Forced-gradient flow sustained by a geothermal well doublet in a porous-fissured reservoir is subject to a tracer test anew, following a significant augmentation of fluid turnover rates. The distinct aromatic sulfonates (N2S and P4S) used as tracers in the first (lower-rate) and the second (higher-rate) test are supposed to be transported conservatively, an thus similarly to each other under this reservoir’s in-situ conditions. Cumulative mass recovery for each tracer can be calculated based on its theoretical ‘single-passage’ signal, deconvolved from its measured signal (eliminating ‘redundant’ contributions from fluid recirculation; to account for flow-rate variability, we set up an ad-hoc deconvolution algorithm). From tracer sampling to date, CMR amounts to ~28% for P4S, and ~70(!)% for N2S – whose first 20-30(!)% mass amounts were swept through the reservoir under the lower-rate flow regime, and its subsequent amounts under the higher-rate regime, reaching 60-65(!)% by the time P4S was added. CMR values for N2S marked by an (!) cannot be told accurately due to short-term flow-meter (instrumental) failures during precisely this transition; not only these particular values for N2S, but its entire subsequent CMR calculation is impeded by the flow-meter data gap. (As a substitute, one may attempt to reconstruct the missing flow-rate data from geothermal power generation data, but here operator-provided information proved insufficient.) CMR for P4S exhibits a significantly lower growth rate than for N2S, even when plotted against cumulative fluid turnover, which should have compensated for flow-rate disparities. More strikingly yet, it shows a marked first-order discontinuity (tangent drop) after reaching ~20% (which would correspond to ~30% N2S after the same cumulative fluid turnover, counted since tracer injection). Since P4S was injected much later after the flow-meter failure, its tangent discontinuity in CMR stays independent upon those missing flow rate data. Four hypotheses which might explain P4S CMR findings are proposed: (i) P4S loss by some physico-chemical processes (maybe related to borehole cleaning / acidizing treatments), i. e., non-conservative tracer behavior? (ii) evidence for large induced fracture(s), or reservoir stimulation, thus stronger P4S dilution? (iii) reservoir compartmentalizing, thus P4S loss into some non-pay zone? (iv) last not least, might flow-rate records be flawed over a time interval significantly larger than originally believed? Accordingly, we consider and evaluate a couple of more specialized monitoring options that would allow to disambiguate (or refute) some induced-fracture or activated-fault or karst-window scenarios. To validate or infirm (i), we suggest to conduct endo-tracer push-pull (single-well dilution) tests during late-tailing stages of both N2S and P4S, allowing a direct comparison as to their possibly non-/conservative behavior. To corroborate or refute either (ii) or (iii), complementary knowledge from geophysical exploration, in-situ stress field characterization, and additional (hydraulic, thermal) data are needed. If all of (i) – (iv) can be excluded, one can still attempt to ‘reconstruct’ the correct flow-rate data based on holomorphicity requirements for the second tracer’s CMR curve. In any case, signal monitoring for both tracers would need to be extended by at least 16 months, counting from the second tracer’s CMR tangent drop (i. e., few months from now), and assuming the higher flow rate is kept approximately constant over the entire duration until the endo-tracer push-pull test, for which then a much slower rate is recommendable.

Press the Back button in your browser, or search again.

Copyright 2021, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.