Case Study: Stratified Gas Hydrate Reservoir with Associated Free Gas

Group ProjectPETE 680: Horizontal well Technology

Presented By,Namit Jaiswal,

Adejoke M Ibironke

Objective1. Gas Hydrates

2. Overview of horizontal well and designer well

3. Case description

4. Results

5. Conclusion

6. Recommendation

Alaska Methane Hydrate Alaska Methane Hydrate Estimated ResourceEstimated Resource

TARN TREND

FREE GAS?

PBU26%

KRU39%

MPU100%

DIU

EILEEN TREND

44 TCF

60 TCF?

GAS HYDRATE& FREE GAS

GAS HYDRATE

(After Collett, 1993)

GAS HYDRATES – AN OVERVIEWCrystalline structures of ice that form cages around

guest molecules

Guests are gas molecules (methane, ethane, CO2,

N2…)

No chemical bond between guest and host lattice

Physically stable with only partial occupancy

Different structures sI, sII, sH, sT, …?

Large amounts of gas molecules are entrapped within these cages

Up to 180 volumes of gas (scf) per volume of hydrate

Gas molecules can penetrate through the hydrate zone to form new gas

hydrates at boundary

Formation and growth occurs only under certain pressure and temperature

conditions

Hydrate formation conditions are high pressure and low temperature

Reserves Estimation

Gas Hydrate Production Methods

Methanol

Dissociated

Thermal Injection

Gas Out

Imperm. Rock

Impermeable Rock

Gas Hydrate

Inhibitor Injection

Gas Out

Imperm. Rock

Impermeable Rock

Depressurization

Free-Gas

Gas Out

Gas Hydrate

Hydrate

Reservoir Dissociated HydrateDissociated Hydrate

Imperm. Rock

Hot Brine or Gas

Gas Hydrate

After Collett, 2000

Gas zone Hydrate zone

Pwf

Peq

Po

t

Radial distance

Pin

Boundary Condition

Algorithm for Performance of a Hydrate Reservoir Evaluation

1. Assume an average pressure pavg, and calculate gas compressibility cg using

avgg p

c 1

2. Using the value of cg , calculate total compressibility and hydraulic diffusivity constant from the known reservoir parameters.

gft ccc  

3. For a desired gas withdrawal rate, solve eqn

1

1

1

22

4exp2

expsc

scHH

H

h

eqin

H

H

sc

scsc

Tp

BSZTEI

ppkhTZTpq

Above equation has on both sides; it requires a numerical scheme to solve. As a special case, when there is no gas flow from the undissociated

hydrate zone, so above equation is simplified to

14exp HH

sc BSh

q

Above equation has on both sides; it requires a numerical scheme to solve. As a special case, when there is no gas flow from the undissociated

hydrate zone, so above equation is simplified to

4. Using the value of , solve below equation (1) and (2

1.......2 tR

2.......10 tRT

pBdt

dRRTpBppK

sc

scH

sc

scH

neqd

5. Using the value R*, Po and solve eq 3. and 4.

trEI

hTkZTpq

ppsc

scsco

1

2

1

1221 42

and for the un-dissociated region is

tr

EI

tREI

pppp

H

H

eqininH

4

4

22*

2222

to obtained pressure profiles as a function of radial distance from the wellbore.

6. From the pressure values obtained in step 5. find pwf and calculate a new average pressure using

2wfo

avg

ppp

7. Using the new value of pavg, calculate new values of cg, ct and .

Compare the new value of with that calculated in step 2. If the new value is within 10% of the old value, use the pressure profile generated in step 5. If not,

go to step 3 and repeat Steps 4-8 till the consecutive values of agree.

Overview of horizontal well technology

What is horizontal well ?

Parameter Effects

1. Skin Factor

2. Payzone thickness

3. Anisotropy

Productivity by Well Testing

1. To obtain reservoir properties

2. To find total producing length

3. To estimate mechanical skin factor

                                                                                            

Gas Reservoir

1. Low Permeability

Vertical HorizontalSmall spacing Single is sufficient

Requires difficult fracturing

May or may not require

Low productivity High productivity

2. High Permeability

Vertical Horizontal

High turbulence Low turbulence

Frac-pac required Not required

High production per unit ht.

Produces less gas per unit length

Low productivity High productivity due to long length

hydrate

Water and Gas Coning

Horizontal well

Vertical well

Case Description

Highly Stratified Gas Hydrate Reservoir

0.35’

2’

6 x 50’

Kh=15 md

VERTICAL WELL

ASSUMPTIONS FOR VERTICAL WELL RESERVOIR

• Temperature for free gas zone is constant.• Average viscosity is used for calculation.• Individual well bore pressure are assumed taken from

literature.• Pressure drop across tubing is negligible.

