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Titel Single-well and inter-well dual-tracer test design for quantifying phase volumes and interface areas in subsurface flow and transport systems
VerfasserIn I. Ghergut, H. Behrens, T. Licha, F. Maier, M. Nottebohm, M. Schaffer, M. Sauter
Konferenz EGU General Assembly 2012
Medientyp Artikel
Sprache Englisch
Digitales Dokument PDF
Erschienen In: GRA - Volume 14 (2012)
Datensatznummer 250070965
 
Zusammenfassung
Technology-relevant georeservoirs in the realm of energy production (such as: spent-radionuclide repositories, gas-storage, geothermal, as well as CCS candidate reservoirs) contain mobile and immobile fluid regions, and often also different fluid and solid phases. The lifetime of a particular reservoir (from a hydraulic, thermal, geomechanical and/or hydrogeochemical point of view) depends on the volumes and/or interface areas of some of these regions and/or phases. Mostly, their lifetime-effective values cannot be measured by geophysical and hydraulic methods. Since they essentially relate to fluid-based transport processes, attempting to measure them by tracer tests is a sensible endeavour. However, in designing and dimensioning such tracer tests, one should keep in mind that not every tracer test is sensitive w. r. to every fluid transport parameter. A certain complementarity exists, w. r. to parameter sensitivity, between single-well and inter-well methods, between equilibrium and kinetic exchange processes, between volume and area parameters. Mobile-fluid volumes can be measured from inter-well conservative-tracer tests, whereas single-well push-pull tests are generally insensitive w. r. to mobile-fluid volumes. Immobile-fluid volumes, in single-phase systems, are rather difficult to measure, by either kind of test. Different-phase volumes can be determined from inter-well tests using partitioning tracers at equilibrium exchange between phases; whereas single-well tracer push-pull tests are rather insensitive w. r. to tracer exchange processes at equilibrium. Im-/mobile fluid, or inter-phase interface areas can be determined from single-well tracer push-pull tests relying on kinetic exchange processes between compartments or phases. Single-well tests are often believed to be more sensitive w. r. to such processes than w. r. to advective-dispersive processes, and than inter-well tests. Inter-well tests are not physically insensitive w. r. to kinetic exchange processes, but they are strongly affected by ambiguity between dispersion and non-advective non-equilibrium processes. (Actually, this ambiguity also impedes upon single-well tests.) An interesting compromise between the advective- or equilibrium-dominated parameter sensitivity regimes, and the advection- or equilibrium-insensitive regimes is obtained when using in-situ tracer creation in a time-dependent manner (from another initially-injected tracer with different phase-partitioning properties), as had been originally proposed by [1] for determining residual-oil saturations. The poster presents a model set-up enabling to directly compare the sensitivities of the different tracer-test methods w. r. to the different parameters for a given system, and to reshape the concept of [1], from its originally intended oilfield application, to a possible new application for CCS site characterization. We illustrate the latter by simulating such dual-tracer tests for the pilot site of a CCS-related ‘MMV Experiment’ (Measuring, Monitoring, Verification) at Heletz in Israel. The target storage formation at Heletz is assumed to consist of a number (3–6) of permeable sandstone layers (with porosities ~12–17%) separated by shale layers (with porosities ~3–5%), whose permeabilities contrast by factors ~103–104. While single-well tracer tests are rather insensitive w. r. to porosity and permeability stratification details, they can yield information about gas-phase saturations and gas-brine interface densities within selected layers; using the in-situ creation of a dual tracer in the sense of [1], the sensitivity of single-well tests in the low-saturation (residual-saturation) range can be enhanced significantly. A inter-well conservative-tracer test, with depth-resolved monitoring at the ‘arrival’ well (as intended at the Heletz site), can additionally yield the effective-porosity profile, which can be used to better constrain the single-well test inversion. With bulk (not depth-resolved) tracer monitoring, a inter-well conservative-tracer test still yields very valuable information, which can be poured into the shape of a flow-storage repartition[2],[3]. Furthermore, considering a CO2 plume with the radius–thickness relationship derived by [4], and ‘integrating’ it ‘over’ the particular-site stratigraphy, with the ‘weighting’ defined by the flow-storage repartition (cumulative distribution function for q against Bφ) that was derived from the inter-well, conservative-tracer test, we get:       A ( t , θ˚ )   -‰ˆÂ Â Â (t/π)1-ˆ•2 × [ μ(θ˚ ) + 1/μ(θ˚ ) ] × Σi (qiBi/φi)1-ˆ•2  ,         from which the evolution of CO2–brine interface areas (A) during early injection regimes (immiscible displacement) can roughly be estimated as a function of time t and temperature θ˚ , with μ(θ˚ ) denoting the mobility ratio between CO2 and brine (mobilities being taken at each one’s saturation). The comparison of CO2 plume volumes and CO2–brine interface areas predicted for the Heletz MMV experiment under different stratigraphy assumptions demonstrates the importance of brine-phase (single-phase!), conservative-tracer tests for characterizing the ‘transport-effective hydrogeology’ of a candidate CCS site, prior to initiating any experiments involving a CO2 phase.    References: [1] TOMICH J F, DALTON R L JR, DEANS H A, SHALLENBERGER L K (1973): Single-Well Tracer Method to Measure Residual Oil Saturation. J Petrol Technol / Transact, 255, 211-218  [2] SHOOK G M (2003): A Simple, Fast Method of Estimating Fractured Reservoir Geometry from Tracer Tests. Geotherm Resour Council Transact, 27  [3] http://www.geothermal-energy.org/pdf/IGAstandard/SGW/2010/behrens.pdf [4] NORDBOTTEN J M, CELIA M A, BACHU S (2005): Transp Porous Med, 58, DOI 10.1007/s11242-004-0670-9 – page 348, rel.(8)    Acknowledgement: Field and laboratory work for implementing the tracer methods at the Heletz site are funded by the EU Seventh Framework Programme FP7 / 2007–2013, within the MUSTANG project (grant agreement no. 227286). The authors express their gratitude to Jac Bensabat (EWRE, Haifa) and Auli Niemi (University of Uppsala), for fruitful discussions during their visit in Göttingen.