building a geomechanical framework for interpreting das ... · —50 m tall pkn fracture...
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This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC
Building a Geomechanical Framework for Interpreting
DAS Measurements
Christopher Sherman, Robert Mellors, Rick Ryerson, Joseph Morris,
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▪ DAS is designed to measure signals at a high spatial resolution (~ 1 m) over large distances (multiple km)
▪ DAS uses the fiber itself as a sensor to measure strain (or strain rate) along its length
▪ Its development has opened up a massive source of data for subsurface characterization / monitoring
▪ Questions:— How can we optimize the performance of DAS?— How do we interpret the data we collect?
Introduction to Fiber Optic Distributed Acoustic Sensors (DAS)
Lumens,
2014
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DAS Examples – Microseismicity
▪ Comparison of traditional geophone and DAS monitoring programs (Hull et al., 2017)
— Sensors located in an offset vertical well, with hydraulic stimulations in a nearby horizontal well
— DAS config: L = 760 m, Lgauge = 10 m, Fs = 2 kHz
— Geophone config not specified (lower resolution)
— Microseismic events recorded during an example stage
• DAS = 31 events (minimum Mw = -2)
• Geophones = 785 events (minimum Mw = -2.68)
• Note: different event detection algorithms used
Depth
Time
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▪ Hydraulic fracture geometry characterization attempts (Jin and Roy, 2017)— Fracture stimulation and DAS in adjacent horizontal wells— DAS configuration: Lsample = 1 m, Lgauge = 5m, Fs = 10 kHz
▪ Waterfall plots— Vibrational energy for a given frequency band— Excited by the opening and fluid flow in fractures?— Tend to be messy and difficult to interpret
▪ Low-frequency strain measurements— Carefully filter the data to estimate near-DC component
of strain rate (this example: f < 0.05 Hz)— Matches the psuedostatic fracturing process
DAS Examples – Waterfall Plots and Low-Frequency Strain
Plan View
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▪ Goal: Develop a framework for interpreting DAS measurements that is robust, quantitative, and grounded in geomechanics
▪ Due to its topicality, focus our initial efforts on hydraulic fracture monitoring
▪ Implement a model of DAS in GEOS (LLNL)— HF modeling from near-wellbore to reservoir scales— Geothermal energy production— Microseismicity— Etc.
Large-Scale Geomechanical Modeling
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▪ Fiber model:— The scales of interest are way too large to explicitly mesh the fiber
• Instead, define a virtual fiber as a set of nodes in the underlying FE mesh • Assume that the fiber is perfectly coupled to the rock and is insensitive to shear
— Record the nodal displacement along the virtual fibers at high frequency— Use central-difference operators to calculate strain and/or strain-rate— Apply an arbitrary gage length applied via a convolutional filter during post-processing
▪ The target DAS signals are often very small (~1 nε/s)— Challenging constraint for large heterogeneous models, explicit discontinuities— We use a combination of implicit/explicit time-stepping to bring the model into an initial
equilibrium state— Before loading, we track the drift/noise in the model and require a S/N of at least 10
GEOS Fiber Modeling
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▪ Instead of looking at a particular case study, focus on a set of idealized models— Geologic model sensitivity— Stimulation design sensitivity— Target low-frequency DAS on three fibers (f << 1 Hz)
GEOS Fiber Modeling
Hydraulic Fracture Pre-existing fracture
network (DFN)
Synthetic DAS
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▪ Base model:— 50 m tall PKN fracture propagating from a horizontal wellbore— In-situ stress state is normal— Fluid injected into a single perforation cluster for 80 minutes at 0.05 m3/s
▪ Increase the complexity of the model to isolate signals of interest in the DAS
Geological Model Sensitivity - Design
Model I
(PKN)
Model II
(+ Stress Barriers)Model III
(+DFN)
Increasing Complexity
+ +H
L
+ …
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Geological Model Sensitivity – HF Generation Examples
The base model (Model I) results
in a simple, height-limited fracture
The addition of the stress barriers (Model II)
and DFN (Model III) introduces significant
complexity into the generated fracture
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Geological Model Sensitivity – DAS Results
The measured strain rate in the
horizontal well is dominated by the
opening of the perforation
Horizontal Well
Vertical Offset
Well (x = 380 m)
Vertical Offset
Well (x = 0 m)
Base Model + Stress Barriers + DFN
Extents
Extension Compression
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Geological Model Sensitivity – DAS Results
Horizontal Well
Vertical Offset
Well (x = 380 m)
Vertical Offset
Well (x = 0 m)
Base Model + Stress Barriers + DFN
Extents
The measured strain rate in the
vertical wells displays this
characteristic Ricker-like pattern along
the length of the fiber
Extension Compression
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Geological Model Sensitivity – DAS Results
Horizontal Well
Vertical Offset
Well (x = 380 m)
Vertical Offset
Well (x = 0 m)
Base Model + Stress Barriers + DFN
Extents
When a fracture breaks through a
stress barrier near a fiber, it results in
this characteristic bend shape
Extension Compression
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Geological Model Sensitivity – DAS Results
Horizontal Well
Vertical Offset
Well (x = 380 m)
Vertical Offset
Well (x = 0 m)
Base Model + Stress Barriers + DFN
Extents
The presence of a DFN introduces
significant complexity into the DAS
response
Model III slice at z = 0 m:
Extension Compression
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▪ Low frequency DAS measurements may be used to constrain fracture geometry
▪ Synthetic DAS measurements may be used to design/optimize field deployments
▪ Simultaneous measurements in horizontal (common) and vertical offset wells (less common) allows best resolution
▪ DAS measurements may also be a useful tool for monitoring the interaction of fractures with barriers
Geological Model Sensitivity – Conclusions
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Future Applications – Machine Learning
▪ Current effort to design machine learning approaches (CNN) to interpret these data
▪ Use our approach to generate a labeled training dataset:
— Length/height of generated fractures
— Location/timing of triggered microseismic events
— Interaction with fracture barriers
— Proppant and Multiphase related phenomena
…
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GEOS THM Coupling Diagram
Fracture
Propagation
Solid
Mechanics
Fracture
Flow
Contact
Mechanics
Porous
Media Flow
Microseismicity
Heat Flow
Effective Stress, Poroelasticity
Thermal stress
Penalty
Barton-
Bandis Leakoff
Convection,
Conduction
LEFMRemeshing
New DAS/DTS
Model
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▪ Begin with Model II (stress barriers) as a reference— Incorporate a more realistic pumping schedule into the design— Modify the fluid leakoff rate into the surrounding formation— Track Changes in the DAS measured along the horizontal fiber FH1
— Compare to distribution of proppant in the generated fracture
Proppant Placement Sensitivity – Design
+
Stress Profile Base Pumping Schedule
+ Leakoff Sensitivity
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Proppant Placement Sensitivity – Results
Leakoff x2
Leakoff x4
Leakoff x8
Leakoff x16
Leakoff x32
Base
Proppant VF Strain Rate (nε/s)
0
0.6
0.3
-10
10
0
Proppant settling
(due to gravity)Tip screenout
(due to finite size of
proppant particles)
Extension Compression
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Proppant Placement Sensitivity – Results
x2
x4
x8
x16
x32
Base
Proppant VF
0
0.6
0.3Tip screenout tends to appear as a deviation
from the characteristic exponential steps.
Otherwise, differences in proppant distribution
are difficult to discern from these particular data