Productivity for vertical gas well is calculated by using following equation:

 

Where,q = gas flow rate, Mscfdk= permeability, mdh = Thickness, ftPe =Pressure at external radius, re, Psia

Pwf = well bore flowing pressure, Psia

Z= average compressibility factorT = Reservoir temperature, Rre = drainage radius, ft

)1(

ln

0007027.0

'

22

w

e

wfe

rrzT

ppkhq

Graph of Pressure Vs production from vertical well

Pwf vs q for vertical well

0

50

100

150

200

250

300

350

0 200 400 600 800 1000 1200 1400 1600 1800Hundredsq (Mscf/d)

h=50ft,s=0 h=20 ft, s=0 h=50ft,s=0.5 h=20,s=0.5

Variation of productivity with payzone thickness.

J vs h plot

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 10 20 30 40 50 60h,ft

J, M

scf/(

day.

psi

2)

J ,s=0 J, s=+0.5

Vertical Fractured Wells

Any technique that helps to create fissures and openings in the reservoir rock of an oil and (or) gas formation, and help increase the flow of oil and (or) gas.

Fracturing can either be Natural - created by faults in the formation) Artificial -This can be

• Pneumatic - by the flow of high pressure compression of air.

• Hydraulic - pumping of fluid under high pressure.

Techniques

Increase the flow rate of gas from low permeability reservoirs.Increase the flow rate of gas from wells that have been damaged.Connect natural fractures or cleats in a formation to the wellbore.Decrease pressure drop around well to minimize sand production.Decrease pressure drop around well to minimize problems with asphaltene or paraffin deposition.Increase the area of drainage.Connects the full vertical extent of a reservoir to a slanted or horizontal well.

Application

Must have a need to increase the productivity index. A thick pay zone.Medium to high pressure.In-situ stress barriers to minimize vertical height growth.It will either be a low permeability zone or a zone that has been damaged (high skin factor).Must have a substantial volume of gas in place.

Candidate Selection

A fractured vertical well behaves much like a horizontal well.Advantages of Fractured Vertical Wells

Can be used in thick formationsNot affected by low vertical permeability

Disadvantages of Fractured Vertical WellsNo control over the fracture orientationPossibility of uncontained fracture growth resulting in excessive gas or water influx.

TYPES OF FRACTURESINFINITE-CONDUCTIVITY FRACTURESUNIFORM FLUX FRACTURESFINITE-CONDUCTIVITY FRACTURES

Assumptions

Drainage volume is box shapedThe well fully penetrates the formationThere is no restricted entry to flowThe production is predominantly stabilized flow for all layersThe effect of non-Darcy flow is ignored The rock property in each layer is the same

Equations

Sr

rZTPPKh

q

w

e

wfe

ln

0007027.0 22

f

ffCD kx

bkF

RESULTComparism Between Two Reservoir Pay Thicknesses

0

200

400

600

800

1000

1200

1400

40000 90000 140000 190000 240000 290000 340000

Gas Flow Rate (Mscf/D)

Pres

sure

(psi

a)

Pay = 50ft Pay = 20ft

Vertical fracture well productivity decreases with pay thickness

Fracture can only be beneficial when permeability is relatively low

For the gas hydrate reservoir, it is expedient to perforate in the free gas zone

Conclusion

SLANT WELLα

•A directionally drilled well, that is inclined at an angle α to the vertical.

•α is usually between 30˚-75˚ to be effective

Definition

α

Reason for use & Areas of Practical Application

To reduce the cost of drilling several wells from a single platformTo allow extraction of oil/gas from areas unreachable conventionallyIn reservoir with down-dipFor formation with low permeability to gas

The Great Lakes, along the shores of Lake Michigan and HuronIn the Coalbed Methane field of Valencia Canyon in Northern San Juan basin of Colorado.In the Greater Green River Basin of Colorado.

