Case Study: Stratified Gas Hydrate
Reservoir with Associated Free Gas
Group Project
PETE 680: Horizontal well Technology
Presented By,
Namit Jaiswal,
Adejoke M Ibironke
Objective
1. Gas Hydrates
2. Overview of horizontal well and
designer well
3. Case description
4. Results
5. Conclusion
6. Recommendation
Alaska Methane Hydrate
Estimated Resource
EILEEN TREND
MPU
100%
FREE GAS?
44 TCF
KRU
39%
DIU
PBU
26%
GAS HYDRATE
& FREE GAS
60 TCF?
GAS HYDRATE
TARN TREND
(After Collett, 1993)
GAS HYDRATES – AN OVERVIEW
Crystalline 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
Depressurization
Gas
Out
Imperm.
Gas
Thermal Injection
Gas
Out
Rock
Imperm.
Hot Brine or
Gas
Rock
Inhibitor Injection
Gas
Out
Imperm.
Methanol
Rock
Hydrate
Gas Hydrate
Dissociated
Hydrate
Free-Gas
Reservoir
Gas Hydrate
Dissociated Hydrate
Dissociated Hydrate
Impermeable Rock
Impermeable Rock
After Collett, 2000
Boundary Condition
Gas zone
Hydrate zone
t
Peq
Po
Pwf
Radial distance
Pin
Algorithm for Performance of a
Hydrate Reservoir Evaluation
1.
Assume an average pressure pavg, and calculate gas compressibility cg
using
1
cg 
p avg
2.
Using the value of cg , calculate total compressibility and hydraulic
diffusivity constant  from the known reservoir parameters.
ct  c f  c g
3.
For a desired gas withdrawal rate, solve eqn
q sc ZTp sc
exp     2k H
hTsc
H
p
2
in
2
 p eq
 exp       4ZTS
  
EI   1 
 h 


1
 H 
H BH
p sc
 1
Tsc
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
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
q sc
exp     4S H BH 1
h
4.
Using the value of , solve below equation (1) and (2
R  2
t .......1
K d  p eq  p0   BH
n
5.
psc dR 
psc
 BH
RTsc dt
RTsc
Using the value R*, Po and solve eq 3. and 4.
q sc 1 ZTp sc
p p 
2k1 hTsc
2
1
2
o
  r 2 

 EI  
  4 1t 
1
t
.......2
and for the un-dissociated region is
p
2
H
 p
2
in
p
2
 p eq



r2


EI


 4 t

R *2  
H


EI 

 4 t 
H


2
in




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
pavg 
7.
po  p wf
2
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
Horizontal
Small 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
Vertical well
Horizontal well
Case Description
Highly Stratified Gas Hydrate Reservoir
6 x 50’
2’
0.35’
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:
q

2
0.0007027 kh pe2  pwf
 re 

' 
 rw 
        (1)
zT ln 
Where,
q = gas flow rate, Mscfd
k= permeability, md
h = Thickness, ft
Pe =Pressure at external radius, re, Psia
Pwf = well bore flowing pressure, Psia
Z= average compressibility factor
T = Reservoir temperature, R
re = drainage radius, ft
Graph of Pressure Vs production from vertical
well
Variation of productivity
with payzone
J vs h plot
thickness.
0.08
J, Mscf/(day.psi 2)
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0
10
20
30
J ,s=0
J, s=+0.5
h,ft
40
50
60
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.
Techniques
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.
Application
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.
Candidate Selection
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.
A fractured vertical well behaves much like a
horizontal well.
Advantages of Fractured Vertical Wells
Can be used in thick formations
Not affected by low vertical permeability
Disadvantages of Fractured Vertical Wells
No control over the fracture orientation
Possibility of uncontained fracture growth
resulting in excessive gas or water influx.
TYPES OF FRACTURES
INFINITE-CONDUCTIVITY
FRACTURES
UNIFORM FLUX FRACTURES
FINITE-CONDUCTIVITY
FRACTURES
Assumptions
Drainage volume is box shaped
The well fully penetrates the formation
There is no restricted entry to flow
The production is predominantly stabilized flow for all layers
The effect of non-Darcy flow is ignored
The rock property in each layer is the same
Equations
FCD 
k f bf
kx f

0.0007027 Kh Pe2  Pwf2
q
r
ln  e   S
 rw 
 ZT
RESULT
Comparism Between Two Reservoir Pay Thicknesses
1400
1200
Pressure (psia)
1000
800
600
400
200
0
40000
90000
140000
190000
240000
Gas Flow Rate (Mscf/D)
Pay = 50ft
Pay = 20ft
290000
340000
Conclusion
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
SLANT WELL
α
Definition
•A directionally drilled well, that is inclined at an
angle α to the vertical.
•α is usually between 30˚-75˚ to be effective
α
Reason for use & Areas of Practical
Application
To reduce the cost of drilling several wells from a single
platform
To allow extraction of oil/gas from areas unreachable
conventionally
In reservoir with down-dip
For formation with low permeability to gas
The Great Lakes, along the shores of Lake Michigan
and Huron
In the Coalbed Methane field of Valencia Canyon in
Northern San Juan basin of Colorado.
In the Greater Green River Basin of Colorado.
Equations
ss    41
2.06
hD  h rw


