Natural helium as a screening tool for assessing caprock imperfections at geologic CO2 storage sites

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Energy Procedia

Energy Procedia 00 (2008) 000–000

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Natural helium as a screening tool for assessing caprock imperfections at geologic CO2 storage sites

Jason Heatha,*, Brian McPhersonb, Fred Phillipsa, Scott Cooperc, Thomas Dewersd aDepartment of Earth and Environmental Science, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, USA

bDepartment of Civil and Environmental Engineering, University of Utah, 122 South Central Campus Drive, 104 CME, Salt Lake City, UT 84112-0561, USA

cCooper Geological Consulting LLC, 76 Raven Road, Tijeras, NM 87059, USA dDepartment of Geomechanics, Sandia National Laboratories, P.O. Box 5800, MS 0751, Albuquerque, NM 87185-0751, USA

Elsevier use only: Received date here; revised date here; accepted date here

Abstract

Natural helium is a screening tool for identifying the presence or absence of caprock imperfections. Imperfections can be manifested as a variety of features or processes, including insufficiently low permeability, preferential flowpaths such as fractures and faults, and the propensity for capillary breakthrough. Theory and simulations detail how various types of imperfections affect the spatial distribution of natural helium above, within, and below caprock in a single-phase, brine-saturated system. Specifically, the distribution of natural helium can reveal the presence of preferential flowpaths through formations with low matrix permeability. The distribution patterns of helium shed insight on the size, shape, location, and connectedness of imperfections in caprock. We show how imperfections associated with characteristic distributions of natural helium will affect the retention of CO2. We discuss the advantages of natural helium, together with temperature distributions, for revealing imperfections and the optimum locations for sampling the natural tracers. This research is being carried out to support design and interpretation of ongoing field-testing by the Southwest Regional Partnership on Carbon Sequestration. Specifically, we are evaluating seal integrity of the Partnership’s Pump Canyon Enhanced Coalbed Methane-CO2 Storage Demonstration, located in the San Juan Basin, New Mexico. The caprock at this site is the Kirtland Formation. This formation is composed of a variety of continental deposits (sandstones, siltstones, mudrocks, and shales) and is ideal for investigating the capability of helium to characterize sealing integrity of a very heterogeneous caprock. We present results of analyses of noble gases and a variety of petrological and petrophysical analyses on core through this caprock. These results are used to investigate the presence of imperfections and their potential impact on CO2 migration and the overall viability of utilizing natural helium as a screening tool. The authors gratefully acknowledge the U.S. Department of Energy and the National Energy Technology Laboratory for sponsoring this project. © 2008 Elsevier Ltd. All rights reserved

Keywords: caprock, CO2 storage, fractures, helium, isotopes, leakage, preferential flow path, seal by-pass, tracers

* Corresponding author. Tel.: +1-505-845-1375; fax: +1-505-844-7354. E-mail address: [email protected].

c© 2009 Elsevier Ltd. All rights reserved.

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Heath et al. / Energy Procedia 00 (2008) 000–000

1. Introduction

Migration of CO2 from a geologic storage formation is a major concern. Such migration, hereafter referred to as leakage [1], may pose impacts to environmental health and could result in unsuccessful reductions of CO2 emissions to the atmosphere [2]. Research has focused on potential leakage through caprock. Caprock is a geologic formation overlying a storage reservoir, which acts as a barrier to prevent a fluid phase from migrating out of the reservoir. The existence of hydrocarbon reservoirs implies that caprocks exist that may similarly contain CO2 for geologic timescales. However, differences between interfacial tension and other properties for hydrocarbon-brine systems and CO2-brine systems necessitate careful study of the actual caprock sealing behavior in CO2-containing systems [3,4].

Issues remaining for caprock research for CO2 storage include obtaining adequate characterization of discrete

leakage pathways, determining timescales and magnitudes of coupled processes that impact the ability of a caprock to impede leakage, quantification of potential fluxes of CO2 through caprock, developing quantitative criteria for what constitutes significant leakage, characterizing the heterogeneity of caprock properties, and prediction of diffuse or localized leakage patterns [3-7]. Risk and performance assessment frameworks are being developed to integrate data and simulate processes to investigate these issues [1,8]. However, these assessments contain uncertainties due to the paucity of available data on caprock properties. Site characterization depends on acquiring key data to lessen uncertainty and to build confidence on whether the sealing behavior of a caprock will be acceptable [1].

Through numerical modeling and a field study, this paper presents a screening tool for building confidence of the

sealing integrity of caprock, which could be used in tandem with other key data for assessing sealing behavior. The tool is based on a characterization of the distribution of natural tracers, which are present in the subsurface at all sequestration sites. The tool is akin to aquifer isolation studies at nuclear waste repositories that use natural tracers [9]; however, the use of the tracers detailed here is in the framework of CO2 storage.

