it was acknowledged that many of the sensors and monitoring tools that performed well in laboratory testing were likely to have limited capacity to cope with depth, temperature and the aggressive environments within which these tools would subsequently be deployed. It is possible that many tools could be modified (i.e. miniaturised, ruggedised) to become low cost, use low amounts of power and be sufficiently sensitive for CCS applications. This project revisits many of these technologies in more detail in order to determine whether sufficient advances have been made to aid deployment of such tools to provide low cost, broad coverage monitoring in the future.

The scope of the previous study has now been expanded, but the approach remains similar. Substances likely to be mobilised have been identified, followed by an evaluation of the tools that might be used in monitoring them in a CCS site. Literature reviews (ISI Web of Science, ISI Derwent Innovations Index [patent search] and other standard search engines) and case study evaluations were effective sources of information. The case studies used were; (1) Pembina, Canada, (2) Ketzin, Germany, (3) Cranfield, USA, (4) Frio, USA and (5) CO2CRC Otway Project, Australia. These case studies gave a good overview of the suite of typical M&V tools currently applied to monitoring CO2.

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2. Substances Mobilised and Suitable Tools

2.1. Flow and Physical Effects

Injection of a large volume of CO2 into a storage reservoir will produce a pressure pulse that propagates to a far larger footprint than the CO2 plume itself[3]. Understanding the in-situ pressure has application to almost all aspects of CO2 storage; from site characterisation to capacity estimates and monitoring and verification. The IEAGHG Monitoring Selection Tool strongly recommends monitoring pressure as part of any CCS project. All of the case studies examined monitor the pressure in the storage reservoir and some also monitor overlying permeable zones [i.e. CO2CRC Otway Project, Australia[4] and the Southeast Regional Carbon Sequestration Partnership (SECARB) CO2 EOR project in Cranfield, USA[5]].

Monitoring pressure is standard practice and a mature technology in both the water resource and oilfield industries. Both industries have developed tools which can be purchased off-the-shelf, are sensitive and robust and can remain in situ for many years. In addition, these systems can be very cost-effective, depending on the level of sophistication required. There are also other physical effects that can be measured such as changes in conductivity, thermal perturbations, density, acoustic properties, electrokinetic potential and flow rates. Fibre optic technologies have the potential to provide sensitive information about the acoustic, flow and temperature properties. However, one of the limitations of all of these monitoring systems is the capacity to produce large volumes of data which can be complex to handle, require sophisticated geological models to interpret, and considerable computing and manpower to process[5].

Evidence of these effects may be monitored for in shallow groundwater systems, permeable zones above the storage reservoir and in some cases, the storage reservoir itself. The significant advantage in these types of tools and systems is their ability to detect changes due to the presence of CO2 some distance away from the CO2 itself. This increases the percentage of reservoir "covered" by the monitoring system, increasing confidence in the containment and allowing for the possibility of early detection and remediation.

Many of these types of tools are routinely used within the oilfield industry for reservoir characterisation, and several of these are being developed as research tools to detect near wellbore changes due to the presence of CO2. For example, a fibre-optic distributed thermal perturbation sensing (DTPS) system was installed at the Ketzin pilot CO2 storage site in Germany. The results from Ketzin found that that CO2 was detectible by measuring changes in thermal conductivity and their modelling indicated that CO2 would be detected as a temperature anomaly at a monitoring well 50m away from the injection well[6]. However, the long-term usefulness of these tools for monitoring is unclear. It is likely that modifications relating to longevity and stability at elevated temperatures and pressures would be required, in addition to cost reduction. In addition, most of these are deployed at the reservoir level; this requires having a well penetrating the reservoir in which the CO2 is stored. Although the wellbore integrity may be maintained through a permanent installation, it does increase the risks associated with long term CO2 storage.

The change in pH associated with the introduction of dissolved CO2 into formation water would make pH seem a likely candidate for monitoring. However, generally, pH sensors are not robust enough for long-term downhole installation. They are generally sensitive and accurate, but are prone to drift and require frequent recalibration. Solid-state pH sensors are considered to have better potential, and can be designed to operate in the range 4-150°C and at depths of 800-1300m[7]. However, there are still problems

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with stability and poor-performance at varied temperatures[8]. Although there does not appear to be any suitable pH sensor for CCS applications at this time, this is an ongoing field of research.

