(Endo-)Tracer-Aided Visibility of ‘Nearby’ Fault Reactivation: Scoping Simulations, II

The effect of permeability windows (PW) within a large-scale fault on the advective-dispersive, matrix-diffusive transport of a generic non-reactive solute (conservative endo-tracer) from a neighboring, stimulated deep reservoir towards a shallow aquifer is explored by numerically simulating a set of worst-case hydrogeological scenarios. A generic conceptual model comprising reservoir, basement, cap rock, aquifer layers, adjacent to or intersected by a sub-vertical normal fault underlies all scenarios. The large-scale fault is represented as a tight core flanked by relatively permeable damage zones. Within core and/or damage zones, PW (putatively associated with fault reactivation by pore pressure increase during unconventional reservoir stimulation) are assumed; their location, vertical extent and physical properties are varied between scenarios. Somewhat counter-intuitively, PW augmenting is not found to increase the overall solute freight to the aquifer, but, in some cases, solute plumes are found to redistribute from the reservoir-adjacent to the opposite side of the fault zone. Bearings on aquifer monitoring system design, and on some current research projects in the ‘georeservoirs’ realm are briefly discussed. Our simulation results should not be mistaken for a prediction of shallow aquifer contamination induced by HF operations. Rather, we are more interested to see whether (endo-)tracer signals detectable by shallow sampling can be used for faultzone characterization at depth, or for telling the transport properties of at least some of its compartments. This would nicely complement Shapiro’s (2015) agenda of using induced (micro)seismicity to infer hydraulic properties of the subsurface (with a prominent example at the KTB site), since inverting hydraulic parameters from seismicity and/or from pressure signals (McDermott et al. 2006) leaves fluid transport parameters yet undetermined (Ghergut et al. 2007). This hidden agenda has let us select and combine hydrostratigraphy, faultzone and PW property ranges yielding ‘non-zero’ (i. e., instrumentally detectable) tracer shallow signals for the scenarios’ most part, for which we were willing to put up with unrealistically high upward fluxes (‘ultra-conservative’ scenarios, in the sense of Sauter and Lange 2012). Thus it’s more about ‘seeing’ the fault zones, than about ‘seeing’ the tracers. Furthermore, at research sites where largescale fault systems are not the ‘undesired, hazardous’ feature, but the very target of a forced-gradient fluid-turnover based exploration (like the KTB and Horstberg sites in Germany, and certain spots beneath the Molasse basin), tracers are expected to convey the fluid transport properties that could not be told from geophysical and hydraulic signals. On the other hand, we coined the term ‘endo-tracers’ – always in conjunction with a georeservoir-typical hydraulic operation sequence on a (more or less well-characterized, maybe still largely unexplored) georeservoir – in order to flexibly denote some either naturally-present, or artificially-introduced, or a combination of both kinds of tracer species, whose subsurface transport (within/to/from the georeservoir and its neighboring geological formations) is subject only to the forced gradients implied by that georeservoir-typical operation sequence (e. g., massive fluid injection for the purpose of georeservoir stimulation, followed by injection and/or production of a possibly different kind of fluid). – I. e. , we are not referring to a purportedly conducted tracer test with its well-tailored design, usually involving some forced-gradients added, and its well-tailored sizing of tracer quantities, added such as to ensure optimum detection, metering, and inversion of tracer signals. Thus the endo-tracer signals, as generated without further intervention, may (if at all detectable and quantifiable) be quite far from optimal for the purposes of georeservoir and/or process characterization; whereas the endeavor to optimize their use – like in tracer tests’ best practice – would have required considerable hydraulic deviations, at odds with the georeservoir’s economic use. 1. ENDO-TRACERS INTO LARGE-SCALE FAULT ZONES: SHALL WE SEE THEM? (PURPOSE OF THIS STUDY) Hydraulic fracturing (HF) has been practiced for many decades – in various, more or less similar forms, depending on georeservoir play type. Concerns about environmental hazards associated with what is nowadays deemed ‘unconventional’ reservoir development technologies (i. e., HF in its various technical formulations) only became acute over the past decade, with an almost exclusive focus on shale-gas reservoir development – from where the same concerns are sometimes simply replicated to the ‘enhanced geothermal systems’ (EGS) realm, implicitly assuming a kind of natural equivalence between ‘enhanced’ or ‘stimulated’, and ‘unconventional’. Most concerns (Myers 2012; Warner et al. 2012; Vengosh et al. 2014; Reagan et al. 2015; Sherwood et al. 2016) revolve around the prospect of shallow aquifers suffering from contamination by HF fluid (HFF) constituents, and/or by reservoir gas, or brine, possibly enriched in some undesired minerals etc., mobilized (Schwartz 2015) from the reservoir matrix along with its permeability stimulation. This (more or less theoretical) prospect may further be aggravated by assuming HF-related processes – putatively, fault reactivation by virtue of pore pressure increase – can induce new, or augment pre-existing migration pathways, e. g. , permeability ‘windows’ (PW) between the stimulated reservoir and a shallow aquifer. Interestingly, concerns about enhanced fluid and solute migration following fault reactivation (cf. ExxonMobil 2012) appear to be disproportionately strong in Germany, compared to world regions naturally more prone to such hazards – likely nourished by vivid memory (Zoback and Gorelick 2012; Scheer et al. 2017; Kissinger et al. 2017) of controversies on CCS in the years preceding the ‘fracking’ debate, and/or of the Preese Hall (Bowland shale) 2011 ‘incident’ (Clarke et al. 2014).