2. Numerical modeling of sealing integrity

2.1. Modeling approach

The modeling approach focuses on the following two general situations: 1) transport of 3He, 4He, and heat in the subsurface under scenarios of pre-CO2 injection; and 2) injection of CO2 under the same conditions of the pre-injection models. These models will help determine if helium and temperature can indicate the presence of preferential flowpaths through a caprock that could cause unacceptable amounts of leakage, and thus potentially serve as a screening tool for determining caprock suitability for carbon sequestration. To model helium tracer transport and potential leakage of CO2 through a caprock, we invoke the TOUGH2 family of software [10], with the ECO2N equation of state module [11] used for the two-phase CO2 fluid injection simulations and the EOSN equation of state module [12] used for 3He and 4He generation and transport simulations.

2.1.1. Criteria of sealing integrity A measure of sealing integrity for the modeling scenarios is based on the percentage of injected CO2 that

migrates through the top of the caprock over the time period of the simulations. Similar criteria have been suggested by researchers attempting to define acceptable leakage rates [13]. For our scenarios, rates of 0.0% yr-1, 0.1% yr-1, 1.0% yr-1, and 7% yr-1 will be examined. Based on emerging criteria from the CO2 storage community [13], we consider leakage over 0.1 % yr-1 unacceptable.

2.1.2. Modeling scenarios To demonstrate numerically the use of helium and temperature as CO2 caprock screening tools, the following

modeling scenarios were utilized: 1) a basin model with hydrological conditions loosely modeled after the San Juan Basin in Northern New Mexico [14], run to steady state flow conditions (1 My); 2) a sub-basin model with boundary and initial conditions taken to represent a sub domain from the basin model results; and 3) a sub-basin model

2904 J. Heath et al. / Energy Procedia 1 (2009) 2903–2910


identical to case 2 but with a single zone of higher permeability. Following the hydrostratigraphy used for the basin model, the simulation domain includes a target storage reservoir 200 m in thickness, a 50-m-thick caprock, and an overlying aquifer 250 m in thickness.

For case 3, the caprock can contain zones of high permeability, which represent seal by-pass systems such as

permeable faults and/or connected fracture networks. The models investigate helium and temperature distributions before injection and then determine the amount of CO2 leakage under the same conditions by “injecting” CO2 into a single cell just beneath the caprock at a rate of approximately 25 tonnes/d. Helium systematics for the models is represented by following typical procedures in the literature, which include a basal flux of helium (3He and 4He) that corresponds to the helium sourced from the underlying rocks at the base of the domain, in-situ production of 4He from uranium (U) and thorium (Th) decay, and groundwaters at recharge areas in equilibrium with atmospheric helium [15]. To approximate the basin scale boundary conditions, the sub-basin models were run with basal cell volumes of essentially infinite extent. This enabled a constant source of overpressure, helium generation, and heat generation. Basal heat production was taken to be 50 mW m-2. Basal helium fluxes were set at 1.0e-23 kg m-2 s-1 for 3He and 1.7e-16 kg m-2 s-1 for 4He, which are values from Mahara and Igarashi [16]. For simplicity, the left and right-hand vertical boundaries were taken to have no-flux conditions and values of pressure, temperature, 3He and 4He aqueous mass fractions at the top of the sub-domains were held constant at values dictated by the basin model.

In all models, reservoir and aquifer porosities were taken to be 0.15, with a value of 0.07 used for the caprock.

Absolute permeability tensors were taken to be isotropic with 10-14 m2 used for the reservoir and aquifer layers and 10-18 m2 used for the caprock. These values were suggested by values for regional aquifers and aquitards in the San Juan Basin in the calibrated groundwater model of Kernodle [17]. Capillary pressure and relative permeability relationships used the Van Genuchten-Mualem model in TOUGH2 [10] for liquid relative permeability and capillary pressure function [18], and the Corey [19] model for gas relative permeability (parameters used in the multiphase flow runs were as follows: λ = 0.4, Slr = 0.2, Sls = 0.99 for all three rock types, 1/Po = 1.60e-7 Pa-1 for caprock and 2.79e-4 Pa-1 for reservoir and aquifer, and Pmax = 1.0e8 Pa for caprock and 1.0e7 Pa for reservoir and aquifer).

