California State University
Long Beach
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 Beach
Georgios P. Tsoflias, Department of Geology, University of Kansas
Abstract
Highly 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
Results
• 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 4: Temperature rise throughout the height of all boreholes.
Introduction
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.
Figure 5: Production well thermal breakthrough for each test.
Figure 1: Dipole flow pattern (left) compared to channelized flow (right).
Surface area in a dipole pattern is significantly greater than channelized flow.
128 S
387 S
293 S
142 S
589 S
Method
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.
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.
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.
Summary
• 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
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.
• Fracture flow models using constant aperture may be overly simplistic at the
local-scale.
This research was funded by grants from the Department of Energy Geothermal Technologies
Program (DE-EE0002767).