2. Well Planning
• Pore Pressure and Fracture Gradient
• Casing Depth Selection
3. Types of Well Planning
• Exploration well planning.
• Development well planning.
• Completion/workover programme planning.
4. Well Planning
• Close interaction between Drilling and the
Exploration, Petroleum Engineering, Design
and Construction Departments is vital to
optimise total project costs. Rig/platform
interaction, well reach and well design
options should be fully evaluated in the
conceptual stage. All platform options and
well design options should be considered and
all possibilities for reducing costs should be
examined.
5. Typical Well Configuration
Typical well
configuration
between exploration
wells and
development well
Figure 1: Well Configuration
6. Pore Pressure and Fracture
Gradient
Parameters like pore pressure and formation
strength determine aspects like:
1. Choice of mud weight profile,
2. Determination of casing setting depths,
3. Design of optimal casing strings,
4. Selection of the drill bit,
5. Cementing additives and procedures.
7. RKB & MSL
• Within drilling engineering, the drilling floor or
rotary table (RKB) is the most often used
reference depth.
• Geologists and geophysicists generally prefer
to use their data in reference to ground floor
or mean sea level (MSL).
8. Well Depth Guidance
• Correct RKB to MSL reference
MSL RKB D h
– ……………………………..(1)
• Convert MSL data to RKB
…………….………………..(2)
RKB
d d D
-
=
d = d D -
hRKB
RKB MSL
D
9. Wildcat to Development Well
Correlation
• Another common problem is when data referenced
to one RKB (e.g. rig used to drill the wildcat well)
has to be applied for further/later calculations (e.g.
drilling development wells from a production
platform). Here the data have to be corrected from
RKB1 to RKB2 .
• Correct from RKB1 to RKB2:
d d D h RKB RKB
= - D 2 1
……………………………..(D 3)
10. Wildcat to Development Well
Correlation
where:
• D [m or ft] ... total depth of point of interest in
reference to RKB
• hRKB [m or ft] ... height of RKB above MSL
• Δh [m or ft] ... difference of elevation of RKB1
to RKB2
11. Geology Prediction
• Normally when a well is to be
drilled, the drilling engineer is
supplied from the geology
department with a sequence
of predicted subsurface
formations, their
characteristics and markers,
as well as knowledge about
where special care has to be
taken.
Figure 2: Typical geological
profile to plan a well
12. Pore pressure Prediction-
Hydrostatic Pressure
• By definition, a hydrostatic pressure is
developed due to the own weight of a fluid at
a certain depth.
……………………………..(4)
p = r g h = 9.81r h
• Or in field units:
……………………………………..(5)
p = 0.052 r fl D
13. Pore pressure Prediction-
Hydrostatic Pressure
Where
• ρfl [ppg] ... density of the fluid causing hydrostatic
pressure
• ρ [kg/m3] ... average fluid density
• D [ft] ... depth at which hydrostatic pressure occurs
(TVD)
• h [m] ... vertical height of column of liquid
• p [psi] ... hydrostatic pressure
• g [m/s2] ... acceleration due to gravity
14. Pore pressure Prediction-
Hydrostatic Pressure
When the burial depth
increases, the overlaying
pressure (overburden
stress) increases. This
decreases the pore space
between the grains and
thus the porosity of the
formation.
Figure 3: Porosity profile with
increasing depth
15. Pore pressure Prediction-
Hydrostatic Pressure
• When drilling a well, formations are often
encountered that are under a different pressure
regime. These formations are named to be
“abnormally pressured”. Abnormal pressures can be
positive (actual formation pressures are higher than
hydrostatic pressure) or negative (actual formation
pressures are lower than hydrostatic pressure).
Sometimes the term “subnormal pressure” is used
when the formation pressure is lower than the
hydrostatic one.
16. Pore pressure Prediction-
Hydrostatic Pressure
• Some mechanisms that lead to abnormally
pressured formations are:
• 1. Compaction effects,
• 2. Artesian System
• 3. Uplift
• 4. Salt Beds
• 5. Differential density effects (Osmosis)
• 6. Salt Domes
17. Pore pressure Prediction-
Hydrostatic Pressure
Compaction effects
A sealing mechanism must be present to trap abnormal
pressure environment. The common sealing mechanism is a
shale section. The shale reduces normal fluid escape,
causing undercompaction and abnormal fluid pressure.