2.2. Modeling results and discussion

Preliminary simulations indicated that a perturbed permeability for the fault flow pathway of 1x, 10x, and 100x gave CO2 leakage rates of 0.0, 1.0, and 7.0% yr-1, and thus these values were used in both CO2 injection and steady-state heat and helium transport simulations to examine sensitivity to permeability perturbation magnitude. Here we show examples for only the 0.0 and 7.0% yr-1 leakage rate examples because helium and temperature spatial profiles for the 1.0% yr-1 rate case were very similar to the 7.0% yr-1 case. Figures 1A and 1B compare steady-state 4He profiles for cases of 7% yr-1 (100x fault zone permeability) and 0.0% yr-1 (1x fault permeability; 3He profiles are similar in shape but differ in magnitude). In both cases, 4He profiles are steeper through the caprock, suggesting that helium tracer vertical profiles through caprock, relative to surrounding aquifers, might be used to infer caprock sealing capacities. In the case of a 100x fault permeability, significant elevations in 4He are observed above the fault zone compared to the no-fault case. It may be possible to discern the presence of such a flow pathway from the presence of the helium “mounding” above a target caprock. A vertical depth profile of 4He through the caprock is convex upward as compared to the linear-with-depth profile of the no-fault case (Fig. 1E).

Responses to CO2 injection for both permeability scenarios are shown in Figure 1C (100x fault permeability

above background) and Figure 1D (1.0x above background). Significant CO2 leakage is observed for the former case, while no leakage is observed in the latter case. Vertical profiles of 4He, temperature, and pressure are shown in Figures 1E-1G for the 7% yr-1 leakage case through the more permeable fault zone (red line) and at an effectively infinite distance (blue line). The 4He profiles (Fig. 1E) demonstrate that a helium perturbation may be used to infer the presence of the leaky caprock. Capability to measure an actual perturbation would depend on uncertainty in data quality due to sampling methodology and the sensitivity and precision of the laboratory analysis [20]. Typical groundwaters have a 4He concentration of ~4.8e-5 cm3 at standard temperature and pressure (STP) per kg water [20,21]. The value of 4.8e-5 cm3 STP kg-1 corresponds to water equilibrated with atmosphere at 10°C. Precision of laboratory analysis for such waters is ~ ±1% [20,22]. If the helium perturbation of Fig. 1E is considered to represent

J. Heath et al. / Energy Procedia 1 (2009) 2903–2910 2905


Fig. 1. TOUGH2 simulation results comparing helium and CO2 leakage through a caprock with and without a permeability perturbation (i.e. a vertical “fault” with 100x background caprock permeability). The zero elevation on the figures corresponds to a depth below ground surface of ~1.5 km. A&B. Profiles of dissolved 4He as mass fraction of aqueous solution (x1010) for “faulted” (A) and unfaulted (B) cases. Caprock is shown as grey horizon. C & D. CO2 gas saturation after one year of injection directly beneath the caprock. E. Comparison of vertical profile in dissolved 4He both running through the fault zone (red line) and at an infinite distance from the fault (blue line) under steady state flow conditions (1 My). F. Temperature profiles taken through the fault zone (red line) and at an infinite distance from the fault (blue line) under steady state flow conditions (1 My). G. Vertical pressure profiles both through the fault (red line) and at an infinite distance from the fault (blue line), showing pressure dissipation through the fault. a real system, the perturbation could be measured by laboratory analysis (1.0e-8 mass fraction of 4He corresponds to 5.6e-2 cm3 STP kg-1). The corresponding temperature perturbation is relatively very small at <0.0001%, which would be very difficult to impossible to measure using temperature logging [23]. In this case, helium measurements could be much more effective than temperature in verifying the presence of a seal by-pass system. The pressure profile through the leaky seal could be interpreted as being hydrostatic, and thus nothing could be inferred from the pressure profile by itself with regard to sealing capacity of the caprock. These models show the potential usefulness of natural helium measurements for CO2 storage sites. Real systems, however, can be affected by changes in the effective diffusion coefficient and thermal conductivities, heterogeneity, and other properties that would affect the distribution of helium and temperature that should be considered in interpreting natural tracer data from field sites.

3. Field-based test of concept

3.1. Field study at a CO2 storage project



The U.S. Department of Energy’s (DOE’s) Southwest Regional Partnership on Carbon Sequestration (SWP) is conducting a CO2 storage project in unmineable coal seams at the Pump Canyon Site near Archuleta, New Mexico. The results of the project will be used to assess CO2 injection for enhanced coalbed methane recovery. The project is a field test of the SWP’s Phase II activities, which are evaluating and validating the most promising concepts and monitoring technologies for CO2 storage in the southwest U.S. [24]. In May, 2008, the site operator, ConocoPhillips, began drilling a new CO2 injection well. Carbon dioxide injection commenced on July 30, 2008, into three coal seams in the Fruitland Formation located at depths between ~ 889.2 m and 956.5 m (below ground surface). The average injection rate for the first two months of injection was 0.549 m3/s (1675 Mcf/d) [25]. Drilling of the injection well provided the opportunity to core and study the Kirtland Formation. The Kirtland Formation conformably overlies the Fruitland Formation and is considered a regional aquitard and reservoir seal [26].

