Merging Single-well and Inter-well Tracer Tests into ...

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ScienceDirect Energy Procedia 59 (2014) 249 – 255

European Geosciences Union General Assembly 2014, EGU 2014

Merging single-well and inter-well tracer tests into one forcedgradient dipole test, at the Heletz site within the MUSTANG project Horst Behrensa, Julia Gherguta *, Jac Bensabatb, Auli Niemic, Martin Sautera a

Applied Geology Dept., Geoscience Centre (GZG), University of Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany b EWRE Ltd., 97a Yefe-Nof St., P. O. Box 6770, Haifa 34641, Israel c Earth Sci. Dept., University of Uppsala, Geocentrum, Villav. 16, 752 36 Uppsala, Sweden

Abstract The Heletz site in Israel was chosen for conducting a CO2 transport experiment within the MUSTANG project, which aim is to demonstrate and validate leading-edge techniques for CCS site characterization, process monitoring and risk assessment. The major CO2 injection experiment at Heletz was supposed to be preceded and accompanied by a sequence of single-well (SW) ‘push-then-pull’ and inter-well (IW) tracer tests, aimed at characterizing transport properties of the storage formation. – Instead of the rather luxurious, multiple-SW and multiple-IW test sequence described in our previous work, we now propose a drastically economized tracer test concept, which lets the fluid sampling stages of SW and IW tests merge into a single production stage, and relies on a forced-gradient dipole flow field at any time of the overall test. Besides cost reduction, this economized design also improves on operational aspects, as well as on issues of parameter ambiguity and of scale disparity between SW and IW flow fields. © 2014 byby Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Austrian Academy of Sciences. Peer-review under responsibility of the Austrian Academy of Sciences

Keywords: CCS; pilot site; Heletz; tracer tests; single-well; inter-well; brine displacement; geologic heterogeneity; phase interface area; residual saturation

* Corresponding author. Tel.: +49-551-399709; fax: +49-551-399379 E-mail address: [email protected]

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Austrian Academy of Sciences doi:10.1016/j.egypro.2014.10.374

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Horst Behrens et al. / Energy Procedia 59 (2014) 249 – 255

1. Introduction: tracer test goals within the MUSTANG project The Heletz site [1,2] in Israel was chosen for conducting a CO2 transport experiment within the MUSTANG project [A multiple space and time scale approach for the quantification of deep saline formations for CO2 storage, www.co2mustang.eu], which aim is to demonstrate and validate leading-edge techniques for CCS site characterization, process monitoring and risk assessment [3,4]. Relevant physical, hydrogeological and hydrogeochemical features of the Heletz site, as determined prior to drilling and testing new wells, are described under www.co2mustang.eu/ Heletz.aspx; this site was found to be particularly suited for evaluating the joint performance of a number of non-fluid-based geophysical as well as fluid-transport based (i.e., tracer) techniques. The major CO2 injection experiment at Heletz was supposed to be preceded and accompanied by a sequence of SW ‘push-then-pull’ and IW tracer tests, aimed at characterizing x CO2 – brine – rock interface processes, following ideas proposed by [5,6], x fluid transport properties of the storage formation, following ideas described in [7,8]. The design and dimensioning of SW and IW tracer tests was supposed to observe a number of general and specific principles [8,9]. Generally, IW tracer tests are used to determine fluid residence time distributions, whose ‘statistical’ moments provide important ‘bulk-reservoir’ information (size, boundaries, heterogeneity). Complementarily, SW tracer push-then-pull tests are used to quantify processes other than advection-dispersion: typically, the exchange of some extensive quantity (mass, energy) between fluid and solid/fluid phases by processes like matrix diffusion or sorption/partitioning, whose rate or amount depends on the density (area per volume) of involved fluid/rock interfaces. Flow-field reversal during the ‘pull’ phase is supposed to largely compensate the effects of flow-path heterogeneity (excepting the hydrodynamic level), and to enhance the effects of tracer exchange processes at fluid/rock interfaces, thus enabling to quantify interface densities from measured tracer return signals; yet this depends on whether the fluid volumes and flow/shut-in durations used in the push-pull test match the system’s homogeneity scale and the (a priori unknown) bulk exchange rates. The sensitivity of tracer signals from both IW and SW tests with respect to interface densities depends upon the type of process that dominates at the space-time scale of the experiment [6,7,9], which can be: fast-equilibrium sorption or partitioning between phases; kinetic exchange, or matrix diffusion with high diffusivity (typical for heat exchange in fractured hard-rock aquifers); slow interface reactions, or matrix diffusion with low diffusivity (typical for most solutes in most rocks, and for heat exchange in unconsolidated, high-porosity aquifers).

