The interplay of non-static permeability and fluid flow as a possible pre-requisite for supercritical geothermal resources

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1 Available online at ScienceDirect Energy Procedia 40 (2013 ) European Geosciences Union General Assembly 2013, EGU Division Energy, Resources & the Environment, ERE The interplay of non-static permeability and fluid flow as a possible pre-requisite for supercritical geothermal resources Philipp Weis*, Thomas Driesner Institute of Geochemistry and Petrology, ETH Zurich, Clausiusstrasse 25, 8092 Zürich, Switzerland Abstract Unconventional geothermal resources at supercritical conditions have been inferred to occur beneath high-enthalpy systems in active magmatic environments, and bear the potential to increase electricity production from a geothermal well by an order of magnitude. The high specific enthalpies of these fluids cannot be explained by simple convection models and a hydrologic divide between two distinct flow regimes may be required. In numerical simulations of porphyry-copper systems, such a hydrologic divide self-organized from an interplay of non-static permeability and fluid flow. The physical principles of these fossil magmatic-hydrothermal systems are closely related to supercritical geothermal systems The Authors. Published Published by Elsevier by Elsevier Ltd. Open Ltd. access under CC BY-NC-ND license. Selection and/or peer-review peer-review under responsibility under responsibility of the GFZ of German the GFZ Research German Centre Research for Geosciences Centre for Geosciences Keywords: Hydrothermal systems, geothermal energy, permeability, fluid flow, ore deposits. 1. Introduction Heat transport in the upper crust is governed by thermal conduction mainly through the solid rock mass and by heat advection with fluids flowing through the pore space [1]. The average system-scale permeability of the rock has a major influence on which of these processes dominates, eventually governing the temperature profile of natural geothermal systems as an expression of the convecting fluid s specific enthalpy [2,3]. Fluids in high-enthalpy systems typically have specific enthalpies near * Corresponding author. Tel.: ; fax: address: philipp.weis@erdw.ethz.ch The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of the GFZ German Research Centre for Geosciences doi: /j.egypro

2 Philipp Weis and Thomas Driesner / Energy Procedia 40 ( 2013 ) MJ/kg (Fig.1). Supercritical systems, such as the one tapped at the IDDP-1 well at Krafla, Iceland, may reach enthalpies in the order of 3 MJ/kg [4]. In an attempt to increase energy production by up to an order of magnitude compared to conventional high-enthalpy systems, this well targeted supercritical conditions in the vicinity of a high-enthalpy system in order to tap an unconventional geothermal reservoir. In 2009, the drills hit rhyolitic magma at about 2 km depth, presumably a solidifying dike associated with magmatic activity at Krafla volcano [5]. Subsequently, dry superheated steam (450 C) could be produced from the well [6]. Flow tests indicated that a production of about 35 MW electricity from the well would be possible [6]. Fig. 1. Special enthalpies of geothermal systems. The black arrows schematically show ideal isenthalpic fluid ascents. A fluid with a specific enthalpy of 1.5 MJ/kg rises as a single-phase ( supercritical ) fluid and reaches the boiling curve at a fluid pressure of about 10 MPa (typically in 1 to 1.5 km depth), while isenthalpic rise of a 3 MJ/kg fluid would produce a so-called superheated steam when rising towards the surface. Isotherms (grey dotted lines) are given in degrees C. 2. The interplay of non-static permeability and fluid flow Given that geothermal up-flow is normally considered to represent isenthalpic convection, the difference in specific enthalpy is too large for supercritical resources simply being the feeder zones of high-enthalpy systems. Rather, a hydrologic divide is required that at least partially separates the two regimes from each other. Here, we present new insights into these processes from our recent numerical simulations of magmatic-hydrothermal systems where such a hydrologic divide proved to be crucial to provide a mechanism for metal enrichment to economic grades explaining the formation of porphyrycopper ore shells [7]. These deposits form within magmatic-hydrothermal systems underneath a volcano and can be regarded as fossil analogues of some types of active high-enthalpy geothermal systems.