The purpose of studying the Kirtland Formation is threefold. First, the Kirtland warrants an assessment of its

ability to contain CO2 since it overlies the injection site. Second, the Kirtland’s heterogeneous lithology affords the opportunity to assess geologic controls on the variability of sealing quality. Third, obtaining fresh core from the Kirtland facilitates the analysis of natural tracers in the pore fluids. This paper presents preliminary results of a containment assessment that incorporates the use of natural tracers to characterize sealing behavior.

3.2. Methodology – coring program, sample preservation, and laboratory analysis

Fresh core was needed for the preservation of noble gases in the pore fluids of the rock. Using logs from the nearest offset wells to the location of the injection well, depths for two 18.3-m (60-ft) sections of core were chosen in the upper and lower shale members of the Kirtland. The two depths for coring were separated by more than 183 m for the examination of the helium gradient and change in 3He/4He across a large portion of the Kirtland. Sample plugs of core were obtained in the field. The plugs were placed in vacuum-tight canisters built following instructions from Dr. Stute at the Lamont-Doherty Earth Observatory and Dr. Solomon at the University of Utah’s Noble Gas Laboratory. Atmospheric gases were purged from the canisters using a procedure developed by Stute [27,28], which is similar to the methods of Osenbrück et al. [29]. Field blanks were obtained for quality control.

Analysis of noble gases was performed at the Noble Gas Laboratory of the University of Utah on a mass

spectrometer. Terra Tek, Inc., in Salt Lake City, UT, performed petrologic analysis using x-ray diffraction (XRD), scanning electron microscopy (SEM), and petrographic descriptions of thin sections. Terra Tek also analyzed samples for total organic content (TOC), obtained a gamma ray log and pressure-decay permeability, effective and total porosity, and fluid saturations. They also slabbed the core to facilitate examination of lithology and fractures. Formation micro-imager logs are being evaluated for the presence and locations of fractures within the Kirtland and Fruitland. Poro-Technology in Sugar Land, TX, is currently performing mercury injection capillary pressure measurements. Unslabbed pieces of core were preserved against drying and delivered to the National Energy Technology Laboratory (NETL) for geomechanical analysis and CO2 adsorption tests.

3.3. Preliminary field-study results

The first section of core retrieved 15.90 m of rock with 9.05 m of Ojo Alamo Sandstone and 6.85 m of upper Kirtland. The second section of core retrieved a total of 2.53 m from the lower Kirtland. The clay matrix of the upper Kirtland contains illite, smectite, mixed layer illite-smectite, chlorite, and kaolinite (Fig. 2). Detrital grains are typically poorly sorted, sand-sized, and include quartz, chert, feldspars, volcanic clasts, and authigenic pyrite. The lithology is sandy, argillaceous mudstone. Induced fractures from drilling, core-handling, or desiccation are present in some thin sections. Samples from the lower Kirtland are argillaceous and silty mudstone (Fig. 2). The clay matrix is composed mainly of chlorite and expandable illite/smectite layers. The total clay content of five samples from the upper Kirtland ranges from 42 to 65 wt%, whereas total clay content varies from 49 to 52 wt% in the lower Kirtland.

The upper and lower Kirtland members have low permeability with average values of 8.3 (±1.2)×10-20 m2 and 8.1 (±0.4)×10-20 m2 , respectively (Fig. 3; uncertainty based on 1 ). Total organic content is low for the Kirtland with values less than 0.3 wt% for seven samples (Fig. 3).



A. B.

Fig. 2. Photomicrographs of Kirtland thin sections. Thin sections were stained magenta by dyed epoxy. A. Upper Kirtland thin section at the depth of 625.47 m. Clay matrix is is smetitic and chloritic. Volcanic (v), chert (ch), and plagioclase (p) clasts are labeled. Scale base is 0.5 mm. Photo under plane-polarized light. B. Thin section from lower Kirtland at the depth of 822.01 m. Matrix is illite/smectite mixed layers, illite, chlorite, with minor smectite and kaolinite. Pink areas indicate micropores probably associated with dehydration. Scale bar is 200 m.

