Reservoir Mechanisms and Problem types

2 WC Reservoir and Problems14-Jun-02

Reservoir Problem types

• Reservoir water flow mechanisms– Displacement efficiency– Vertical sweep efficiency– Areal sweep efficiency

• “Good” water, “Bad” water

• Problem types

• Well and facilities

• Scale

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Displacement efficiency

• Fraction of oil displaced from rock swept with water

• Oil available = (1 - Sor - Swi)

• Total oil = (1- Swi)• Not target for water control• Sor can be changed by

surfactant flood Swi

Sor

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Areal sweep efficiency

• Fraction of horizontal section swept by water

• Dependent on– Areal permeability

distribution– Permeability anisotropy– Deposition environment– Flood pattern– Fluid mobility

• Limited target for water control

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Five Spot Pattern

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Effect of Mobility Ratio

M=0.4

Breakthrough

83%

WOR = 0.6

87%

WOR = 4.7

96%

M=1.4

Breakthrough

65%

WOR = 0.5

70%

WOR = 2

82%

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Areal Sweep Efficiency

0.1 0.2 0.3 0.5 1 2 3 5 100.5

0.6

0.7

0.8

0.9

1

Mobility Ratio

Ea

at

bre

akth

rou

gh

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Aerial Sweep Problems

Original oil water contact

Proposed injector

Proposed oil producers

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Vertical sweep efficiency

• Fraction of vertical section swept by water

• Dependent on– Vertical permeability profile– Permeability contrast– Deposition environment– Gravity segregation– Fluid mobility

• Prime target for water control

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Homogeneous zone

InjectorProducerProducer

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Discontinuous Zones

InjectorInjectorInjector Producer Producer

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Sandstone Geology

Vertical stacking Isolated stackingLateral stacking

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Permeability Profile Brent

0 2 4 6 8 10 12(9,700)

(9,600)

(9,500)

(9,400)

(9,300)

(9,200)

(9,100)

(9,000)

Permeability, Darcies

De

pth

, ft

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RFT Pressures

5,2005,400

5,6005,800

6,0006,200

6,400(9,700)

(9,600)

(9,500)

(9,400)

(9,300)

(9,200)

(9,100)

(9,000)

Pressure, psi

Dep

th, f

t

Initial reservoir pressure

Upper Jurassic

Rannoch

Etive

NessTarbut

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Favorable Permeability Profile

Injector

Producer

Oil

Sea level

Deposition

Fine

Coarse

Incr

easi

ng

ener

gy

Gamma Perm

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Unfavorable Permeability Profile

Gamma Perm

Water level

Deposition

Coarse

Fine

Ero

sion

Injector

Producer

Oil

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"Good" Water• Needs to be produced with oil• Cannot be shut-off without shutting off oil

• Down-hole separation may be a solution

Injector Producer

t1

t2

t3

t4

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Relative Permeability

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

Water saturation, fraction pore space

rela

tive

per

mea

bili

ty f

ract

ion

Swi

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Good WaterProduced with hydrocarbon

Cannot be shut-offwithout loosing hydrocarbon

What Are The Water Problem Types?

Simple Complex

3 4 5 6 7 8 9 101 2

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Tubing / Casing / Packer Leak

Leak in the wellbore

– Detect with spinner or temperature log

Solution case specific

– Shut-off fluids Leak Matrix

– Mechanical shut-off

01

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Problem in or near wellbore

– Poor cement

– Cave due to sanding

– Channel in formation

Solution is case specific

– Cement in channel

– Gel in formation

– Resin

Near-Wellbore Flow

02

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OWC moving up Bottom water uniform

sweep

– OWC move up slowly

– Low vertical permeability

Solution

– Abandon perforations from the bottom Bridge plug

(through tubing) Cement plug

03

23 WC Reservoir and Problems14-Jun-02

High Perm. Layer - No Crossflow Vertical sweep

problem

Edge water

– Water flood or aquifer

– Thief layer

– High perm streak

Shut off layer

– Producer or Injector

– Mechanical or fluid

04

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Fissures to an Injector

Areal sweep problem

– Rapid breakthrough

– Test with tracers

Treat at injector

– Use ‘flowing gel’

05

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Fissures to a Water Layer Oil bypassed

– Natural fissures

– Hydraulic fracture

– Fault

– Water flow in fissures

– Oil in matrix

Treat producer

– Flowing gel

– Over displace

06

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Water Coning Bottom water

– Near oil/water contact

Critical oil rate

– Depends on Kv

– Depends on gel radius

Laterals

Treat with layer of gel

– Near bottom perforations

– Large volumes required

– Long delay times

07

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Poor Areal Sweep Areal sweep

problem

– Heterogeneous

– Anisotropic

– Sand channel

– Fingering due to mobility

Treat at injector

– Polymer flood

– Often not economic

Aquifer

08

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Gravity Segregated Layer Vertical sweep

problem

Edge water

– Water flood or Aquifer

– One main zone

– High vertical permeability

– Local coning

Laterals

Modify profile

– Producer

09

29 WC Reservoir and Problems14-Jun-02

High Perm. Layer - with Crossflow Vertical sweep

problem

Edge water

– Water flood or aquifer

– Thief layer

– High perm streak

Shut off layer

– Deep gel penetration

– Difficult to place

– Often not economic

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30 WC Reservoir and Problems14-Jun-02

Which Reservoir?