3 104 Philipp Weis and Thomas Driesner / Energy Procedia 40 ( 2013 ) Porphyry copper systems The numerical process model used for simulating the formation of porphyry-copper deposits is an implementation of the CSMP++ modelling software solving for heat and mass advection and heat conduction in a continuum, porous medium approach [7]. The model configuration has been kept as generic and simple as possible in order to be able to identify first-order processes that control the ore formation. The state and characteristics of the hydrothermal system during ore deposition are constrained by numerous field and fluid inclusion studies [8]. Only after the numerical model considered dynamic permeability feedbacks mimicking hydraulic fracturing and the transition from brittle to ductile rock behaviour in a geologically reasonable way, a hydrothermal system explaining all of these constraints developed [7]. To this end, we assembled a dynamic permeability module from existing permeability models with the following hydrological key parameters: Permeability has a depth-dependent profile characteristic for tectonically active crust [9]. This profile is linked to a critically stressed brittle crust [10] resulting in a failure criterion at nearhydrostatic fluid pressures [11]. Permeability decreases and stress state relaxes at high temperatures describing an increase in ductile behaviour of the rock [12], leading to a near-lithostatic failure criterion [11]. We assume a brittleductile transition within the temperature interval between 370 C and 500 C [2]. If fluid pressure exceeds a stress-state dependent failure criterion [11] permeability temporarily increases to values of up to two orders of magnitude higher [9] until fluid overpressure is released. With this module in place, ascending volatiles, expelled from a cooling upper crustal magma chamber, establish a hot fluid plume under near-lithostatic pressures that is surrounded and cooled by convection of colder meteoric fluids (Fig. 2, left). Copper enrichment to economic grades can be inferred at a hydrologic divide separating these two flow regimes, where fluid pressure and temperature drop abruptly within about 200m only Supercritical geothermal systems The physical principles, flow rates and permeability and depth ranges of our simulations of porphyry copper systems are identical or comparable to conventional natural and enhanced geothermal systems [7,13], but temperatures are considerably higher, such as in supercritical geothermal systems. The expression supercritical relates to the critical point of pure water and is slightly ill-termed in this context because the two-phase vapour-liquid coexistence field widens dramatically towards higher temperatures and pressures when adding salt (NaCl) as in our porphyry simulations [14]. The same permeability model describing a gradual change from brittle to ductile rock behaviour [12] inferred for the concept of targeting supercritical geothermal systems at Krafla volcano [4] is one of the key parameters in our porphyry model for the self-organization of the hydrologic divide. We assumed a different strain rate for our simulations, leading to a different temperature interval for the brittle-ductile transition (in [4] referred to as semi-brittle conditions). Despite these differences, our study can still provide valuable insights into how this permeability model behaves when integrated within a fluid flow model. Plotting the temperature profile of our porphyry simulations onto the schematic diagram of [4] shows that the targeted supercritical conditions can indeed be reached or rather even exceeded (Fig. 2, right), albeit only below the hydrologic divide that self-organizes as an interplay of non-static permeability and the two flow regimes (magmatic and meteoric).

4 Philipp Weis and Thomas Driesner / Energy Procedia 40 ( 2013 ) Fig. 2. Transferring insights of the hydrology of porphyry-copper systems [7] (left) to unconventional geothermal systems at supercritical conditions [4] (right). The left panel shows a snapshot of a porphyry system after 5,000 years of simulation time (figure modified from [7]): magmatic volatiles expelled from a magma chamber ascend through the overlying host rock (black arrow). The system is cooled by convection of meteoric water (grey arrows). Isotherms are plotted in grey in degrees C; black lines show the pore fluid factor (fluid pressure divided by lithostatic pressure) with the values 0.4 and 1.0 outlining the hydrologic divide between the hot, near-lithostically pressured and the colder, near-hydrostatically pressured flow regimes. The grey area at the tip of the divide outlines the onset of copper enrichment. The right panel shows a schematic profile of targeted supercritical geothermal resources (grey oval). This figure is taken and modified from [4] with the temperatures of the left panel s centre line plotted on top of it with E representing the cupola and injection point of magmatic fluids and F and G showing the upper and lower bounds of the brittleductile transition. Points A to D are redrawn from [4] and show typical isenthalpic rise (B to A) as in figure 1, the critical point of pure water (C) and the temperature rise at the bottom of high-enthalpy geothermal systems (B to D) Potential constraints A number of characteristics of porphyry systems may indicate some constraints for supercritical geothermal systems: The area at supercritical conditions is dominated by magmatic volatiles where fluid pressures are high enough to create sufficient permeability in rock that otherwise behaves in a ductile manner [7]. The simulations relate copper enrichment to a stable, sharp hydrological front with a drastic drop in pressure and temperature [7]. Porphyry-copper systems are spatially and temporarily associated with felsic dike intrusions [8]. These observations may indicate that supercritical geothermal systems may develop in conjunction with increased magmatic activity. Magmatic volatile degassing may be crucial to develop a region at elevated temperatures and dike emplacements might be a common feature for these systems. The targeted semi-brittle conditions [4] may only exist within a very narrow region and form a sharp hydrologic divide with predominantly ductile rock underneath which might impose additional engineering challenges. The

