Interwell Tracer Analyses of a Hydraulically Fractured Granitic Geothermal Reservoir

Field experiments with fluorescent dye and radioactive tracers (Br(82) and I(131)) have been employed to characterize a hot, low-matrix-permeability, hydraulically fractured granitic reservoir at depths of 2440 to 2960 m (8,000 to 9,700 ft). Tracer profiles and residence time distributions (RTD's) have been used to estimate sweep efficiencies and fracture volumes and to diagnose normal and pathological flow behavior both in injection and in production wells. The effectiveness of one- and two-dimensional (1- and 2D) theoretical dispersion models using single and multiple porous, fractured zones with velocity- and formation-dependent effects is discussed with respect to actual field data. Introduction Field tests of a hot dry rock (HDR) geothermal reservoir are being conducted by the Los Alamos Natl. Laboratory at the Fenton Hill site near the Valles Caldera in the Jemez Mountains of north central New Mexico. We are exploring the creation of artificial permeability in otherwise impermeable crystalline rock by hydraulic pressurization of the formation. A two-well (injector and producer) system allows for closed-loop circulation of pressurized water through the fractured reservoir. Heat extraction rates are controlled by the rate of thermal conduction through the rock surrounding fluid-filled fractures. Extensive testing of this hydraulically fractured reservoir in low-matrix-permeability granite at 150 to 200 degrees C (302 to 392 degrees F) and at depths of 2 to 3 km (6,500 to 10,000 ft) has been conducted during the past 5 years to characterize fracture initiation and propagation, flow distribution and impedance, reservoir size, fluid loss caused by permeation, geochemistry, and induced seismic effects. The results of several major tests where heat was extracted at rates ranging from 1 to 5 MW(t) are discussed in previous papers (Murphy and Tester, Tester and Albright, Murphy et al., and Zyvoloski et al).This paper focuses on the use of tracer techniques to characterize flow distributions in geothermal reservoirs, particularly in injection and production zones near wellbores and within the fractures themselves. Flow fractions, fracture volumes, RTD's, and the degree of fluid mixing within the fractured region can be determined by suitable tracer methods. For example, wellbore data from temperature, spinner, caliper, and borehole televiewer logs were used in conjunction with I(131) and Br(82) tracer logs under fluid injection and production conditions to construct a consistent geometric model to account for normal and pathological flow behavior within the fractured reservoir, behind casing, and at various borehole-to-fracture connections. In addition. because of the relatively small volume of the combined wellbore and fracture system [160 m (40,000 gal)], flow-through residence times were short, so repeated tests could be run. RTD's were determined in response to a tracer pulse injected into Well EE-1 and produced in Well GT-2, which was connected directly to the fractured region.The RTD provides a direct measure of the mean reservoir size and of the distribution of flow velocities in the connected system. As Wagner, Wagner et al., and Ogata point out, RTD tracer techniques can be very useful to the reservoir engineer in characterizing complex flow systems. For example. for our particular application to fractured HDR geothermal systems. tracer techniques were used to identify injection and production zone profiles and well casing cement integrity. Furthermore, the tracer-determined flow distributions and fracture volumes can be related to effective heat transfer areas. This is particularly useful in estimating thermal capacities and production lifetimes of actual HDR geothermal reservoirs. SPEJ P. 537^