CONTROL OF FRACTURE PROPAGATION
AND FLUID FLOW
Geophysical Monitoring of Subsurface Electrical
Transient Signals for Fracture Detection and
Characterization
John Pritchett
Senior Scientist, Leidos Inc.
Member, GEA Board of Directors
July 22, 2014
Washington, D.C.
CONTENTS OF TALK
1. Introduction to GEA and the geothermal industry.
2. Role of rock fractures in geothermal systems.
3. Natural vs. synthetic geothermal reservoirs.
4. Monitoring geothermal fracturing operations.
5. Applications to other processes and industries.
THE GEOTHERMAL ENERGY ASSOCIATION
(“GEA”)
• Trade association for the U.S. geothermal industry.
• Total membership, 112 companies (10 international).
• Offices: 209 Pennsylvania Ave SE, Washington DC.
• Mission: to represent the interests of the industry to
all levels of government (federal, state, and local).
Plate Tectonic Processes
Spreading Center
Oceanic
Continental
Plate
Plate
Convection
Plate Boundaries
“Ring of
Fire”
Geothermal Power Plants
NAMEPLATE GEOTHERMAL GENERATING
CAPACITIES IN MEGAWATTS, APRIL 2014
From the “2014 Annual U.S. and Global Geothermal Power Production Report”
by Benjamin Matek of GEA
WORLD:
UNITED STATES:
United States
Philippines
Indonesia
Mexico
Italy
New Zealand
Iceland
Japan
3442
1904
1333
1005
901
895
664
537
California
Nevada
Utah
Hawaii
Oregon
Idaho
New Mexico
Alaska
All others
1332
All others
TOTAL
12013
TOTAL
2711
566
73
38
33
16
4
1
0
3442
ESTIMATED TEMPERATURE @ 6 KM DEPTH
Map courtesy SMU Geothermal Laboratory, Dallas, TX
GEOTHERMAL’S CONTRIBUTION TO THE
ANNUAL AVERAGE ELECTRICAL SUPPLY
USA
WORLD
470 GW
2310 GW
Existing geothermal capacity 3.4 GW
(0.7%)
12 GW
(0.5%)
Electricity consumption
Geothermal’s potential with
no new technology
* U.S. Geological Survey
** Bloomberg New Energy Finance
30 GW*
(6.4%)
196 GW**
(8.5%)
U. S. COMMODITY MARKET VALUES
ONE BARREL OF CRUDE OIL:
ONE BARREL OF HOT BRINE:
RATIO: 644 : 1
$103.08 *
$ 0.16 **
* for West Texas Intermediate (WTI) crude on
July 18, 2014 per NASDAQ.
**contains 130 kg of pressurized 200°C brine;
generates 1.8 kWh of electricity at $90/MWh.
TYPICAL RESERVOIR GEOLOGY
• OIL AND GAS:
Soft sedimentary rocks (sandstones, shales,
sedimentary composites, etc.). Porosity usually >
10%, average rock permeability > ten millidarcies.
Intergranular permeability usually suffices to support
production > 200 bbl/day (1 ton/hour) of oil per well.
• GEOTHERMAL:
Volcanic/metamorphic hard rocks (basalts, andesites,
rhyolites, granites, etc.). Porosity usually < 3%,
average intact rock permeability ~ a few microdarcies.
Extensive natural fractures are essential to supply
commercial geothermal fluid production rates.
EARTHQUAKES, VOLCANOES AND
GEOTHERMAL PROJECT PROSPECTS
• Successful geothermal project locations are strongly
correlated with regions of intense tectonic activity
and/or volcanism.
• Such regions often have relatively high subsurface
temperatures at depths accessible by drilling.
• Equally important, ongoing tectonic activity serves to
create and maintain dense networks of natural rock
fractures which provide the fluid conduits required to
permit commercial geofluid production at rates
exceeding 55,000 bbl/day (300 tons/hour) per well.
SEEKING HEAT, NOT JUST FLUID
ESSENTIAL ATTRIBUTES OF A
SUCCESSFUL CONVENTIONAL
GEOTHERMAL PROJECT
•
•
•
•
•
•
Abundant natural heat flow and local tectonic activity.
High subsurface temperatures at accessible depths.
Large water/steam-filled fractured permeable reservoir.
Impermeable upper seal layer to prevent leak-off.
Nearby electricity market and/or grid connection.
Absence of endangered species, national parks, etc.
Simultaneous occurrences of all of these attributes
are rare, so successful geothermal projects are
sparse and geographically restricted.
