Baijayanta Ghosh
Reservoir Domain Champion
Testing Services, Schlumberger
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Scope and Application of Pressure
Transient Tests in CBM and Shale Gas
reservoirs
Agenda

Brief introduction to Pressure Transients
– With reference to Shale Gas/CBM reservoirs
The bigger picture
– where Pressure Transient Tests fit in unconventional gas
exploitation/reservoir management

Techniques and applications of Pressure Transient Analyses
in – conventional and the latest
– Shale Gas reservoirs
– Coal Bed Methane (CBM) reservoirs

Conclusion
– Hardware enablers for better Pressure Transient tests
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
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Specially with reference to Shale Gas and CBM reservoirs
BRIEF INTRODUCTION TO
PRESSURE TRANSIENT TESTS
Physics of Gas flow in
SG & CBM reservoirs
Coal Bed Methane
Shale Gas Reservoirs
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Pressure Transient
Analysis (PTA)
Deconstructed
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Determine
-Permeability (k)
-Initial Reservoir Pressure (Pi)
-Skin (S, D|Q|)
Detect
-Boundaries
Type Curve Analysis
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vs.
Derivative Analysis
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Gringarten, 2011
THE BIGGER PICTURE
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Where Pressure Transient Tests fit in unconventional gas exploitation/reservoir
management
 Applications of Pressure Transient Tests

Workflow to optimize FDP
in SG Reservoirs
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Adsorption Isotherm and
CBM Reservoirs
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SHALE GAS
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Pre-frac PTA
Diagnostic Formation Integrity Test, Perforation Inflow Diagnostic Test
Post-frac PTA
G-function based Mini-frac, Flow-back & BU Test
Advanced Interpretation Models
Coupled Geomechanical model, non-linear adsorption
Case Studies – Shale Gas

Pre-frac evaluation
– Recommended to perforate large
section in extreme underbalanced and
using deep penetrating charges

Post-frac flow and BU
– Determining p, k, S, Drainage Area
to compute EUR
– 9-stage frac horizontal well
– Experience: drainage area =f(k)
– forecast model for multi-stage frack
well = sum(combination of n no of
PSS single stage frack well model)
– Recalibration of PTA kh using
Production Analysis (Declien Curve)
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– Important to determine virgin
conditions and to eliminate
incomplete frac fluid clean-up effects
– DFIT – Diagnostic Fracture Injection
Test
– Perforation Inflow Diagnostic test
Case Studies – Shale Gas

Comparison between DFIT &
Conventional Analysis

Advanced Interpretation Models
– Type curves for shale gas reservoir
response with non-linear
adsorption and Klinkenberg effect
– Coupled geomechanical &
reservoir flow simulator to models
– Change in pore volume and
permeability
– Failure near the fracture and away
from wellbore
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– Log-log derivative approachexample: stage frac horizontal well
– inadequate, only kmin and xf|max
can be determined
– DFIT– sleek water fracture and
closure analysis using G-function &
independent after closure IFOT
COAL BED METHANE
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Pre-frac & Pro-frac PTA
Injection Falloff Test, Slug Test
Advanced Interpretation Models
Pressure dependent reservoir properties
Case Studies – CBM IFOT

Injection at pressure > frac pressure
– Reservoir simulator that models frac
growth must be used to match pressure
transient due to

Accounting for free gas (above
Critical Desorption Pressure)
– System compressibility =
– pore volume compressibility +
– fluid saturation X fluid compressibility +
– apparent compressibility due to gas
sorption on the surface of the coal
– The effects of sorption compressibility
are shown to have a significant impact
on skin but not on permeability.
Neglecting sorption compressibility will
lead to a more (-)ve skin value
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– fracture growth
– leak-off
Case Studies – CBM IFOT
Recommended practice

Otherwise use numerical
reservoir simulator
capable of handling


Effect of injecting at a
pressure higher than coal
fracture pressure
Unstable pressure before
start of injection
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Advanced interpretation:
Pressure dependent
properties

– Uses Palmer & Mansoori model
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

Effect of stress dependent permeability –
magnifies skin effect [Fig. 13]
Injection Falloff Tests done in Colorado,
US – SDPP model [Fig.16]
For boundary detection both DST & IFOT
recommended – stress dependent
change in trend of derivative is opposite
directions [Fig. 17 & 18]
Modeling gas block formed at reservoir
pressures below critical desorption
pressure
– Skin effect – Hawkin’s equation
– Radial composite behaviour
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Pseudo-pressure based approach to
model stress dependent porosity &
permeability – rock mechanical
parameters regressed
Applications in Production data
Analysis
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Type curve used for Production Data
Analysis needs to be of the
corresponding model as determined
from the flow regime analysis of PTA
Conclusion
Pressure Transient Tests have become more
integrated to the engineering workflow
 We have the interpretation tools for sufficiently
accurate transient analysis


We have the hardware enablers
– High resolution electronic gauges
– Downhole shut-in devices
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– We are getting better – more robust models that handle
detailed complex phenomena
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bghosh@slb.com
THANK YOU