All ten sample preservation canisters contained high concentrations of methane compared to other gases. Extraction of methane removed other gases except 3He, 4He, and 20Ne. 4He data given in Fig. 3 are preliminary. These data are the first run of the samples on the mass spectrometer after waiting a sufficient period of time for gases to be transported from the pore fluids. A second run will be made to confirm the quantitative release of noble gases from the pore fluids. Three samples are not shown on Fig. 3 because they had very high concentrations that require corrections for the effects of non-linearity during the analysis. The corrections are currently being made. One sample had high 20Ne and R/Ra values, which indicated leakage of the canister. The questionable leaky sample is also not shown on Fig. 3. The uncertainty associated with sample collection and laboratory analytical procedures is being determined. Previous work using similar methods estimated helium losses of less than 20-30% before preservation of core samples [29]. Core analysis by Terra Tek indicate that gas saturations of core samples may have ranged from ~11 to 19 % (of pore volume) in the upper Kirtland and ~23 to 24% (or pore volume) in the lower Kirtland. Due to the gas saturations, the 4He are preliminarily plotted as cm3 of 4He at STP over the pore volume. The R/Ra ratios (i.e., 3He/4He of the samples over 3He/4He in the atmosphere) differ between the upper and lower Kirtland with average values of 0.151 (±0.003; uncertainty based on 1 of samples) and 0.122 (±0.001), respectively (Fig. 3). The 4He concentration has the lowest values in the lower Kirtland based on the current data.

3.4. Discussion of sealing integrity

Sealing integrity of the Kirtland will be properly assessed when geomechanical and capillary-entry pressure results can be incorporated into the data set. We make preliminary observations about the likely sealing behavior. The low permeability of the upper and lower Kirtland, the amount of clay minerals, and the poor sorting of the sediments indicate that the capillary-entry pressure of these rocks is probably high. The porosimetry measurements underway will test this assumption. Geomechanical testing at NETL will infer ductile versus brittle behavior under in situ and injection-altered stress conditions.

The current 4He data do not display a trend of increasing concentration with depth. The flux of 4He from

formations deeper than the Kirtland was expected to cause an upward gradient of 4He. Conceptual models explaining higher concentrations in the upper Kirtland include the impact of higher U and Th concentrations in the upper Kirtland than the lower Kirtland and/or the transport of fluids with high 4He concentrations deeper than the lower Kirtland through a seal by-pass system (e.g., fault or other upward preferential flowpath). We make no definitive statement as to interpretation of the helium data until the rest of the samples have been analyzed and magnitudes of uncertainty have been properly assessed. The presence of a separate gas phase may affect interpretation of the helium data as expedient transport may be through flowpaths of interconnected gas phase as



Fig. 3. Summary of currently available core-related data from the ENPG Com A Inj. #1 well. Data includes R/Ra values, 4He concentration (normalized by pore volume), and gamma ray log on core. All scale bars are linear except for permeability, which has a log scale. compared to brine.

4. Conclusions and ongoing work

Numerical modeling demonstrates the potential usefulness of natural tracers, especially helium, for indicating the presence of a seal by-pass system. Natural helium is much more sensitive to seal by-pass features than temperature. The helium perturbation should be looked for in overlying aquifers and caprocks during site characterization and drilling of monitoring and injection wells to build confidence in the sealing behavior of the caprock. Further generic models are investigating the impacts of more complex seal by-pass systems, variability in effective diffusion coefficients and bulk thermal conductivity, and the magnitude of boundary conditions that drive fluid flow. The field study is not far enough along for a strong argument as to the sealing behavior of the Kirtland. On-going work will further assess the uncertainty of the helium data and then use the data to test conceptual models of fluid flow through the Kirtland using TOUGH2 modeling. The models will be calibrated with the helium data. A focus will be to answer the question as to whether the tracer data could indicate a seal by-pass through the Kirtland or are supportive of alternative hypotheses.

Acknowledgements

We thank Dr. Martin Stute, Dr. Kip Solomon, and Alan Rigby for providing guidance on building canisters for noble gas sampling in the field and assistance in understanding helium systematics. We thank William Holub, Randy Everett, Lee Harris, and Dr. Reid Grigg for their assistance with field work. We thank ConocoPhillips,



especially Ryan Frost and Eddie Pippin, for helping plan the coring program. Dr. Stefan Finsterle provided assistance with using the noble gas module for TOUGH2. The authors gratefully acknowledge the U.S. DOE and the NETL for sponsoring this project. Support for the first author has come from the Petroleum Recovery Research Center at New Mexico Tech. The first author is currently a student intern at Sandia National Laboratories, which has provided additional support.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S.

DOE’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

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Natural helium as a screening tool for assessing caprock imperfections at geologic CO2 storage sites