ENHANCED GEOTHERMAL SYSTEMS (EGS)
• Emerging technology, still in feasibility-study stage.
• Approach - create synthetic fractured geothermal
reservoirs by artificial means, e.g. hydraulic fracturing
(“fracking” for geothermal).
• EGS should substantially increase geographic areas
where geothermal projects are feasible.
• Activities so far restricted to pilot studies and EGS
“augmentation” of conventional geothermal projects.
• Long-term objective - large-scale EGS “greenfield”
projects generating hundreds of megawatts apiece
distributed over large parts of the country.
THE NEED FOR FRACTURE MAPPING AND
CHARACTERIZATION
• Hydrofracturing is the preferred technique for enhancing
permeability and creating EGS circulation systems.
• Hydraulic connections between production/injection wells
must be good enough to permit fluid flow, but not too
good - short circuits cause premature cooling.
• Accurate fracture mapping is essential. Trial-and-error
drilling costs will preclude EGS commercial feasibility.
• Microearthquake (MEQ) monitoring is state-of-the-art, but
MEQ’s cannot always distinguish permeable fractures and
MEQ event locations may be poorly resolved.
DOWNHOLE TRANSIENT
SELF-POTENTIAL (“SP”) MONITORING
• Fracture pressurization will cause propagation of an
electrokinetic SP (self-potential) signal from the fracture
surface outward into the surrounding reservoir rock.
• Downhole transient SP monitoring in nearby observation
wells can provide additional fracture mapping data.
• Combining MEQ and SP provides better drilling targets
than either alone.
• Accurate fracture mapping and remote sensing of fracture
permeability reduces EGS project development costs.
ORIGINS OF SP IN THE EARTH
Causes of natural voltages (“SP”) in the earth are:
• Underground temperature gradients
(“the earth as a thermocouple”),
• Heterogeneous rock chemical composition
(“the earth as a battery”), and
• Electrokinetic effects caused by underground
fluid circulation through pores and fractures
(“the earth as a dynamo”).
ELECTROKINETIC EFFECTS
• At a rock/water interface, potential differences cause
charge separation in the molecular-scale electrical
double layer, with negative ions preferentially attached to
the rock surface and excess positive charge in the fluid.
• The degree of charge separation is proportional to the
potential difference or adsorption potential , which
depends on temperature and on fluid and rock chemistry.
• Fluid motion will carry separated positive charge, causing
a weak distributed electric current (the drag current).
• The drag current creates an electrokinetic potential
gradient according to Ohm’s Law.
TRANSIENT SP CHANGE
• Temporal changes in temperature and composition are
relatively slow. Rapid SP changes arise from electrokinetic
effects caused by underground fluid flow transients.
• So observing temporal SP variations provides information
about the subsurface flow and how it changes with time.
• Hydrofracturing creates highly transient outward flows in
the rock surrounding the fracture, causing SP changes
during fracture pressurization and subsequent falloff.
RESULTS FROM SCOPING STUDY
• About ten years ago, theoretical calculations were
carried out to appraise the feasibility of using
downhole transient SP monitoring to help
characterize geothermal hydrofractures for EGS.
• Numerical reservoir simulation techniques were used.
A “base case” calculation was followed by over 70
more involving key parameter variations.
• Since that study, SP-transient prediction capability
has been incorporated into new geothermal reservoir
simulation software.
THE BASE CASE
BASE CASE COMPUTED RESULTS
REMOTE SENSING
• In the Base Case, after one month of pressurized
injection, the volume within which the SP transient
signal amplitude is greater than 100 millivolts is
more than one kilometer in diameter.
• The practical detection limit is 10 millivolts or less.
• At the same time, the contact interface between the
in-situ reservoir fluid and the new injected fluid has
only moved outward about two meters into the
reservoir from the fracture surface.
EFFECTS OF PERMANENT STIMULATION
OTHER APPLICATIONS
• SP-transient monitoring of hydrofracturing in EGS
reservoirs has not yet been attempted at full-scale
owing mainly to a lack of suitable project sites.
• The same basic approach may also prove useful for
other applications requiring subsurface fluid flow
management, e.g. monitoring “seal integrity” for:
– “Fracking” operations for augmented oil and gas production.
– Underground greenhouse-gas sequestration.
– Subsurface toxic waste disposal.
SEAL INTEGRITY MONITORING
• Purpose: to provide
early warning of toxic
materials leaking into
the overlying potable
groundwater supply.
• First step: evaluate
potential utility with
scoping calculations
analogous to those for
the geothermal case.
Thank you!