Nomenclature FBA HTO IW NDS NMS SRG SW

fluorobenzoic acid salt tritiated water inter-well naphthalene disulfonic acid salt naphthalene monosulfonic acid salt Sulforhodamine G single-well

2. The original plan for SW and IW tracer tests at Heletz The originally-intended tracer test sequence to be conducted at the Heletz site consisted of four separate experiments [1,7]:


SW1: prior to CO2 injection: dual-tracer SW push-then-pull test (monopole divergent followed by convergent flow field), similar to that reported in [9], using several tracers with contrasting sorption and diffusion properties, aimed at characterizing fluid-rock interfaces and estimating fluid-rock interface densities (also serving as an aid in tracer species selection, dimensioning and instrumentation for all subsequent tests); IW1: prior to CO2 injection: brine-phase dual-tracer IW circulation test (forced-gradient, divergent-convergent dipole flow field), similar to that reported in [8], using two tracers with contrasting sorption or diffusion properties, aimed at estimating storage reservoir size, determining the flow-storage repartition of the brine phase, and characterizing reservoir-scale heterogeneity; SW2: prior to the main CO2 transport experiment, but including small-sized CO2 slugs: dual-tracer, SW multiplepush of alternating brine/CO2 slugs, followed by prolonged push stage, using single-phase tracers as well as phasepartitioning tracers, in the sense of [5,1,3], aiming at a dynamic characterization of CO2-brine-rock interfaces; IW2: during the main CO2 transport experiment: dual-tracer IW injection-extraction test (forced-gradient, divergent-convergent dipole flow field), in the sense of [2,4], using single-phase tracers as well as phase-partitioning tracers, aimed at quantifying the storage capacity, characterizing brine displacement processes (Fig.1), and determining flow-storage repartitions under two-phase flow conditions.

Fig. 1. Brine-phase tracer spreading during a so-called ‘brine displacement’ test in conjunction with CO2 injection.

2.1. Remark on resident-mode (Ketzin) versus flux-mode (Heletz) sampling Unlike the brine-phase spiking conducted at the Ketzin site in Germany (http://www.co2sink.org), where only downhole fluid sampling was possible at few discrete times (volume of sampled fluid = volume of one sample bottle, yielding so-called ‘resident’ [10] values of tracer concentration, inconsistent with the reservoir-scale transport equations), the Heletz experiment offers the advantage of fluid extraction at well-defined rates, rendering measured values of tracer concentrations (actually, tracer fluxes, cf. [10]) consistent with the transport equations from which parameter inversion is endeavored. Forced-gradient extraction of fluid is not meant to be representative of how a CCS site would be operated in reality, but it ensures the meaningfulness of measured experiment quantities. The MUSTANG experiment was not worthwhile being conducted just to see ‘when’ CO2 will arrive in a certain distance; its aim is not to mimic CCS, but to quantify transport processes, which is not possible without well-defined fluid and solute fluxes. Moreover, fluxes

