1. Drilling Depleted Reservoirs
Maurice Dusseault
Module 6-C: Depletion and Changes in Stress: Drilling Depleted Reservoirs: How depletion affects stresses in a generally predictable manner. The changes in stress in a general 3-D stress field, and changes in the deviatoric effective stress during depletion. What are the assumptions behind predictions of stress path?

2. Depleted Reservoir Effects
Depletion of a zone has two major effects:
The lateral total stress, sh, drops
The effective stresses, sh, sv rise
This results in:
Drop in PF in the depleted zone
Increase in confining stress = stronger rock
Consequences:
Slower drilling because rock is tougher
LC and blowout risks go up substantially
More casing strings, LCM squeezes, …
More common recently (deeper targets)

3. Reservoir Depletion sh]o
stress
sh]o
lateral stress
redistribution
pressure
depletion
in stratum
Pressure depletion leads to an increase in sv, sh and a decrease in sh (Sh). This is the Poisson effect, related to n/(1-n)
(n = symbol for Poisson’s Ratio)
Increased s means the rock is stronger and tougher to drill. Different bits may be required.
The sh “lost” in the reservoir zone must be redistributed above and below the reservoir po pf
Dsh
sh
depleted sand Dp
stress
concentration
sh]f
depth

4. Depletion Effect on sh h stress trajectories h concentration
wellbore
h concentration
far-field stresses
h along
wellbore
At a large scale, horizontal stresses are also redistributed above and below the reservoir. This should lead to much better fracture containment if a thermal or fracture process is used after a period of CHOP.
final h
Zone after production (Dp)
Operational consequences: low pfrac in reservoir
-higher pfrac above reservoir
-better fracture containment
initial h Z

5. Depletion and sh (or PF)
The reservoir shrinks because of the drop in pore pressure
sh“arches” around the reservoir
In addition to a drop in PF, there is an increase in sh above and below the zone
Effects on vertical stress are negligible:
Because reservoirs are usually laterally extensive and thin in comparison to L
PF can be expected to be low
Most serious in HTHP wells, multiple zones

6. Shear, Fracture Opening
sv
Zone of pressure decline, -Dp
sh
Pressure decline leads to an increase in the shear stress
This can lead to shearing, which causes fractures and fissures to open
This leads to increases in permeability, better reservoir drainage
Exceptionally, it can lead to casing problems, etc.

7. Other Depletion Effects
In some rare cases, the reservoir enters a condition of shear failure (or collapse)
This may have beneficial effects on the reservoir itself (e.g. Ekofisk in North Sea)
Shearing in a dense rock opens fractures
Permeability goes up
Of course, there may also be issues of casing shear (Dusseault et al., 2001)
Free gas development in the reservoir
Seismic velocities change, stiffness too …
Dusseault MB, Bruno MS, Barrera J, Casing shear: causes, cases, cures. SPE Drilling & Completion Vol.16 Num. 2, , June 2001

8. Depletion Failure Path, M-C Plot
t – shear stress Y
Mohr-Coulomb plot
This example is for
a normal fault case, assuming sv constant
before Dp, initial in situ conditions
shear
after Dp
n
h]b
h]a
v]b
v]a
a & b stand for after and before Dp
Remember, these are effective stresses, not total stresses

9. Irreconcilable MW in Depletionpo
stress
sh]i
MW limits cannot be reconciled! Special measures are required.
It is impossible to find a MW that both controls the pore pressure in thin adjacent undepleted sands, and also avoids LC in the depleted zone
This effect is greatest in deep HPHT reservoirs that are massively depleted (large oil reservoir drawdown)
undepleted sand
lateral stress
depletion
MW to control po in undepleted sands
depleted sand
max. MW to avoid fracturing
undepleted sand
depth

10. A Typical GoM Case A was discovered and depleted to 0.2po50
100
150
A was discovered and depleted to 0.2po
Now, we want to drill through to B & C
Or, A & B depleted, and our target is C
Small gas sands are not depleted and present blowout hazard
Lowered PF in zones present LC hazard
stress, pressure, MPa 1
pore pressure, po 2
thick shale
sequence
lateral stress, hmin 3
vertical stress, v 4
gas sand
Target A
gas sands 5
Target B
gas sand
Target C 6
depth - kilometres