5 106 Philipp Weis and Thomas Driesner / Energy Procedia 40 ( 2013 ) earthquakes recorded at Reykjanes, Hengill and Krafla may as well represent some non-static permeability feedbacks where overpressured (magmatic) fluids episodically manage to break through otherwise ductile rock rather than representing a steady region under semi-brittle conditions [4]. As these systems are located in a mid-ocean ridge basaltic setting where magmatic fluid release is expected to be lower than in supra-subduction systems forming porphyry copper deposits, additional hydrologic factors may be involved that are so far not covered by our model (e.g., host rock alteration, effects of volcanic stratigraphy etc.). 3. Conclusions The simulations of fossil porphyry-copper systems cannot be transferred one to one to the active hydrothermal system at Krafla volcano: after all, there s no indication that an ore deposit is being formed there today. However, the physical and geological processes of both systems are very closely related. The intimate interplay of dynamic permeability responses to temperature and pressure conditions invoked by fluid flow may therefore also be a crucial pre-requisite to describe unconventional geothermal systems at supercritical conditions. References [1] Ingebritsen SE, Sanford WE, Neuzil CE. Groundwater in geologic processes. 2nd ed. Cambridge University Press; [2] Hayba DO, Ingebritsen SE. Multiphase groundwater flow near cooling plutons. J Geophys Res 1997; 102: [3] Driesner T, Geiger S. Numerical simulation of multiphase fluid flow in hydrothermal systems. In: Liebscher A, Heinrich CA, editors. Fluid-Fluid Interactions, Reviews in Mineralogy, 65, Mineralogical Soc Amer; 2007, [4] Fridleifsson GO, Elders WA. The Iceland Deep Drilling Project: a search for deep unconventional geothermal resources. Geothermics 2005; 34: [5] Elders WA et al. Origin of a rhyolite that intruded a geothermal well while drilling at the Krafla volcano, Iceland. Geology 2011; 39: [6] Elders WA, Fridleifsson GO, Bignall, G. Report of an IDDP-ICDP Workshop to Plan a 5 km deep borehole (IDDP-2) into the Root Zone of an Analog to a Black Smoker on Land at Reykjanes, Iceland. SAGA Report 2012; 9: 33p. [7] Weis P, Driesner T, Heinrich CA. Porphyry-Copper Ore Shells Form at Stable Pressure-Temperature Fronts Within Dynamic Fluid Plumes. Science 2012; 338: [8] Sillitoe RH. Porphyry copper systems. Econ Geol 2010; 105: [9] Ingebritsen SE, Manning CE. Permeability of the continental crust: dynamic variations inferred from seismicity and metamorphism. Geofluids 2010; 10: [10] Zoback MD, Townend J, Grollimund B. Steady-state failure equilibrium and deformation of intraplate lithosphere. Int Geol Rev 2002; 44: [11] Cox SF. The application of failure mode diagrams for exploring the roles of fluid pressure and stress states in controlling styles of fracture-controlled permeability enhancement in faults and shear zones. Geofluids 2010; 10: [12] Fournier RO. Hydrothermal processes related to movement of fluid from plastic into brittle rock in the magmaticepithermal environment. Econ Geol 1999; 94: [13] Ingebritsen SE. Modeling the Formation of Porphyry-Copper Ores. Science 2012; 338: [14] Driesner T, Heinrich, CA. The system H2O-NaCl. Part I: Correlation formulae for phase relations in temperature-pressurecomposition space from 0 to 1000 degrees C, 0 to 5000 bar, and 0 to 1 X-NaCl. Geochim. Cosmochim. Acta. 2007; 71: