spe 131772 new pore-scale considerations for shale gas in place calculations
TRANSCRIPT
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SPE 131772New Pore-scale Considerations for Shale Gas in Place CalculationsRay Ambrose - Devon Energy, Robert Hartman - Weatherford
Laboratories, Mery Diaz-Campos, I. Yucel Akkutlu and Carl H.
Sondergeld - University of Oklahoma
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Overview
⢠Introduction
⢠Comparison of petrophysical models
⢠Physical evidence
â Conceptual model
â Sorbed phenomenon
â SEM images of porosity in gas-shale
â Movie of 3D model
â Equation showing correction to free pore-space
⢠Sorbed phase density characterization
⢠Examples of effects
⢠Conclusion
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Introduction
⢠With the current industry-wide calculation method for total gas volumes in organic shale reservoirs, the amount of gas storage has been overestimated.
⢠In this paper, that amount is quantified.
⢠In the past we have not accounted for the volume of measured free-space consumed by the sorbed gas component.
⢠By accounting for the volume consumed by the sorbed gas component, the space available for free gas is reduced.
⢠Examples show that this volume can be significant.
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Comparison of Petrophysical Models
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Organic Content
Connected Pore
Volume Containing
Free Oil, Gas, and Water
Non-Clay
Grain Volume
Dry Clay
Volume
Bound (Clay)
Water Volume
Isolated Pore Volume
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Organic Content
Connected Pore
Volume Containing
Free Oil, Gas, and Water
Non-Clay
Grain Volume
Dry Clay
Volume
Bound (Clay)
Water Volume
Isolated Pore Volume
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Organic Content
Connected Pore
Volume Containing
Free Oil, Gas, and Water
Non-Clay
Grain Volume
Dry Clay
Volume
Bound (Clay)
Water Volume
Isolated Pore Volume
Sorbed Gas
Volume
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Organic Content
Connected Pore
Volume Containing
Free Oil, Gas, and Water
Non-Clay
Grain Volume
Dry Clay
Volume
Bound (Clay)
Water Volume
Isolated Pore Volume
Sorbed Gas
Volume
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Old Petrophysical Model
⢠Old petrophysical model
â Provided basis for calculating volumes.
â Assumed free gas porosity and organics were independent.
â Bulk volume determined from mercury displacement (Vb).
â Grain volume determined from helium pycnometry (Vg).
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ecti
ve V
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Vo
lum
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Organic Content
Connected Pore
Volume Containing
Free Oil, Gas, and Water
Non-Clay
Grain Volume
Dry Clay
Volume
Bound (Clay)
Water Volume
Isolated Pore Volume
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Organic Content
Connected Pore
Volume Containing
Free Oil, Gas, and Water
Non-Clay
Grain Volume
Dry Clay
Volume
Bound (Clay)
Water Volume
Isolated Pore Volume
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New petrophysical model
⢠Porosity for free-gas and volume consumed by sorbed gas is interconnected.
⢠Evidence has existed in the method used to measure adsorption isotherms.
â Un-corrected (raw) isotherms bend downward.
â Sorbed volume must be accounted for in order to correct for this phenomenon.
â This correction is called the Gibbs correction.
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Organic Content
Connected Pore
Volume Containing
Free Oil, Gas, and Water
Non-Clay
Grain Volume
Dry Clay
Volume
Bound (Clay)
Water Volume
Isolated Pore Volume
Sorbed Gas
Volume
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Organic Content
Connected Pore
Volume Containing
Free Oil, Gas, and Water
Non-Clay
Grain Volume
Dry Clay
Volume
Bound (Clay)
Water Volume
Isolated Pore Volume
Sorbed Gas
Volume
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Simplified conceptual model
Void space
measured by
porosity
measurement
Sorbed mass
measured by
adsorption
experiment
+
= Total GIP
Void space
measured by
porosity
measurement
Sorbed mass
measured by
adsorption
experiment
+
-
Free gas
volume taken
up by sorbed
gas= Total GIP
Old Methodology New Methodology
Void space
measured by
porosity
measurement
Sorbed mass
measured by
adsorption
experiment
+
= Total GIP
Void space
measured by
porosity
measurement
Sorbed mass
measured by
adsorption
experiment
+
= Total GIP
Void space
measured by
porosity
measurement
Sorbed mass
measured by
adsorption
experiment
+
-
Free gas
volume taken
up by sorbed
gas= Total GIP
Old Methodology New Methodology
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Adsorption isotherm evidence âGibbs correction
Sorbed Methane Storage Capacity
0
5
10
15
20
25
30
35
40
45
50
0 500 1000 1500 2000 2500 3000 3500 4000
Pressure (psia)
Sto
rag
e C
ap
ac
ity
(s
cf/
ton
)
Corrected Isotherm (Langmuir)
Raw Gibbs Isotherm Data
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SEM evidence of large portion of porosity within the kerogen
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Free pore-space correction
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Sorbed phase density
⢠Previously published work all centers around 0.375-0.4233 g/cm3
for sorbed methane density.
â Dubinin (1960) suggested adsorbate density related to van der Waals co-volume constant b.
â Haydel and Kobayashi (1976) reproduced co-volume constant value experimentally.
â Menon (1968) suggested the value was the liquid density.
â Other authors have had different theories, but all are within the range of 0.375-0.4233 g/cm3.
⢠Sorbed phase densities when fluid is above the critical temperature are difficult.
â Difficult to separate the gas from the sorbed phase.
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Molecular dynamics simulation
⢠We took a molecular dynamics simulation approach
â Utilizes principals of Newtonian mechanics.
â Models fluid-solid, fluid-fluid interactions at pressure and temperature to determine values that are difficult to find experimentally.
â Study effects on sorbed phase density
⢠pore-size
⢠temperature
Molecular simulation cell consisting of graphite walls and
OPLS-UA methane
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Molecular dynamics calculated
density profiles â damped oscillations
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Sorbed phase density as a function of temperature
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Example of effects
⢠Typical gas shale
⢠Shale A shows a decrease of 14.2% of free gas and 11.6% of total gas*
⢠Shale B shows a decrease of 30.2% of free gas and 17.1% of total gas*
⢠* Compared to old industry standard calculation methods
Shale A:
(low sorption capacity)
Shale B:
(high sorption capacity)
0.06 0.06
Sw 0.35 0.35
So 0.0 0.0
Bg 0.0046 0.0046
Ë M 20 lb/lb-mol 20 lb/lb-mol
GsL 50 scf/ton 120 scf/ton
p 4000 psia 4000 psia
T 180 oF 180
oF
pL 1150 psia 1800 psia
b 2.5 g/cm3 2.5 g/cm
3
s 0.37 g/cm3 0.37 g/cm
3
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Conclusions and Future Work
â˘The disregard of the volume that is consumed by the sorbed phase in gas shale leads to inadvertently high values of total gas-in-place due to double counting of the volume available for free gas.
â˘Evidence of a finite volume consumed by the sorbed phase has been around a long time. It has been used to correct raw isotherm data.
â˘The sorbed phase mass is determined in the sorbed gas experiment. Properly characterizing the sorbed phase density is critical in accounting for the volume consumed by the sorbed phase mass.
â˘A new method is shown to properly account for this volume and remove it from the space available for free gas.
â˘Future work in publishing compositional effects on sorbed gas quantities, taking the Langmuir isotherm portion of the equation and substituting a multi-component isotherm model.
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