Several tracer injection campaigns were completed at the testbed from October 2018 to November 2019. Non-reactive (conservative), reactive (sorbing), and thermally degrading tracers were injected, either exclusively or in combination, and their recovery rates from multiple producers were collected and analyzed. Each tracer test was injected at the 164-notch and targeted characterization of hydraulic fractures that might have connected to natural fractures providing flow paths to E1-P, several monitoring holes, and drift E1-I.
The activities to create hydraulic fracture(s) at 164-notch were commenced during May 22-25, 2018. Previously, we showed that the hydraulic fracture(s), two natural fracture zones, (OT-P Connector and PDT-OT Connector), E1-P, and at least 3 monitoring holes (E1-OT, E1-PST, and E1-PDT) were involved in defining dominant flow paths at one time or the other during the flow tests (Figure 2). Also, the Shallow fracture zone/weep depicted in Figure 2 indirectly served as a flow path during flow test at 164-notch. After the top section of the E1-OT was sealed, water leaking to this hole at depth moved up and diverted into the Shallow fracture zone, and ultimately seeped to the drift as a weep near E1-P.
All tracers were conducted with a nominal injection rate of 400 mL/min. Starting late October 2018, the testbed was under a constant flow regime with some interruptions. When there was an interruption in injection prior to a tracer test, a steady flow regime was established before injection of tracer solution. Starting early May 2019, the testbed was subjected to a chilled water injection although there may have been some brief durations where regular mine water was injected due to mechanical failures of the chiller(s).
Table 1. Tracer Injection Dates, Types, and Recovery Location
During early tracer tests, a large volume of relatively low-strength tracer solution was injected using a Quizix pump. This injection mechanism was initially modified by directly pulling a higher strength shear tracer solution into one of the cylinders of the Quizix pump or pulling the tracer solution in the injection line hose. Finally, a dedicated ISCO pump was installed and used to push tracer solutions directly into the main injection flow line. With the exception of the early October 2018 tests, a 5-minute injection pulse of 100 mL high-strength tracer solution was transmitted at a rate of 20 mL/min into the main flow line. Immediately after the injection of the tracer solution, 100 mL of clean water was injected using the same injection mechanism (20 mL/min) as chase water. During the tracer and chase water injection period, the main injection flow rate was 420 mL/min. The chase water was not injected during the last two tracer tests (October 22, 2019 and November 19, 2019). In these tests, the ISCO cylinder was rinsed and the rinse water was collected and analyzed for tracer that was stuck in the ISCO pump cylinder and connecting tubes. This mass was used in tracer injection-recovery mass balance equations.
Fig. 2. Natural fracture zones (or weeps) in the testbed. At least five fracture zones or weeps are identified in the testbed.
A suite of tracer compounds, including both conservative and sorbing tracers, was used for the tracer work in the testbed. Earlier tests included different strains of synthetic DNA; however, use of synthetic DNA was discontinued in later tests. In general, all tracer cocktail solutions were prepared so that they should include at least one fluorescing tracer for near real-time detection in the drift. Rhodamine-B, fluorescein, C-dot, and phenol acetate were used as fluorescing tracers. The ability to detect tracer in the drift not only provided real-time analysis of the tracer breakthrough data at multiple producers but also helped in modifying the sampling strategy and experiment operation in real time.
The C-dot is a nanoparticle (3-5 nm in diameter) tracer that consists of a carbon core decorated with a highly fluorescent polymer. The phenol acetate was used as a thermally degrading tracer, with phenol as a degrading product. In many tracer tests, the tracer cocktails also included solute tracers such as Cl, Br, and K (Table 1).
Immediately after injection of tracer solutions, high frequency samples were collected from major producers (Table 2). The E1-P (production well) below the lower packer (PB) and E1-P interval (PI) flowed consistently during all tracer tests and was sampled for tracer recovery. However, some leaky monitoring wells (e.g., E1-OT, E1-PST, and E1-PDT) flowed intermittently. For example, a leak on E1-OT was significant during the 2018 tests. Repairs at the end of 2018 fixed this issue such that OT flow was not significant during the 2019 tests. All producers with significant flow were regularly sampled and analyzed for tracer recovery.