….(6)
0.465 / 1.0 / ( ) B 1 B P = psi ft D + psi ft D - D
Where:
D1 = depth of interest below the barrier, ft.
DB = depth of the barrier, i.e., low permeability section, ft
P = formation pressure at D1, psi
18. Pore pressure Prediction-
Hydrostatic Pressure
• While burying of the sediments, formation water is
expelled with increasing depth and temperatures
due to reduction in pore space and diagenesis of
the rock materials.
• As long as the permeability and the effective
porosity of the rock is high enough so that the
formation water can escape as quickly as the
natural compaction takes place, the formations are
normally pressured. The (vertical) pressures acting
inside formations can be modelled as:
19. Pore pressure Prediction-
Hydrostatic Pressure
……………………………………(7)
ob z p s =s +
Where :
σob [psi] ... overburden stress
σz [psi] ... vertical stress supported by the grain-to-grain
connections
p [psi] ... formation pore pressure
20. Pore pressure Prediction-
Hydrostatic Pressure
• The bulk density [ppg] of a formation is
estimated by equation:
……………………………….(8)
r r f r f b g fl = (1- ) +
where:
ρg [ppg] ... grain density
ρfl [ppg] ... formation fluid density
f [1] ... total porosity of the formation
21. Casing Depth Selection
Type of casing :
• Conductor casing
• Surface casing
• Intermediate casing
• Production casing.
22. Casing Depth Selection
A. Conductor casing
Applications:
- to provide a fluid conduit from the bit to the surface.
- to minimize hole caving.
B. Surface Casing
Applications:
- cover freshwater sands
- maintain hole integrity by preventing caving
- minimize lost circulation into shallow, permeable zones
- cover weak zone that are incompetent to control kick-imposed
pressure
- provide a means of attaching of BOP
- support the weight of all casings run below the surface pipe
23. Casing Depth Selection
C. Intermediate casing
Applications:
- used to isolate salt zones or zones that cause hole problems, such as
heaving and sloughing shales.
D. Production casing.
Applications:
- isolate the producing zones from the other formations.
- provide a work shaft of a known diameter to the pay zones.
- protect the production tubing equipment.
24. Setting Depth Design Procedures
• Casing seat depth are directly affected by
geological conditions. In some cases, the
prime criterion for selecting casing seats is to
cover exposed, severe lost circulation zones,
differential sticking problems, pressure
depletion, controlling abnormal pressure and
preventing their exposure to weaker shallow
zones.
25. Setting Depth Selection for
Intermediate and Deeper Strings
Criteria:
1.Let the mud weight control formation pressure
without fracturing shallow formations (this
considerations are made from bottom to up)
2.Differential pressure sticking determination to be
made to verify the casing string will become
stuck when running it to the well (this
considerations are made from top to bottom)
26. Setting Depth Selection for
Intermediate and Deeper Strings
General values for the amount of differential
pressure that can be tolerated before sticking
occurs:
- Normal pressure zones: 2,000 – 2,300 psi
- Abnormal pressure zones: 3,000 – 3,300 psi
27. Setting Depth Selection for
Intermediate and Deeper Strings
Equation to determine the new intermediate
depth if sticking is a concern:
…………………………..(9)
DP = (MW -9)(0.052)(D)
Where :
MW = mud weight, lb/gal
D = depth to deepest normal zone, ft
DP = differential pressure, psi
28. Setting Depth Selection for
Intermediate and Deeper Strings
An arbitrary limit of 2,000 – 2,300 psi is normally
used for DP. The mud weight, MW from equation
(9) can be used to locate the depth where the DP
value will exist:
……………………………………….(10)
MW -TM = P
Where :
MW = mud weight, lb/gal
TM = trip margin, lb/gal
P = formation pressure, lb/gal
29. Surface Casing Depth Selection
The most problem at this section is a “kick”. Kick imposed
equivalent mud weights are the cause for most underground
blowouts.
………………….(11)
ö
æ
=
EMW total depth kick + D ÷ ÷ø
( M) OMW
depth of erest
ç çè
int
Where :
EMWkick = equivalent mud weight at the depth of interest, lb/gal
total depth = deepest interval, ft
depth of interest = ft
DM = incremental kick mud weight increase, lb/gal
OMW = original mud weight, lb/gal
