| Keywords |
phase interface area, residual saturation, equilibrium, partitioning, reactive, sorption, thermo-sensitive, single-well, push-pull, SWIW, CCS, Heletz |
| Abstract |
A semi-analytical approximation to tracer signals is proposed for evaluating the chances of unambiguous tracer test interpretation for a CCS pilot site, given the multiple constraints imposed upon tracer test execution at any deep geological formation regarding maximum pressure buildup admissible, fluid turnover rate, injection-fluid availability and conditioning facilities, fluid disposal capacity, test duration, tracer quantities/species available, tracer metering costs, etc. At the Ketzin site in Germany, such constraints acted prohibitively towards conducting any tracer tests prior to CO2 injection. In contrast, at the Heletz site in Israel, a set of single-well (SW) and inter-well (IW) tracer tests to be conducted prior to CO2 injection is being regarded as indispensable for georeservoir characterization. An advantage of the proposed approach, expressing tracer signals in terms of fluid volume recovery fraction (rather than time), is its independence upon IW distance and upon fluid injection/production rates. It roughly predicts conservative- and sorptive-tracer signals, fluid mixing and outflow temperature evolution in IW and SW tests, for stratified reservoirs whose high- and low-permeability layers have thicknesses of same magnitude order. Thus, besides the CCS context, it can be extended to a range of geothermal applications, including some aquifer-dominated systems in the Upper Rhine Rift Valley, S-German Malm-Molasse Basin, N-German Sedimentary Basin, but excluding fracture-dominated (petrothermal) systems. IW tracer signals are sensitive to storage capacity (ε), but suffer from ambiguity between longitudinal (Pe) and vertical (Prf) heterogeneity, and from all-parameter ambiguity at early times. Pe-ε ambiguity vanishes with increasing observation time. SW signals carry all-time Pe-ε ambiguity. IW signal inversion can be freed from Pe-Prf ambiguity, exploiting SW signals’ insensitivity to Prf. In both IW and SW, Pe-ε ambiguity is resolvable by using sorptive tracers, and/or fluid temperature differences, alongside with conservative tracers. Generally, SW signals suffer from poor sensitivity towards rapid-equilibrium exchange of tracer between solid and/or fluid phases, and from ambiguity between such processes and plainly advective transport. This renders parameters like porosity, phase volume (saturation) and phase interface area difficult to invert from SW pull signals of conservative, equilibrium-partitioning or sorptive tracers. Two possible workarounds are discussed: (A) using the time-dependent, in-situ release of a second tracer, from the originally injected tracer, with contrasting partitioning or sorption properties; (B) exploiting the temperature dependence of sorption in conjunction with the injection (push) of a sufficient volume of cold water into a hot reservoir. The idea of (A) is inspired by Tomich et al. (1973), who used the in-situ release of an alcohol by hydrolysis from a injected ester, in order to make residual-oil saturation determinable from a SW push-pull test. Method (B) was analyzed in more detail by Ghergut et al. (2012). Parameter sensitivities in SW and IW tests using conservative, sorptive and partitioning-reactive tracers are illustrated with test design simulations for the CCS candidate site at Heletz (Israel), based on realistic stratigraphy data sets that were formulated for model benchmarking within the EU project MUSTANG (cf. Bensabat and Niemi, 2009). |