2.2. Geophysical Effects

There are a wide range of geophysical monitoring tools, many of which have been evaluated previously for monitoring CO2 presence, anisotropy in rock properties or effective stresses from fluid pressure changes[9]. Geophysical techniques, especially 4D-seismic, have been employed at a number of CCS sites for the monitoring of CO2 plumes. However, there is less activity in the geophysical measurement of substances other than CO2 (e.g., pressure, permeability, conductivity, porosity or temperature). These types of geophysical measurements are more typically associated with shallower surveys with applied examples associated with agriculture or the minerals exploration industry[10, 11]. Thus, aside from a review of the more conventional geophysical techniques, other alternatives that have been less readily employed in monitoring CCS sites were evaluated. These include tools such as magnetic resonance sounding (MRS), ground penetrating radar (GPR), various magnetic (airborne, ground and gradiometry) and magnetotelleurics. Some of these tools look at changes in particle size, porosity, changes in salinity or water content of soils and formations and may be a way to measure substances mobilised by CO2 but can also in some instances measure CO2 as well. MRS has a limited penetration distance and can be used to look either at the shallow subsurface (45-175m) or deployed in a bore hole to look at free versus bound water or saline water intrusion[12] as well as other applications [1]. GPR can be applied in a similar manner and has been tested at natural analogue sites[13]. Magnetotellurics in the form of controlled source magnetotellurics (CSMT) has been employed successfully at the Ketzin pilot CO2 site[14].

Depth or penetration distance to target is, and will continue to be a challenge for the deployment of most geophysical tools, but they can be applied to relatively shallower reservoir intervals, above reservoir or groundwater zones for monitoring. Costs related to 4-D seismic surveys might be substituted by some of the other monitoring techniques, especially if they can be permanently installed, such as passive microseismic arrays, currently in use at the CO2CRC Otway Project.

2.3. Geochemical Effects

For this study, geochemical effects were sub-divided into the measurement of anions, cations and pH. In general, the potential suite of elements and compounds that are to be found in formation fluids and minerals and required to be measured are reasonably well understood from the mineral diagenesis and shallow groundwater fields (Table 2). However, added to these are flue gas or process gas contaminants.

The chemistry of formation fluids sampled from the deep subsurface can be compromised by changes in pressure and temperature as the fluids are brought to the surface. As a result, these samples may not be entirely representative of the chemistry in the subsurface. However, samples taken at the wellhead (or via other sampling tools) are cheaper and easier to acquire than the more pristine samples from the U-tube subsurface measuring tool[15] that retain the pressure of the formation fluid to surface. Furthermore, many of these tools are costly to deploy and operate, and tend to be used exclusively in demonstration sites.

Down-hole monitoring of deep groundwater or storage reservoirs has a number of key requirements for this form of monitoring to be feasible. Factors (common to all downhole tools) such as the ability to withstand elevated temperatures and pressures, long-term stability, ease or lack of calibration, small size for emplacement in a well, low detection limits and analyte specificity are important. But only a small

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number of geochemical analytical techniques are capable of more than a few of these features. Probably the analytical technique with the most promise for remote in situ monitoring of changes in alkali, alkaline earth, transition metal and metalloid concentration are ion selective electrodes (ISE). ISEs are sensors that measure the activity of a specific ion that is present in a solution and presents that data as an electrical potential. Some of the major disadvantages of ISE include interferences from competing ions, longevity (currently only months) and the ability to be deployed in deep, high pressure and temperature environments. In the longer term, deployment of appropriate tools such as ISEs down hole could be one way to reduce or remove the sampling costs from the M&V chain by receiving regular data transmissions from the subsurface rather than transferring fluid samples to a laboratory for analysis.

