Balancing reservoir creation and seismic hazard in enhanced geothermal systems

Geophysical Journal International Geophys. J. Int. (2014) 198, 1585–1598 GJI Seismology doi: 10.1093/gji/ggu221 Balancing reservoir creation and seismic hazard in enhanced geothermal systems V. Gischig,1 S. Wiemer1 and A. Alcolea2,∗ 1 Swiss Seismological Service, ETH Zürich, Switzerland. E-mail: Suisse AG, Zürich, Switzerland 2 Geo-Energie Accepted 2014 June 10. Received 2014 June 9; in original form 2013 December 17 Key words: Geomechanics; Permeability and porosity; Fracture and flow; Statistical seismology. 1 I N T RO D U C T I O N Hydraulic stimulation of reservoirs has become a standard technique for increasing the permeability of enhanced geothermal systems (EGS; Häring et al. 2008; Petty et al. 2013), hydrothermal systems (Majer et al. 2007) and oil and gas reservoirs (de Pater & Baisch 2011). During stimulation, fluid is injected into the rock mass at high pressure. The reduction in effective normal stress induces (i) slip along pre-existing fractures if frictional resistance is overcome (i.e. hydroshearing), and/or (ii) fracture opening if tensional strength is exceeded (i.e. hydrofracturing; Cladouhos et al. ∗ Now  C at: TK Consult AG, Zürich, Switzerland. 2013). For enhancing permeability hydroshearing tends to be more efficient, because shear-dilation associated with slip increases fracture aperture in an almost irreversible manner due to dislocation of asperities on the fracture walls. Hydroshearing is usually accompanied by micro-earthquakes because dynamic slip along fractures radiates seismic energy. Hence, induced seismicity is an essential ingredient for permeability enhancement, but the associated nuisance and potential seismic hazard and risk is a serious threat for EGS projects. A number of past projects involving fluid injection into a reservoir have been affected by the negative impact of seismicity (e.g. waste water injection (Ellsworth 2013; Keranen et al. 2013) or EGS projects Häring et al. (2008)). During EGS stimulation, most of the events are micro earthquakes of magnitude <2.5 that are usually not felt. However, because the size of induced The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society. 1585 SUMMARY Fracture shear-dilatancy is an essential process for enhancing the permeability of deep geothermal reservoirs, and is usually accompanied by the radiation of seismic waves. However, the hazard and risk perspective of induced seismicity research typically focuses only on the question of how to reduce the occurrence of induced earthquakes. Here we present a quantitative analysis of seismic hazard as a function of the two key factors defining an enhanced geothermal system: The permeability enhancement, and the size of the stimulated reservoir. Our model has two coupled components: (1) a pressure diffusion model and (2) a stochastic seismicity model. Permeability is increased in the source area of each induced earthquake depending on the amount of slip, which is determined by the magnitude. We show that the few largest earthquakes (i.e. 5–10 events with M ≥ 1.5) contribute more than half of the total reservoir stimulation. The results further indicate that planning and controlling of reservoir engineering operations may be compromised by the considerable variability of maximum observed magnitude, reservoir size, the Gutenberg–Richter b-value and Shapiro’s seismogenic index (i.e. a measure of seismic reactivity of a reservoir) that arises from the intrinsic stochastic nature of induced seismicity. We also find that injection volume has a large impact on both reservoir size and seismic hazard. Injection rate and injection scheme have a negligible effect. The impact of site-specific parameters on seismicity and reservoir properties is greater than that of the injected volume. In particular, conditions that lead to high b-values—possibly a low differential stress level—have a high impact on seismic hazard, but also reduce the efficiency of the stimulation in terms of permeability enhancement. Under such conditions, target reservoir permeability can still be achieved without reaching an unacceptable level of seismic hazard, if either the initial reservoir permeability is high or if several fractures are stimulated. The proposed methodology is a first step towards including induced seismic hazard analysis into the design of reservoir stimulation in a quantitative and robust manner. 1586 V. Gischig, S. Wiemer and A. Alcolea (1) What event magnitudes are most efficient in enhancing permeability? (2) How predictable are seismic hazard and target reservoir size, given the uncertainties in our knowledge of stress distribution and fault locations, and the inherent randomness of the processes controlling stimulation? (3) What level of induced seismic hazard should be anticipated for a certain target size of the enhanced reservoir? (4) What conditions lead to successful permeability enhancement without exceeding predefined hazard seismic levels? 2 METHOD Our modelling approach consists of two components: (1) a fluid flow model that computes transient pressure disturbances and (2) a stochastic seismicity model that uses the transient pressure field as input to induce earthquakes at potential earthquake hypocenters (i.e. the so-called seed points). Our study is limited to 2-D. Thus, the model represents an equivalent continuum of a medium containing a single large fracture plane that accommodates all flow and slip, and that is optimally oriented in the ambient stress field. As we assume a strike-slip stress regime for all our models, the model represents a subvertical fracture that is penetrated by a subhorizontal borehole. The open-source code SUTRA is used as flow simulator (Voss & Provost 2010). The stochastic seismicity model is presented by Goertz-Allmann & Wiemer (2013) and Gischig & Wiemer (2013). These two basic components are explained in detail below. 2.1 The fluid flow model SUTRA (Saturated-Unsaturated TRAnsport; Voss & Provost 2010) is a finite element code that solves the fluid flow as well as the solute or energy transport equations. For the purpose of this study, we simplified the code to only solve for pressure diffusion (i.e. fluid mass balance) in a saturated porous medium. The governing equation is:   ρk ∂p =∇· ∇ p + Q p, (1) ρ Sop ∂t μ where ρ (kg m−3 ) is fluid density, μ (Pa · s) dynamic viscosity, Sop (Pa−1 ) is specific pressure storativity, k (m2 ) the intrinsic permeability tensor, p (Pa) is pressure and Qp [kg (m3 · s)−1 ] the mass source of fluid per unit volume of the medium. The 2-D model presented here represents a single fracture surface or a fracture zone contained within the 2-D modelling domain. Since our model is an equivalent continuum representation of a fractured medium, it assumes that fracture connectivity is between sufficiently high to allow hydraulic communication in the entire 2-D domain. Although the fracture is assumed to be subvertical (...truncated)