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Scope and Application of Pressure Transient Tests in CBM and Shale Gas reservoirs. Baijayanta Ghosh Reservoir Domain Champion Testing Services, Schlumberger. Agenda. Brief introduction to Pressure Transients With reference to Shale Gas/CBM reservoirs The bigger picture

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scope and application of pressure transient tests in cbm and shale gas reservoirs

Scope and Application of Pressure Transient Tests in CBM and Shale Gas reservoirs

Baijayanta Ghosh

Reservoir Domain Champion

Testing Services, Schlumberger

agenda
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
physics of gas flow in sg cbm reservoirs
Physics of Gas flow in SG & CBM reservoirs

Coal Bed Methane

Shale Gas Reservoirs

pressure transient analysis pta deconstructed
Pressure Transient Analysis (PTA)Deconstructed
  • Determine
  • Permeability (k)
  • Initial Reservoir Pressure (Pi)
  • Skin (S, D|Q|)
  • Detect
  • Boundaries
derivative analysis
Derivative Analysis

Gringarten, 2011

the bigger picture
Where Pressure Transient Tests fit in unconventional gas exploitation/reservoir

management

Applications of Pressure Transient Tests

The Bigger picture
shale gas
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

Shale gas
case studies shale gas
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)
case studies shale gas1
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
coal bed methane
Pre-frac & Pro-frac PTA

Injection Falloff Test, Slug Test

Advanced Interpretation Models

Pressure dependent reservoir properties

Coal bed methane
case studies cbm ifot
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
case studies cbm ifot1
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
advanced interpretation pressure dependent properties
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
applications in production data analysis
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

conclusion
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