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Scope and Application of Pressure Transient Tests in CBM and Shale Gas reservoirs
Baijayanta GhoshReservoir Domain ChampionTesting Services, Schlumberger
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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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BRIEF INTRODUCTION TO PRESSURE TRANSIENT TESTS
Specially with reference to Shale Gas and CBM reservoirs
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Physics of Gas flow in SG & CBM reservoirsCoal Bed Methane Shale Gas Reservoirs
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Pressure Transient Analysis (PTA)Deconstructed
Determine-Permeability (k)-Initial Reservoir Pressure (Pi)-Skin (S, D|Q|)Detect-Boundaries
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Type Curve Analysis
vs.
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Derivative Analysis
Gringarten, 2011
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THE BIGGER PICTURE
Where Pressure Transient Tests fit in unconventional gas exploitation/reservoir management Applications of Pressure Transient Tests
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Workflow to optimize FDPin SG Reservoirs
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Adsorption Isotherm and CBM Reservoirs
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SHALE GAS
Pre-frac PTA
Diagnostic Formation Integrity Test, Perforation Inflow Diagnostic TestPost-frac PTA
G-function based Mini-frac, Flow-back & BU TestAdvanced Interpretation Models
Coupled Geomechanical model, non-linear adsorption
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Case Studies – Shale Gas Pre-frac evaluation
– Important to determine virgin conditions and to eliminate incomplete frac fluid clean-up effects
– DFIT – Diagnostic Fracture Injection Test
– Perforation Inflow Diagnostic test– 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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Case Studies – Shale Gas Comparison between DFIT &
Conventional Analysis– Log-log derivative approach-
example: 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
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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COAL BED METHANE
Pre-frac & Pro-frac PTA
Injection Falloff Test, Slug TestAdvanced Interpretation Models
Pressure dependent reservoir properties
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Case Studies – CBM IFOT Injection at pressure > frac pressure
– Reservoir simulator that models frac growth must be used to match pressure transient due to – fracture growth– leak-off
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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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 Pseudo-pressure based approach to
model stress dependent porosity & permeability – rock mechanical parameters regressed– Uses Palmer & Mansoori model
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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Applications in Production data Analysis
Type curve used for Production Data Analysis needs to be of the corresponding model as determined from the flow regime analysis of PTA
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Conclusion
Pressure Transient Tests have become more integrated to the engineering workflow
We have the interpretation tools for sufficiently accurate transient analysis– We are getting better – more robust models that handle
detailed complex phenomena We have the hardware enablers– High resolution electronic gauges– Downhole shut-in devices