We adopted two methods to collect liquid samples from producers for analyses. The first method was manually collecting 'grab' samples in 10 mL amber sampling tubes. Typically, these samples were each collected in less than a minute, and were subsequently capped and labeled with the collection location and sampling time. Immediately after the collection of these samples, the fluorescing tracer included in that particular tracer injection cocktail was analyzed in the drift. When phenol acetate was also included in the tracer cocktail solution along with C-dot, the analysis of the thermally degrading (kinetically controlled) product was conducted first, followed by the analysis of the co-injected C-dot.
The second method of sampling was using fraction collectors to collect samples from the three locations. The fraction collectors were set to advance at a specified time interval and a peristaltic pump was used to control the rate of sampling such that the 10 mL sampling tube would be filled during the sampling interval. Unlike the grab samples, which represent a sample concentration at the moment of sampling, the fraction collector samples represent an integrated sample concentration over the sampling interval.
Table 2. Injection and Outflow Rates of Producers During 2019 Tracer Tests
All fluorescing tracers [C-dots, fluorescein, rhodamine-B, and phenyl acetate (phenol as degrading product)] were analyzed using an Ocean Optic spectrophotometer system (Ocean Optic FIA-SMA-FL-UTL cell, PX-2 pulsed xenon lamp, QEPRO spectrophotometer). For each fluorescing tracer, the optimum excitation and emission radiation wavelength was identified with a tracer solution prepared in background water (pre-tracer injection PI water). For each sample, three scans of average fluorescence count were recorded with a 5 to 30 second integration time for each scan. Background fluorescence values were established for each producer and corrected either during analysis (for PI samples) or during data processing. Prior to each analysis, a series of calibration standard solutions of the target analyte were prepared using the background (PI) water for construction of the calibration curve. Consequently, the recorded fluorescence count of each sample was converted to tracer concentration using the relationship between fluorescence counts and concentration as illustrated in Figure 3.
Table 3: Excitation and Emission Wavelengths
For solute tracers (Cl, Br, K, etc.), all time-stamped samples were shipped to a laboratory for analysis with ion chromatography (IC) and inductively coupled plasma optical emission spectroscopy (OCP-OES). Some of the samples were analyzed at the Center for Advanced Energy Studies (CAES) in Idaho Falls, ID whereas some additional samples are currently being analyzed at LLNL in Livermore, CA.
Fig. 3. C-dots calibration curve constructed for July 24, 2019
Tracer Arrival Time Adjustment
Initial tracer detection at the production wells at the EGS Collab testbed occurred in less than an hour. Such a short duration test allowed us to account for all possible time delays associated with the injection and sampling. Universally for tracer tests and producers, one common time delay was the time taken by tracer pulse to get to the injection point (164-notch interval). Given the flow rate (400 mL/min) and length of the tube from the drift to the injection target, we found this time delay to be about 3 minutes.
Similarly, we also found the travel time for water from where it left the rock/fracture and entered the outflow drain tubes at depths (e.g., for PB and PI) to the sampling location on the drift. Since the outflow rates of both PB and PI varied over time, the time delays for PB and PI samples were different for different tests. For PB and PI, the out bound time delays ranged about 9 to 32 min and 6 to 65 min, respectively, and we adjusted respective time delays for each sample from each test. On the other hand, for outflowing monitoring wells (e.g., E1-OT, E1-PDT, E1-PST, etc.) the out-bound time delays were unknown (water travelled through the grouted hole with unknown flow channel length/volume) and such time delays were not adjusted. Finally, when the fraction collector was used to collect samples, the time delays associated with the travel time for water from the producer outlet to the drip point were also adjusted in the reported time. Unlike the grab sample with instantaneous sampling at an associated time stamp, the fraction collector collected water samples over a duration (usually a 20 minute duration). For these samples, a mid-interval time stamp was used as sampling time.