Equations 100log5641 865.106.2

Ds hs

vhwD kkrhh

tantan 1hv kk

sww srr exp

wewevs rrrrJJ lnln

Result:A Graph of Productiovity Index Ratio of Slant to Vertical Wells

vs Slant Angle For Pay Thickness = 20ft

0.995

1

1.005

1.01

1.015

1.02

1.025

1.03

1.035

30 35 40 45 50 55 60 65 70 75 80

Slant Angle (degree)

Js/J

v

Kv = 0.0001md Kv = 0.001md Kv = 0.01md Kv = 0.1md

A Graph of Productivity Index Ratio of Slant to Vertical Wells vs Slant Angle For Pay Thickness = 50ft

0.995

1

1.005

1.01

1.015

1.02

1.025

1.03

1.035

1.04

30 35 40 45 50 55 60 65 70 75 80

Slant Angle (degree)

Js/J

v

Kv = 0.0001md Kv = 0.001md Kv = 0.01md Kv = 0.1md

Conclusion

The slant well is highly dependent on the vertical permeability

When the kv is low, then productivity will be low

Gas migrates vertically upward and because the kv is very low the productivity turned out low

The result shows that the slant well has a higher productivity than the vertical well

Model of the given field

Staircase Horizontal Well

A steady state equation for gas flow in a Horizontal section

Where,q = gas flow rate, Mscf/day kh= permeability, mdh = Thickness, ft Pe =Pressure at external radius, re, Psia

Pwf = wellbore flowing pressure, PsiaZ= average compressibility factorT = Reservoir temperature, Rre = drainage radius, ft r'w =effective wellbore radius, ft = SQRT (kh/kv) Sm = mechanical skin factor

5.0

44/225.05.05.0

ehrLa

mw

g

wfeh

srh

Lh

LLaa

zT

pphkq

2ln

2/2/

ln

0007027.022

22

Assumptions

Negligible pressure drop.

Permeability of each zone is same

No production from vertical sections.

Open hole completion to increase hydrate production in long run.

onacceleratifrictiongravityt PPPP

Production Plot for L=500 ft. for staircase horizontal well

Impact of vertical permeability on staircase horizontal well production

0

50

100

150

200

250

300

350

0 50000 100000 150000Flowrate (Mscf/d)

Pwf (

psi)

kv=0.0001 md

kv =0.001mdkv =0.01md

kv =0.1 md

Productivity Plots for staircase.

Veritical permeabilty Vs Gas flow rate

0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14Millions

Flowrate (Mscf/d)

vertic

al p

erm

, kv(

md)

h=50 ft h=20 ft

Veritical permeabilty Vs Gas flow rateL=1000 ft

0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.05 0.1 0.15 0.2 0.25 0.3MillionsFlow rate (Mscf/d)

vertic

al p

erm

, kv(

md)

h=50 ft h=20 ft

Multilateral Wells

InitializationFor stratified well the partial differential equation, Pj denoting the pressure in the jth layer,

)()()(22

2

2

2

wwHoj

jtjj

jxj

jyj

jx zzxxyqBt

pc

yp

kyp

kxp

k

Cases :

Communication Shell Barrier

Multilateral well in hydrate reservoir with no communication

Where,F=4,2,1.86 and 1.78 for n=1,2,3,4.m =number of levels.For this case, m=6 and n=1.

A Steady state equation for Multilateral Well

mw

eg

wfeh

smrh

mnLh

LFr

zT

pphkq

2lnln

0007027.0 22

Productivity Plot for L=500 ft. for multilateral well

Veritical permeabilty Vs Gas flow rateL=500 ft

0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14Millions

Flowrate (Mscf/d)

vert

ical

per

m, k

v(m

d)

h=50 ft h=20 ft

Productivity Plot for L=1000 ft. for multilateral.

Veritical permeabilty Vs Gas flow rateL=1000 ft

0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.05 0.1 0.15 0.2 0.25 0.3Millions

Flowrate (Mscf/d)

vert

ical

per

m, k

v(m

d)

h=50 ft h=20 ft

Sinusoidal Well

Pressure Drop Equations

Comparison

• 1)  A single undulating well outperforms up to total of three horizontal wells drilled in each isolated layers.

• 2) In order for a cased horizontal well to achieve the performance of an open hole horizontal well, high shot densities and long penetration lengths are needed.

• 3)  Multiple hydraulic fractures are more attractive for cased horizontal wells to achieve substantial increase in flow rates. There are optimum number of hydraulic fractures in order maximize production rates with cost consideration.

• 4)   In multiple thin-bedded sand layers, drilling horizontal well in each layer may not be feasible. Hydraulic fracturing of horizontal well to reach out to remaining sand layer may not easily achievable either. In such reservoir, drilling an undulating well seems to be more suitable completion technique.

Conclusion:

Conclusion

Recommendation

Productivity per unit length is highest in vertical well\

Overall productivity is highest for multilateral well

Incorporating in some way to productivity equation

Namit