Field Measurement of Fracture/Matrix Heat Exchange using Fiber Optic Distributed Temperature Sensing

Adam J. Hawkins and Matthew W. Becker, Department of Geological Sciences, California State University, Long BeachAdam J. Hawkins and Matthew W. Becker, Department of Geological Sciences, California State University, Long BeachGeorgios P. Tsoflias, Department of Geology, University of Kansas Georgios P. Tsoflias, Department of Geology, University of Kansas

IntroductionIntroduction

California State University

Long Beach

AbstractAbstractHighly channelized flow in fractured geologic systems has been blamed for early thermal breakthrough and poor performance of geothermal circulation systems. An experiment is presented in which the effect of channelized flow on fluid/rock heat transfer is measured. Hot water was circulated between two wells (7-14 m separation) completed in a single bedding plane fracture. The elevation of rock matrix temperature was measured using Fiber Optic Distributed Temperature Sensing (DTS). Between wells with good hydraulic connection, heat transfer followed a classic dipole sweep pattern. Between wells with poor hydraulic connection, heat transfer was skewed toward apparent regions of higher transmissivity (or larger aperture). Heat transfer between fracture and matrix was compared with saline tracer circulated between the same wells. Saline distribution was imaged using surface Ground Penetrating Radar. The results suggest that flow channeling can have a significant impact on heat transfer efficiency even in single bedding plane fractures

In geothermal reservoirs characterized by tight interconnected fractures, water flows unevenly in response to injection and pumping (Figure 1). Uneven flow or flow “channeling” can lead to sudden declines in production well temperatures, known as “premature thermal breakthrough” and, therefore, is a critical design parameter for effective geothermal reservoirs. Predictions of thermal breakthrough that assume constant permeability may provide inaccurate results in the presence of flow channeling, because it minimizes the surface area available for heat exchange and thus shortens the time to thermal breakthrough.

This research was funded by grants from the Department of Energy Geothermal Technologies Program (DE-EE0002767).

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MethodMethod

ResultsResults

SummarySummary

Hot water (40 ºC) was circulated through a single sub-horizontal bedding plane fracture while temperature was recorded in the extraction well and heat exchange was recorded in the sandstone rock matrix using Fiber Optic Distributed Temperature Sensing (DTS) (Figure 2). Two experiments (Test 1 and Test 2) were conducted with two different well pairs. Heat exchange in the rock matrix (Figures 3 & 4) as well as temperature rise in the pumping well (Figure 5) was recorded in each test.

DTS is a relatively new means of thermometry that has high thermal, temporal, and spatial resolution. Distances of up to 5 kilometers can be measured with thermal and spatial resolutions up to 0.01 ºC and 1 m, respectively. Spatial resolution was increased for this experiment by wrapping fiber optics around a threaded PVC pipe with a resulting spatial resolution of 2.1 cm.

• Heat diffusion into the rock matrix was monitored by the thermal sensor rods (Figures 3 &4)

• Test 1 recorded temperature rise in the production well after 3.23 hours of circulation at 4 L/min. At 8 L/min temperature rose in the production well after 43 min (Figure 5).

• Test 2 recorded temperature rise in the production well after 24 min of circulation at 8 L/min (Figure 5).

• Test 2 resulted in a breakthrough time nearly half that of Test 1 at an equal flow rate.

• A comparison of matrix temperatures to a GPR imaged saline tracer survey (Tsoflias et al. NS51B-1834) suggest flow channelization led to anisotropic heat exchange and thermal breakthrough (Figure 6).

• Results of Test 1 concur with an analytic model of dual porosity heat transport (Figure 7).

Figure 1: Dipole flow pattern (left) compared to channelized flow (right). Surface area in a dipole pattern is significantly greater than channelized flow.

Figure 2:Cross-sectional view (left) and three-dimensional view (right) of the experimental setup.

Figure 3: Temperature change throughout the length of each borehole and throughout time is shown above. A map view of the well field is shown at the upper left corner of the figure. Boreholes that did not experience more than half a degree change are not shown in this figure.

Figure 4: Temperature rise throughout the height of all boreholes.

Figure 5: Production well thermal breakthrough for each test.

Figure 6: Comparison to GPR tracer test, Spatial distribution of heat exchange, and residence time from the analytical model (only Test 1) are shown for Test 1 (left) and Test 2 (right).

Figure 7:Comparison of predicted normalized temperature vs. observed normalized temperature for one borehole (b5). Residence time and Peclet number were minimized via the conjugate gradient method. Minimized residence time for five boreholes are shown in Figure 6.

• Flow channelization and circulation rates determine the onset of production well temperature rise.

• Heat exchange generally followed the path of tracer transport.

• Heat exchange and tracer transport exhibited strong anisotropy

• Fracture flow models using constant aperture may be overly simplistic at the local-scale.