Table 2. Examples of possible geochemical analytes that might be mobilised and monitored for. (a) cations, (b) anions or (c)

hydrocarbons. Cation Anion HydrocarbonAlkali & alkaline earths Chloride Methane Iron Sulfide/sulfate Benzene, Ethylbenzene Lead Fluoride Toluene Copper Nitrate Naphthalene Mercury Anthracene REE (rare earth elements) Phenanthrene

In the case of anion sensors, ISEs often use polymer receptors or membranes that could be compromised at greater pressures and temperatures, and while these tools may have great sensitivity and selectivity, they are, like the cation ISEs prone to drift and require repeated calibration. Cation sensors also suffer from the additional issues of matrix matching, need for calibration with specialised fluids or internal reference materials and limited dynamic range characteristics. Fortunately there is rapid development in miniaturisation and movement towards increased lifespan of tools using “lab-on-chip” designs or solid state devices[16].

There are many possible analytes in the formation waters (Table 2) that could be mobilised and may be site/formation specific. In addition, it has to be decided whether it is more important to measure relative or absolute changes in ion concentrations?

2.4. Biological Impacts

So far there has been limited testing in the CCS domain of tools for biological monitoring. These are approaches used in ecological studies and, for example, in bio-prospecting for mineral exploration[17]. Biological monitoring is in its infancy in terms of development, uptake and application in the CCS M&V domain. For example, of the 270 papers presented as orals at the 2010 Greenhouse Gas Technologies Conference, approximately 2% investigated biological monitoring. However, there are a variety of tools that can measure biological/microbiological changes in soil communities that have been applied to situations other than CCS (Fig. 2) but which might have application to M&V. The tools can be specifically designed to look at the changes in vegetation, ecology or microbiology. Many of these tools have been designed to observe changes in bacterial communities, for example, where there may be differences in soils and soil communities that overlie zinc deposits[17]. These responses are related to increased mobilisation of Zn, but similar results could be achieved for other substances e.g. transition metals mobilised by CO2 acidification near the surface.

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Simple biological monitoring using soil bacterial counts has been investigated at BP’s CCS operation at In Salah[18] in Algeria and at the carbon storage test facility at Ketzin, Germany[19], with the latter study also showing changes in community structure. Bacterial counts are less likely to be useful for monitoring as they tend to be conservative. By comparison, bacterial community structure analysis has been shown to change along physicochemical gradients, including a natural CO2 seep[20]. Conventional microbiological approaches can be time consuming, however, other biosensors using staining or fluorescence markers can rapidly determine community structure. For example the PhyloChip® microarray can detect up to 32,000 unique versions of 16S RNA providing a rapid determination of community structure and also shows species that cannot be cultured by traditional laboratory methods[17]. The resulting microarray data can show changes between baseline and subsequent potential leakage sites. Specific target biosensors can also be developed to trace associated contaminants once these are identified (e.g., SOx, SF6, CH4) as has been shown in BTEX biosensors from petroleum leaks[21, 22].

Figure 2. An example of the possible biological tools that could be deployed as a part of an M&V strategy at CCS sites and how they can be integrated. Modified after Maphosa et al, 2010[23].

These types of tools will require to be “tuned” to the specific challenges of measuring either increased CO2 or related changes that will affect vegetation or microbial communities, before they may be deployed in earnest. Natural analogue or shallow release sites to test and calibrate these tools are essential. Microbiological surveys are underway at the ZERT shallow release site in Montana, USA, with other biological monitoring tools (L. Spangler, Pers Comm, 2011).

2.5. Hydrocarbons and Organics

Hydrocarbons and organic species are important to monitor for the following reasons; 1) they are present in abundance in depleted oil and gas fields (pre-tested structural/stratigraphic traps, for example at the CO2CRC Otway Project; 2) are present in fields undergoing enhanced oil or gas recovery (such as at

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Weyburn); 3) the well known solvent properties of supercritical CO2 may strip hydrocarbons[24] from organic-rich seals or laminae present in clastic or carbonate storage formations, and 4) organics may be entrained in capture gas contaminants[25] from the burning or utilisation of fossil fuels.

Fortunately there are numerous tools that could potentially be deployed from the environmental monitoring and petroleum industry[26]. These tools include some with piezoelectric (mass/gravimetric), optical (infrared, fluorescent etc.) and electrochemical/electrical (e.g. resistance or potentiometric) sensors. There is often a trade-off between the selectivity and sensitivity of hydrocarbon sensors. Most sensors can achieve ppm sensitivity, however the species of interest are often present at ppb levels in the environment. The study has found that sensors employing absorbing membranes tend to be less stable at higher temperatures/pressures as they are prone to fouling or degradation of the membrane. By contrast, fluorescence based detection methods are very robust because both the light source and photometer remain stable while also having appropriate levels of sensitivity. Some oceanographic fluorometers can be deployed to up to 6000m water depth and the light source remains stable for 2 years. Downhole testing capability is unknown, nor is the ability of the tools to withstand heat or high CO2 environments.