  56
1.865
kh kv
   tan 1 k v k h tan 
log hD 100


rw  rw exp  s s 
J s J v  ln re rw  ln re rw 
Result:
A Graph of Productiovity Index Ratio of Slant to Vertical Wells
vs Slant Angle For Pay Thickness = 20ft
1.035
1.03
1.025
Js/Jv
1.02
1.015
1.01
1.005
1
0.995
30
35
40
45
50
55
60
65
70
Slant Angle (degree)
Kv = 0.0001md
Kv = 0.001md
Kv = 0.01md
Kv = 0.1md
75
80
A Graph of Productivity Index Ratio of Slant to Vertical Wells vs Slant
Angle For Pay Thickness = 50ft
1.04
1.035
1.03
Js/Jv
1.025
1.02
1.015
1.01
1.005
1
0.995
30
35
40
45
50
55
60
65
Slant Angle (degree)
Kv = 0.0001md
Kv = 0.001md
Kv = 0.01md
Kv = 0.1md
70
75
80
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
q

2
0.0007027 k h h p e2  p wf
 a 

 g zT ln 


 

2
a 2  L / 2    h   h

 ln 
L/2
  L   2rw
a  0.5L0.5  0.25  2reh / 4 


4



  s m 



0.5
Where,
q = gas flow rate, Mscf/day kh= permeability, md
h = Thickness, ft Pe =Pressure at external radius, re, Psia
Pwf = wellbore flowing pressure, Psia
Z= average compressibility factor
T = Reservoir temperature, R
re = drainage radius, ft r'w =effective wellbore radius, ft
 = SQRT (kh/kv) Sm = mechanical skin factor
Assumptions
 Negligible pressure drop.
Pt  Pgravity  Pfriction  Pacceleration
 Permeability of each zone is same
 No production from vertical sections.
 Open hole completion to increase hydrate
production in long run.
Production Plot for L=500 ft. for
staircase horizontal well
Impact of vertical permeability on
staircase horizontal well production
kv=0.0001 md
kv =0.001md
kv =0.01md
350
kv =0.1 md
300
Pwf (psi)
250
200
150
100
50
0
0
50000
100000
Flowrate (Mscf/d)
150000
Productivity Plots for staircase.
Veritical permeabilty Vs Gas flow rate
Veritic al permeabilty Vs Gas flow rate
L=1000 ft
0.12
0.1
vertical perm, kv(md)
vertical perm, kv(md)
0.12
0.08
0.06
0.04
0.02
0
0
0.02
0.04
0.06
0.08
Flowrate (Mscf/d)
0.1
0.12
0.14
Millions
0.1
0.08
0.06
0.04
0.02
0
0
0.05
0.1
0.15
0.2
Flowrate (Mscf/d)
h=50 ft
h=20 ft
h=50 ft
h=20 ft
0.25
0.3
Millions
Multilateral Wells
Initialization
For stratified well the partial differential
equation, Pj denoting the pressure in the jth layer,
k jx
2 p j
x
2
 k jy
2 p j
y
2
 k jx
p j
y
2
  j c jt
p j
t
 Bo q H ( y) ( x  x w ) ( z  z w )
Cases :
Communication
Shell Barrier
Multilateral well in hydrate
reservoir with no
communication
A Steady state equation for
Multilateral Well
q

0.0007027k h h p e2  p wf2

  Fre   h   h 

  s m 
 g zT ln    
 ln 
  L   mnL   2mrw 

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.
Productivity Plot for L=500 ft. for
multilateral well
Veritical permeabilty Vs Gas flow rate
L=500 ft
vertical perm, kv(md)
0.12
0.1
0.08
0.06
0.04
0.02
0
0
0.02
0.04
0.06
0.08
Flow rate (Mscf/d)
h=50 ft
h=20 ft
0.1
0.12
0.14
Millions
Productivity Plot for L=1000 ft. for
multilateral.
Veritical permeabilty Vs Gas flow rate
L=1000 ft
vertical perm, kv(md)
0.12
0.1
0.08
0.06
0.04
0.02
0
0
0.05
0.1
0.15
0.2
Flow rate (Mscf/d)
h=50 ft
h=20 ft
0.25
0.3
Millions
Sinusoidal Well
Pressure Drop Equations
Comparison
Conclusion:
•
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.
1)
Conclusion
Productivity per unit length is highest in vertical well\
Overall productivity is highest for multilateral well
Recommendation
Incorporating in some way to productivity
equation
Namit