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measured at one borehole should reflect reservoir-scale fluid motion, and not just wellbore-scale flow gradients induced by the particular device used to collect fluid samples. Additionally, experiences made with detecting and quantifying certain organic tracer species in fluid samples from IW tracer tests conducted in deep georeservoirs in Germany (2003–2012) led to suggesting new procedures for tracer pre-quantification to be applied ‘on site’ (uphole and/or downhole, more or less ‘in situ’) alongside with the ongoing programs of fluid monitoring during dipole circulation. Furthermore, time-integral passive sampling with solid-phase enrichment is especially recommended with a view at very high dilution of brine-phase tracers expected for the so-called ‘brine displacement’ test stage (Fig.1). 3. Modified SW and IW test design Instead of the rather luxurious {SW1, IW1, SW2, IW2} test sequence described in our previous work [1-4, 6-9], we now propose a drastically economized tracer test concept (self-explanatory in Fig.2), which lets the fluid sampling stages of SW and IW tests merge into a single production stage, and relies on a forced-gradient dipole flow field at any time of the overall test. Besides cost reduction, this economized design also improves on operational aspects, as well as on issues of parameter ambiguity and of scale disparity between SW and IW flow fields: (i) the new design renders SW test results more representative for the aquifer sector (‘angle’) actually interrogated by the IW dipole test (cf. Fig.3); (ii) the new design saves time and costs on the SW test (fluid sampling for SW ‘pull’ now being conducted simultaneously with IW-related sampling and monitoring), while allowing for a considerably longer duration of SW ‘pull’ signals than had originally been endeavored, whose late-time tailings help improve the quantification of nonadvective processes and parameters, which are of great relevance to mid- and long-term trapping mechanisms (‘residual trapping’, ‘mineral trapping’); (iii) the quasi-concomitant execution of fluid injection/production for the IW and SW tests considerably reduces the overall hydraulic imbalance that was originally associated with the SW test, thus preventing formation damage and supporting hydrogeomechanical stability; (iv) the new design reduces the imbalance between injected and produced fluid volumes at any time to a minimum, thus eliminating the need for large-capacity tanks (and water supply) to provide ‘push’ fluid for injection and to store ‘pull’ fluid during production within the SW test (saving on costs again). 3.1. Remarks on parameter sensitivity Unlike the typical deep-georeservoir application suggested by [9], the triple-scale SW push-pull defined in Fig.2 is not meant to detect thermo-hydro-mechanically- and/or hydrogeochemically-induced changes to hydrogeologic properties of the target storage formation. Instead, it is expected to provide information on hydrogeological heterogeneity, in terms of a possibly scale-dependent, equivalent ‘Peclet number’ (cf. semi-analytical simulations in [8], the modified-SW test sensitivity to this parameter being approximately similar to what was seen in Fig.2 of [8], with slight improvement: ~erfc(Pe1/2...), for Peclet numbers, versus ~erfc(.../R1/4), for retardation factors).


Fig. 2. Merged SW and IW test design, using the available two wells at the Heletz site within the MUSTANG project.

Fig. 3. Reservoir regions captured by dipole vs. monopole test, in homogeneous single-continuum vs. porous-fissured reservoir.

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Fig. 4. Role of transverse diffusion versus advective fluxes during early and late stages of a SW push-then-pull test in a heterogeneous medium with features of highly contrasting fluid mobility.

Concomitant use of solutes and heat (fluid temperature) as tracers is supposed to enable concomitant realization of those different asymptotic regimes (I) – (IV) identified by [9], from which effective values of pore width b and spacing a can be determined independently of each other – at least in principle. The main underlying idea (summarizing what was not stated explicitly by [9]) is that the ratio between advective and intra-particle diffusion fluxes is proportional to the product of pore width and spacing (ab) at early times, whereas at late times it is proportional to the squared spacing (a2), becoming independent of aperture (b); and the ‘early’-to-‘late’ transition occurs much earlier for heat, than for solute tracer transport. This kind of behavior is not limited to homogeneous formations of infinite lateral extension with spatially-invariant b and a, but can also be seen in irregular distributions of a finite number of thin conductive layers (like those expected at Heletz) with varying thickness and spacing, as illustrated in Fig.4. Finally, this kind of heterogeneity represents one more reason for giving preference to the merged SW and IW test design. Acknowledgements Field and laboratory work for implementing the tracer methods at the Heletz site are funded by the EU under FP7, within the MUSTANG project (grant no. 227286). Modeling work (parameter sensitivity analyzes, tracer test design) was funded by Baker Hughes (Celle, Germany) and by the Science and Culture Ministry of Lower Saxony (MWK Niedersachsen, Germany) within the gebo project (Geothermal Energy and High-Performance Drilling, www.gebo-nds.de).


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