11. Gradient Plot, GoM Case
1.0
2.0
Same as the previous plot, only now in mud density units
Colored lines are the original state of po, s
2.0 = 16.7 ppg MW
Original target(s) A&B are depleted
The goal is to drill to Target C without:
Too many casings
Lost circ, blowout
Challenging!
gradient plot, mud density 1
16.7 ppg po
hmin 2 v
thick shale
sequence 3 po 4
gas sand
Target A
gas sands 5
Target B
gas sand
Target C 6
depth - kilometres

12. What Happens in the Reservoir
Reservoir is thin, extensive, so sv constant
No horizontal strains can take place
Thus, the horizontal total stress changes are governed only by the Poisson’s ratio effect and the change in pore pressure σv σh σh
ex = ey = 0 po σv

13. New Fracture Gradient Calculation
Determine depleted pore pressure in ppg
Determine pore pressure change in ppg
New PF is fracture gradient is :
PF = PF]i + Dp·(1-2n)/(1-n)
Where PF]i is the initial fracture gradient
Dp is the change in reservoir pressure in ppg (remember, this is a –ve number)
n is Poisson’s ratio, a dimensionless value
You can do these in any units (density, ppg, MPa, psi), as long as you are consistent

14. All Stresses After Depletion
All the new stresses can be calculated
New pore pressure is pf, change, Dp = po - pf
Initial sv]i = sv – po, final is sv]f = sv – pf
Horizontal effective stress change =
Dsh = Dsv·n/(1-n) = -Dp·n/(1-n) (Dp is –ve)
New horizontal effective stress =
sh]f = sh]i - Dp·n/(1-n) (where Dp is –ve)
PF = sh]f/z (density units where water = 1)
Careful about the signs (+ve, -ve) of terms

15. Example of Depletion Calculation
po = 14 ppg, pf = 4 ppg, Dp = -10 ppg
PF]i = shmin = 16 ppg, assume n = 0.2
PF]f = PF]i + Dp·(1-2n)/(1-n)
PF]f = 16 ppg +(-10·(1 - 20.2)/( )) ppg
= 16 ppg – 7.5 ppg = 8.5 ppg
This is almost the gradient of H20!
Lost circulation will be unavoidable!
There will still be some thin 14 ppg gas sands above or below the zone!
Let’s discuss choice of Poisson’s ratio

16. Choice of Poisson’s Ratio
In very shaley sands, n ~ 0.28 – 0.32
In clean sands, n ~ 0.23 – 0.25
In fractured reservoirs, n ~ 0.18 – 0.22
Extremely fractured, as low as
You can use geophysical log estimates (~)
Better, you can use calibrations based on real measurements, methods have been published
It is best to calibrate to your own basin, so collect the data as it becomes available

17. Back to Our Example
1.0
2.0
Now, there is no MW possible to drill through the zones without exceeding PF or below po
We need to find a new strategy for drilling
gradient plot, mud density 1
16.7 ppg po
PF = hmin 2 v
thick shale
sequence 3 po 4
Intact thin gas sands at original po
Two depleted reservoirs, low PF
New target reservoir
Risks of blowouts, LC 5 6
depth - kilometres

18. The Casings & Liners Option
Requires expensive casing strings or liners
This approach usually results in two extra strings for each depleted zone
Clearly, it is prohibitively expensive if there is more than one zone with active charged gas sands between
Are there other options?
Yes, but there are some risks involved, or, there is additional time involved
Let’s look at some options

19. Option One: Use Casings/Liners
1.0
2.0
Drill to below A with 16 ppg MW, set casing
Lower MW to 13 ppg, drill to B, set casing
Raise MW to 16.7, drill to C, set casing
Lower MW to 13.5 ppg, drill to D, set casing
Raise MW to 16.7 ppg, drill to TD, set production casing/liner
Now you have only a 5” hole and a 3.5” “casing”
gradient plot, mud density 1
16.7 ppg PF 2
13.0 ppg 3 4 A B 5 C D 6
depth - kilometres

20. Case with no Overlying Gas Sands
This is a favorable condition:
Drill to bottom of depleted zone w. low MW
LCM squeeze in depleted zone to raise PF
Raise MW to drill ahead, resqueeze if needed
LCM squeezing is performed as much as necessary to raise MW to po in gas sands
However, remember the shales!
po is still high in the shales
If MW < po(shale), massive sloughing possible
This option is similar to underbalanced drilling
Thin undetected gas stringers remain a risk!

