TRACER TEST ANALYSIS FOR CHARACTERIZATION OF LAUGALAND GEOTHERMAL FIELD
This paper presents the tracer data analysis obtained from the single-phase, conservative tracer experiments conducted during a two-year reinjection project at Laugaland geothermal field in N. Iceland. Three tracer tests were conducted during the two-year period (1997-1999). The tracers injected were sodium fluorescein in Test 1 and Test 3 and potassium iodide in Test 2. We concentrated on Test 1 only, the reasons for which are explained in the paper. Axelsson et al. (2000; 2001) have conducted a comprehensive analysis of the Laugaland reinjection project tracer test data, including both qualitative and quantitative aspects using multi-flow-channel models. In this study, we focus only on the quantitative analysis of tracer test data using analytical interpretation methods . It has been proved extensively that the first temporal moment analysis leads to an estimation of the tracer swept pore volume in a closed boundary with a balanced injection/production rate scenario. But in this paper, we prove that the first temporal moment does give the swept pore volume, even for open boundaries with an unbalanced rate scenario such as existed at the Laugaland geothermal field. The produced tracer concentration history was used to estimate the interwell reservoir relative flow capacity versus storage capacity (F-C plot). The F-C plot is a simple and powerful semi-quantitative tool to evaluate the conceptual model’s relative flow and storage geometries. The relative flow and storage capacities calculated from the flow channel geometry obtained from multi-flow-channel modeling (Axelsson et al. 2001) for one production well LN-12 agreed well with the continuous F-C plot obtained from the tracer return data. The second temporal moment of tracer production history was used to derive an equivalent dispersion coefficient. The reservoir interwell heterogeneity was characterized using the Lorenz coefficient derived from the F-C plot. The Lorenz coefficient for well pair LJ-08/LN-12 was 0.509, indicating that it is moderately heterogenous. Reservoir information was obtained for interwell LJ-08/LN-12 and, to some extent, LJ-08/TN-04. The tracer data for other wells could not be analyzed, since the streamlines were not constant, and deconvolution techniques need to be developed to do meaningful interpretation. Recommendations for possible future Laugaland reservoir modeling are offered in order to obtain a better description of the reservoir.
Analysis of tracer test data, and injection-induced cooling, in the Laugaland geothermal field, N-Iceland
Abstract The Laugaland geothermal system in N-Iceland is hosted by low-permeability fractured basalt and its productivity is limited by insufficient recharge, even though substantial thermal energy is in-place in the 90–100 °C hot rocks of the system. The purpose of a 2-year reinjection experiment, completed in late 1999, was to demonstrate that some of this energy could be extracted economically through long-term reinjection. A comprehensive monitoring program was implemented as part of the project, including three detailed tracer tests. More than 1400 tracer samples were collected during the tests. Tracer return data indicate that the injected water travels through the area bedrock by two modes: first, along direct, small volume flow-paths, such as fractures or interbeds; second, by dispersion and mixing throughout a large volume of the reservoir. Based on the tracer test results, and assuming 15 l/s average future reinjection, the temperature of water produced is predicted to decline by 1–3 °C in 10 years. It can be asserted, in spite of measurement uncertainties, that the 2-year reinjection experiment did not cause a temperature decline greater than about 0.5 °C, conforming to predictions. It is estimated that future reinjection at 15 l/s will enable an increase in energy production amounting to about 24 GWh th /year, which equals roughly 1 4 of the average yearly energy production at Laugaland during the last decade. Reinjection has continued after the experiment and is already an important part of the management of the Laugaland geothermal system.
Energy extraction from fractured geothermal reservoirs in low‐permeability crystalline rock
Two hot dry rock geothermal energy reservoirs were created by hydraulic fracturing of Precambrian granitic rock on the west flank of the Valles Caldera, a dormant volcanic complex, in the Jemez Mountains of northern New Mexico. Heat was extracted in a closed-loop mode of operation, injecting water into one well and extracting the heated water from a separate production well. The first reservoir was produced by fracturing the injection well at a depth of 2.75 km, where the indigenous rock temperature was 185°C. The relatively rapid decline in temperature of the water produced from the first reservoir, 100°C in 74 days, indicated an effective fracture radius of about 60 m with an average thermal power extracted of 4 MW. A second, larger reservoir was created by refracturing the injection well 180 m deeper. Downhole measurements of water temperature at the reservoir outlet as well as temperatures inferred from chemical geothermometry showed that the thermal decline of this reservoir was negligible; the effective heat transfer area of the new reservoir must be at least 45,000 m2, nearly 6 times larger than the first reservoir. In addition, reservoir residence time studies employing visible dye tracers indicated that the mean volume of the second reservoir is 9 times larger. Other measurements showed that flow impedances were low and that downhole water losses from these reservoirs should be manageable. The geochemistry of the produced water was essentially benign, with no scaling problems apparent. Moreover, the level of induced seismic activity was insignificantly small.