The Effectiveness of Tertiary EOR in Heterogeneous Reservoirs

Ann Muggeridge1 Bilal Rashid1,2, Attar Ahmed1, Dastan Sartekenov1, Yingfang Zhou1

1Imperial College, 2BP Exploration

Overview

• EOR in heterogeneous reservoirs •  Influence of viscosity vs gravity vs dispersion •  Permeability heterogeneity and EOR •  Dimensionless numbers

• Definition of a heterogeneity index •  The vorticity of the displacement front (Heller, 1966)

• Approach: numerical simulation • Results • Conclusions

What is EOR?

Enhanced Oil Recovery or Improved Oil Recovery? Ø Often used synonymously Ø EOR: injection of a fluid that does more than maintain pressure

o Miscible gas o Polymers o  Low salinity water o Etc.

Ø IOR: also includes better engineering and project management o  Infill wells o Horizontal wells o Etc

Many EOR processes first used in 60s, 70s

EOR: improve recovery factor above 30-40%.

•  Eps, pore scale displacement efficiency

•  Ems, macroscopic sweep efficiency

•  Ed, drainage efficiency

•  Ec, economic constraints

EOR in Heterogeneous Reservoirs

Following Smalley et al., ”Reservoir Technical Limits: A Framework for Maximizing Recovery from Oil Fields”, SPE109555, 2007

cdmsps EEEERF ×××≈ 5.0~9.017.08.0 ×××=

EOR in Heterogeneous Reservoirs

Macroscopic sweep is affected by

Viscosity ratio Density differences

Diffusion/capillary effects Permeability heterogeneity

EOR in Heterogeneous Reservoirs

EOR schemes: •  more sensitive to geological heterogeneity than water

flooding(?) Ø Apply post waterflood

o  better understanding/less risk

Miscible gas injection, M=20

EOR in Heterogeneous Reservoirs

To capture effects of uncertain heterogeneity 1.  Generate multiple equi-probable realisations of permeability distribution

•  3D •  Millions of grid blocks

2.  Perform flow simulations in each model •  Different well patterns?

3.  Generate probability distribution for performance •  Best case, worst case, most likely case?

Often impossible: ×  Excessive CPU time ×  Memory requirements

Ø Quick screening/ranking tool

EOR in Heterogeneous Reservoirs

Quick screening/ranking tool for heterogeneity •  Dimensionless numbers

Viscosity  ratio  d

oMµµ

=  Gravity  number  

(e.g.  Fayers  and  Muggeridge  1989)  ( ) L

H

MqgAkN

ogv 11−

Δ=

µ

ρ  

Diffusion  number  (Lake  and  Hirasaki,  1981)  

K kL t2 1N = 14TD h Hv k1

φ

φ

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

 

Heterogeneity  number    

?  

 

Dimensionless Heterogeneity Index

The motion of a test particle on an isoconcentration surface: Taking the curl…

Flood front moving in this direction

Heller (1966)

Consider the vorticity of the displacement front

M Mobility ratio G The gravity to viscous ratio

°  Crane (1963)

NTDTransverse Dispersion Number

°  Lake & Hirasaki (1981)

Hv Vorticity heterogeneity index

Dimensionless numbers from vorticity

⎟⎟⎠

⎞⎜⎜⎝

⎛

d

o

µµln

Approach: reservoir simulation

2D Horizontal and vertical cross sections from SPE10 Model 2 Finely gridded: 220 × 60, 85 × 85 Layer 20

Layer 81

1.  Perform 1 phase flow simulation (fast) 2.  Calculate vorticity field 3.  Calculate heterogeneity index 4.  Compare with B/T and recovery at 1PVI from full flow simulation

12

Approach: reservoir simulation

Results

Heterogeneity Index: Secondary Gas Injection Statistics of permeability (no flow)

Vorticity based approach)

Flow based, Shook and Mitchell (2009)

•  Viscosity contrast

−  Instabilities at the front

•  Contribution to front deformation

−  as ln M

Mobility

Viscosity Ratio and Heterogeneity Index

Results Summary

Heterogeneity relatively unimportant when: Mobility Ratio

»  M >> 10

More complex interactions: −  Depends on:

−  Density contrast

−  Permeability which can vary spatially e.g. layering

−  Direction of flow vs gravity vs permeability gradient

Gravity and Heterogeneity

19

Flow-regimes with Gravity, M > 1

Horizontal unstable G >> 1 Lighter fluid above heavier

Viscous fingering G << 1 Lighter fluid above heavier

Intermediate stable G ~ 1

Fayers and Newley, 1988

Gravity and Realistic Heterogeneity: Phase Diagram

!

Gravity dominated Viscous dominated

Homogeneous

Heterogeneous

Gravity

tongue

Breakthrough tim

e (PV

)

Results Summary

Heterogeneity relatively unimportant when: Mobility Ratio

» M >> 10

Gravity » G > 100

For secondary miscible gas flooding

EOR (low salinity example): •  more sensitive to geological heterogeneity than water

flooding(?) Ø Research evidence:

o EOR most effective if applied first o Better recovery regardless of heterogeneity

EOR and Heterogeneity

Tertiary low salinity water injection Secondary low salinity injection

EOR (miscible gas example): •  more sensitive to geological heterogeneity than water flooding(?)

Ø Research evidence: o  Little correlation between secondary waterflood and tertiary gas flood

performance

EOR and Heterogeneity

Oil recovery at secondary water breakthrough

Oil

reco

very

at t

ertia

ry g

as b

reak

thro

ugh

Tertiary polymer injection Ø Incremental oil recovery similar irrespective of heterogeneity

Tertiary EOR and Heterogeneity

Heterogeneity index Heterogeneity index

Heterogeneous Homogeneous

Oil

reco

very

(PV

)

Heterogeneous Homogeneous Incr

emen

tal o

il re

cove

ry

(PV

)

Waterflood to 90% watercut 0.5 PV tertiary polymer

Conclusions

Vorticity based approach: »  Dimensionless heterogeneity index.

§  Useful for ranking different geological realizations. »  Unified formulation to understand the effect of these processes on oil

recovery and sweep. §  Heterogeneity §  Density differences §  Viscosity differences

EOR and heterogeneity » Secondary EOR gives better response than tertiary EOR

§  Miscible gas, polymer and low salinity examined » Little correlation between waterflood response and tertiary

response » Similar incremental recovery for tertiary polymer irrespective of

heterogeneity

Ongoing work

§  Extend calculation of vorticity heterogeneity index to 3D, non-line drive cases Ø Evaluate different well patterns

•  Stabilising influence.

−  Smooths the front and reduces the impact of instabilities

•  NTD varied orders of magnitude

Dispersion and Heterogeneity

Dispersion and Heterogeneity

Dispersion and Heterogeneity

Recovery at 1 PVI as a function of Hv

Heterogeneity Index and Gravity-Viscous Number

L9

L12

L59

L79

!Gravity dominated Viscous dominated