effect of capillary pressure on oil production of low
TRANSCRIPT
Effect of capillary pressure on oil production of low-permeability, tight and shale
reservoirs
Yu-Shu Wu1, Shuyu Sun2 and Yi Xiong1
1-Colorado School of Mines; 2-KAUST Energy Modeling Group (EMG)
Department of Petroleum Engineering Colorado School of Mines
Outline
• Introduction
• Mathematical Model
• Vapor-Liquid Equilibrium (VLE) Calculation
• Case Studies
• Conclusions
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Characteristic of Tight Oil Reservoirs • Small pore and pore-throat size
– nm to µm
3
Pore-throat size distribution Nano-scale SEM image of Bakken matrix rock
(Honarpour et al., 2012)
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Pore Confinement Effect • Effects on fluid phase behaviors in nano-pores
– Capillary pressure
– Surface forces (structural, electrostatic, adsorbtive, van der Waals)
Contributions from surface and capillary forces to the effect on saturation pressure (Firincioglu et al. (2012))
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Effect of Capillary Pressure • Effect on Vapor-Liquid Equilibrium (VLE)
– Phase envelops under effect of capillary pressure
Phase envelop of binary mixtures in 10 nm and 20 nm pores under capillarity effect (Nojabaei et al.,2013).
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Motivations and Objectives
6
• Quantitatively analyze the effect of capillary pressure on: – Fluid flow behaviors – Production performances
• Develop a mathematical model to capture the effect of capillary pressure with capabilities to: – Numerically solve a multiphase, multi-component bubble-
point system – Accurately calculate vapor-liquid equilibrium (VLE)
including the effect of capillary pressure
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Outline
• Introduction
• Mathematical Model
• Vapor-Liquid Equilibrium (VLE) Calculation
• Case Studies
• Conclusions
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Compositional Model
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Evaluation of Capillary Pressure
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• Young-Laplace Equation
• J-function
2 coscP
rσ θ
=
( )14
1
cN
i o i gi
x yσ ρ ρ=
= −∑
( ) ( ) /cos
cP S kJ S β
βφ
σ θ=
Discretized Equations • Equations for each grid block
10
where
and and
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Outline
• Introduction
• Mathematical Model
• Vapor-Liquid Equilibrium (VLE) Calculation
• Case Studies
• Conclusions
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VLE Calculation With Capillary Pressure
12
PR-EOS
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VLE Calculation- Procedure
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Saturation Pressure Calculation
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VLE Calculation Example – Eagle Ford Oil
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• A major tight oil formation in U.S. • Hydrocarbon system with 14 components
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Effect of Capillary Pressure on VLE– Eagle Ford Oil (I)
16
Bubble point pressure suppression
Light components in oil
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• Suppressed bubble-point pressure • More light components remained in oil phase
Effect of Capillary Pressure on VLE– Eagle Ford Oil (II)
17
Oil viscosity vs. Pore radius Oil density vs. Pore radius
• Decrease of oil viscosity and density as increase of capillary pressure (smaller pore radius)
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Outline
• Introduction
• Mathematical Model
• Vapor-Liquid Equilibrium (VLE) Calculation
• Case Studies – Bakken tight oil reservoirs
• Conclusions
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Numerical Studies on Capillarity Effect
• A single-porosity porous medium – matrix rock
• A double-porosity fractured reservoir
19
(Nojabaei et al.,2013) Bakken oil composition
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A Single-Porosity Matrix Rock
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Initial pressure of saturated production = 2700 psi
Effect of Capillary Pressure on VLE – Gas Saturation
• Postponed appearance of gas phase
• Less gas saturation
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x = 1.0 m x = 15.0 m
x = 30.0 m
• Molar fractions of surface production: light, intermediate and heavy
Molar fraction of C1 and C2
Molar fraction of C3, C4 and C5-C6
Molar fraction of C7-C12, C13-C21, and C22-C80
Effect of Capillary Pressure on VLE – Composition (I)
• Less light components produced • More intermediate and heavy
components produced
22
• Molar fractions at reservoir condition ( x=15.0 m): light, intermediate and heavy Effect of Capillary Pressure on VLE – Composition (II)
• More light, less intermediate and heavy components remained in the reservoir
23
• Reservoir pressure at x=1.0, 15.0 and 30.0 m Effect of Capillary Pressure on VLE – Reservoir Pressure
• Quicker decrease of reservoir pressure
24
• Oil compositions vs. reservoir pressure: light, intermediate, heavy
Effect of Capillary Pressure on VLE – Liquid Phase (I)
• Suppressed bubble-point pressure
• More light but less intermediate and heavy components remained in oil phase at saturated condition
25
• Oil properties vs. reservoir pressure
Effect of Capillary Pressure on VLE – Liquid Phase (II)
• Less oil density and viscosity at saturated condition
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A Double-Porosity Fractured Reservoir A
B
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Effect of Capillary Pressure on VLE – Day 1
Without capillarity effect
Capillarity effect has no impact on fracture continuum
Fracture pressure
Fracture pressure
Fracture gas saturation
Fracture gas saturation
With capillarity effect
28
Effect of Capillary Pressure on VLE – 10 Years With capillarity effect Without capillarity effect
Gas saturation of fracture system
Gas saturation of matrix system 29
Effect of Capillary Pressure on VLE – Within SRV (B)
Reservoir Pressure of Fracture Reservoir Pressure of Matrix
Gas Saturation of Fracture Gas Saturation of Matrix
Effect of Capillary Pressure on VLE – Outside SRV (A)
Reservoir Pressure of Fracture Reservoir Pressure of Matrix
Gas Saturation of Fracture Gas Saturation of Matrix
Effect of Capillary Pressure on VLE – Production Performance
Oil Production Rate Accumulated Oil Production
Gas-Oil Ratio Accumulated Gas Production
Effect of Capillary Pressure on VLE – Sensitivity on Production Performance
Oil Production Rate Accumulated Oil Production
Gas-Oil Ratio Accumulated Gas Production
Outline
• Introduction
• Mathematical Model
• Vapor-Liquid Equilibrium (VLE) Calculation
• Case Studies
• Conclusions
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Conclusions (I)
• The effect of capillary pressure on VLE suppresses the saturation pressure and results in more light components dissolved in oil phase, which influences the oil properties, such as density and viscosity.
• The effect of capillary pressure on VLE leads to lower gas
saturation at reservoir condition, less gas and more oil produced at surface, and larger molar fraction of light components remained in the reservoir.
• The effect of capillary pressure on VLE also leads to the different evolution of reservoir pressure during the production.
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Conclusions (II)
• Capillarity effect on VLE is not observed at early production with saturated condition in the double-porosity reservoir, when the production is mainly from fracture continuum.
• The capillarity effect on VLE has larger influence on
suppression of gas production than on growth of oil production in this simulation case.
• It is recommended to investigate capillary pressure model for
nano-pores with more experimental and theoretical work.
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Questions
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Backup
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Values of Capillary Pressure in Case I
Values of Capillary Pressure in Case II
100
120
140
160
180
200
220
240
260
0 10 20 30 40 50 60
Capi
llary
pre
ssur
e (p
si)
Time (year)
Pcgo_A Pcgo_B
Higher_Pcgo_A Higher_Pcgo_B