21. Perhaps UB drilling will help in this zone
No Overlying Gas Sands
Drill down to top of zone with as low MW as possible
Add LCM to mud before you enter zone
Build mud & LCM to keep up with losses
Set casing below zone
Increase MW to handle po in gas sands
Drill to Target
Perhaps UB drilling will help in this zone
Depleted
Gas sands
Target

22. Depleting the Thin Gas Sands
If the original wells are still available…
Perforate into all the thin gas sands identified on logs or drilling records
Deplete with maximum drawdown to reduce PF to the same as adjacent depleted zones
Drill with low MW, set casing above target, increase MW, drill into target… Risks?
Some thin sands may be missed
Depletion may be a lengthy process
Drill immediately after depletion and set casing as quickly as possible (low recharge time)

23. Living with Some Gas Influx
If the gas sands are tight and thin…
Gas influx rate may be manageable during drilling with the MW less than the pressure in the gas sands (underbalanced)
Also, shales cannot be so weak that they slough into the hole with the low MW used!
Mud de-gassers, gas collection from closed flow lines, etc. are safety measures
This can be used only with great caution in the right circumstances
Unlikely in sloughing shale regions

24. Is it Possible to Avoid the Zones?
If the depleted zones are in a fault block, or are limited channel sands, avoidance…
Using seismics, drill into zone to avoid the other zones
Good stratigraphic/seismic data are vital
Well trajectory for avoiding the depleted sequence
Gas sands
Depleted
Target
Faults isolating the productive sequence

25. Can Old Wells be Re-Used?
If the old wells are in good condition, they can be plugged carefully and used to drill
Multilaterals are also possible…
Squeeze with non-shrinking cement, perhaps patches too
Repeat squeeze perhaps?
Drill ahead (with bicenter bit?)
Install liner (expandable liner to save hole size?)
Complete the well

26. LCM Squeezes in Depleted Zones
Drill to just above the depleted zone with as low a MW as feasible
Place high LCM mud at bit, and drill into zone OB; LC will occur, but will also tend to “pack off” the depleted zone
Drill cautiously through zone, continuing to allow fracturing with the high LCM mud
Once through zone, it may be advisable to close annulus and slowly squeeze extra LCM into the depleted zone
Measure LOT in depleted zone for safety

27. How Does a LCM Squeeze Work?
hole
shmin
sHMAX
Zone of increased sq
Multiple fractures
Short, wide fractures created
This increases the stresses locally around the wellbore
Effect can be increased by repeated squeezes to make the tangential stresses higher in a larger zone around well
However, remember that does not increase stresses beyond the extent of the fractures. If a high po is encountered lower, LC may take place
sq must go up as material packed in

28. Some Rig Capabilities Needed
For drilling HPHT wells, and for drilling deep through depleted zones
High & variable load capacity for riser and mud storage on offshore rigs
High pump capability
Large diameter kill line and choke line needed for quick response
Early kick detection system
Pressure sensors on BOP at mud-line level
Gas handler on marine riser
Pressure measurement devices on MWD
LCM pill mixing facility for LCM squeezes

29. Depleted Zone Options Summary
Use the old wells if possible
Deepen wells after zonal isolation
Can be used to deplete charged thin sands
Can the new well trajectory avoid zones?
Live with gas cut mud (if shales are strong)
Drill with designed LCM material in mud
Use LCM squeeze in depleted zones
Place strategic casings and liners
Use bicentered bits, expandable casings to maintain reasonable hole diameter with depth
Combine one or more of these options

30. Lessons Learned- Depleted Cases
Depleted reservoirs present singular challenges for drilling
Options include avoidance, use of LCM, multiple casings/liners, use of old wells…
The risks involved are higher than in conventional reservoir development
Careful surveillance during drilling
Special drilling equipment…
Bicenter bits and expandable liners represent a useful, albeit costly approach