Many of the tools that are available are relatively non-selective in the compounds they measure; however for the purposes of CCS M&V this may not be important. Those that are more selective are likely to be less robust as these tend to contain membranes. Few tools have been tested for long periods in appropriate conditions and so drift and longevity questions remain prior to deployment in a commercial CCS site.

3. Conclusions

While there are numerous substances of a chemical and physical nature that can be measured as a proxy for CO2 movement and many different tools that could be used to measure these substances at standard temperatures and pressures, far less tools exist that can actually withstand some of the conditions required for their deployment at CO2 storage sites.

Many of the tools able to withstand the higher temperatures and pressures in the injection horizon come from the petroleum industry. For example, pressure gauges are designed for the harsh environmental conditions within the injection horizon and can also be applied to zones above the injection horizon. Groundwater monitoring tools developed within the water resource industry are also suitable for application to monitoring zones above the injection horizon and can measure pressure, temperature and formation water conductivity. Oilfield and water resource industry tools are available off the shelf and are suitably sensitive and robust for deployment in a number of settings. Nevertheless, there are distinct gaps in the availability of other deployable tools with sufficient robustness and longevity to have the same utility for CCS operations, such as those measuring electrical conductivity, density or acoustic perturbations Currently the tools available for pH monitoring are limited to short term shallow groundwater monitoring as the longevity and stability of the instruments are limited. However, this is an important geochemical parameter to indicate the presence of CO2 and new advances in measuring pH have been made in recent years.

The inorganic geochemical changes associated with the presence of dissolved CO2 are reasonably well understood and this knowledge can be applied to monitoring in both injection horizons and overlying aquifers/groundwater zones. There have been a number of advances in solid state and ion selective electrodes that are increasingly being ruggedised for deployment in more extreme settings. Their stability and longevity need further testing, as well as a good program design to determine where and when to deploy such instrumentation for monitoring purposes.

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There are numerous tools for detecting hydrocarbons and organics. Many of these tools are used in offshore settings in shallow and deep marine environments and have good robustness with respect to pressure; however they may not be quite so effective in much warmer downhole settings. Fouling or degradation of some membranes may be an issue for deployment in certain settings, but these tools are also being improved and taken out of the laboratory for testing and deployment.

Some areas of research and tool development are more mature than others, such as CO2 monitoring with geophysics, while geophysical monitoring of other substances is not as well developed and may have significant limitations. Further changes in how geophysical tools are developed or deployed or the limits of their resolution in the deep subsurface may produce incremental improvements. Alternatively, there are some geophysical tools that have only been tested to a limited degree in the CCS domain (for example magnetotellurics or CSMT, ground penetrating radar or magnetic resonance sounding) that may provide suitable shallow monitoring alternatives and could be used to observe changes in salinity.

Other areas of research and application have significant potential for development, for example, in the case of biological monitoring with low cost arrays and other tools arriving on the market. Many of theexisting approaches have been used successfully in ecological studies and bio-prospecting for mineral exploration. Tools like the Phylochip® could provide a non-invasive approach to monitoring that may prove effective for stakeholders and landowners that are often adversely affected by large grid testing during soil gas or geophysical surveys. Further testing is required to evaluate the effectiveness of some of these biological tools as well as the economic viability of such testing.

The tools and measurements must still be integrated with the models prepared for a CO2 storage site in order to understand the meaning of the information acquired. Meaningful thresholds for measured parameters alerting a possible change in the physical, chemical or biological properties during the assurance M&V still have to be developed for each site in order for these tools to be of overarching benefit to CCS programs.

Acknowledgements

The team would like to thank the IEA GHG for funding the study “Feasibility of Monitoring Tools for Substances Mobilised by CO2”. Report reviewers both internal and external are thanked for their valuable suggestions and contributions.

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