Section 2 - Well Construction Basics.ppt

Section 2 - Well Construction Basics.ppt

1. 1
WELL CONSTRUCTION
BASICS

2. 2
Well Construction
Well construction is an engineering design process that starts
with a clear understanding of the objectives to be achieved. As a
minimum, the following data are required to commence the design
and planning process
•Well depth
•Location – land, offshore (water depth)
•Geology – lithology, structural features, hazards, stability
•Formation Pressure profile
•Fracture Gradient profile
•Downhole target(s) – vertical or directional well
•Type of well – exploration, delineation, appraisal, development
•Evaluation requirements – sampling, coring, logging, testing
•Type of fluid and anticipated flow rates

3. 3
Well Construction
Any given well requires several strings of
casing, starting with the largest diameter pipe
on top. Subsequent strings of casing are
progressively smaller in diameter as they must
pass through the inside of the previous casing.
The diameter of the deepest casing is
generally dictated by the size of the
production tubing required for anticipated
flow rates.
The primary purpose of each casing string is to
isolate one geological regime from the other
and enable further drilling with full control of
anticipated downhole pressure conditions of
the next interval. An understanding of
downhole pressure variations is essential for
selecting the setting depth of each casing.
Conductor
Surface
Intermediate
Production

4. 4
Basic Concepts for Well Construction and Drilling
Pressure
Hydrostatic Pressure
Pressure Gradient
Conversion of Units
Buoyancy
Pore Pressure
Fracture Gradient
Mud Weight Window
Casing Seat Selection
Wellbore Schematic

5. 5
WHAT IS PRESSURE?
Pressure is the force exerted per unit area.
Units of measurement:
psi, bar, kgf/cm
2
, kPa
1 atm (standard) = 14.696 psi = 101.325 kPa
1 bar = 100 kPa = 0.987 atm = 1.02 kg/cm
2
Pressure may be stated as gauge, or absolute
Atmospheric pressure at sea level is taken as
0 psig (gauge), or 14.7 psia (absolute)
Pounds per Square Inch, or psi, is the most common
oilfield unit for pressure. Some other countries use
metric units or a hybrid system.
Pressure

6. 6
WHAT IS HYDROSTATIC PRESSURE?
A vertical column of fluid (gas or liquid) will exert
a pressure. This is called hydrostatic pressure.
Consider a 12” x 12” x 12” cube of water weighing
62.4 pounds (its density). The pressure on the bottom
surface will be 62.4/144 = 0.433 pounds/sq.inch (psi)
If the height of the column is doubled:
Pressure on bottom surface = (2 x 62.4)/144 = 0.866 psi
For every one foot of increase in vertical height there is
an increase of 0.433 psi.
This is called PRESSURE GRADIENT
Hydrostatic Pressure

7. 7
The total weight of the water column is reduced to half by
splitting the cube. But the pressure on bottom does not
change, nor does the gradient of 0.433 psi/ft. Units for
density are:
pounds per gallon (ppg) – most common in oilfield
pounds per cu.ft. (pcf)
gm/cc (Sp.Gr.)
The density of water is 8.33 ppg
Hydrostatic Pressure at any point depends only on the vertical
height and density of the fluid, or
P
h
(psi) = 0.052 x fluid density in ppg x True Vertical Depth in
feet (TVD)
Pressure Gradient

8. 8
1 cu. ft. = 7.481 U.S. gallons, and 1 sq. ft. = 144 sq. in.
To convert gradient from psi/ft to pounds per gallon, divide
by the factor (7.481/144), or 0.052
To convert gradient from pounds per gallon to psi per foot,
multiply by 0.052
Exercise:
Convert 12.0 ppg to psi/ft_______
Convert 1.0 psi/ft to ppg_______
Conversion of Oilfield Pressure Units

9. 9
Well ‘A’ Well ‘B’
TD 10,000’
TVD 10,000’
MD 12,000’
TVD 10,000’
Mud Weight = 10.0 ppg in both wells
Bottomhole Pressure Well ‘A’ =
Bottomhole Pressure Well ‘B’ =
Hydrostatic Pressure in Wellbore -Exercise
TD = Total Depth
MD = Measured Depth
TVD = True Vertical Depth

10. 10
Pressure gradient of compressible fluids is complex as it
depends on several factors, such as specific gravity of the
gas, bottomhole and surface pressures, temperature,
compressibility factor, and height of column. The
following relationships could be used for approximation
where precision is not critical:
0.1 psi/ft for a gas column down to 10,000’, and 0.15
psi/ft if deeper
Pressure Gradient of a Gas Column

11. 11
Buoyancy
Buoyancy is the loss of weight when a body is immersed in liquid.
It depends on the density and volume displaced.
In drilling applications it is important to know the buoyant
weight of steel drill pipe, casing and downhole tools when they
are lowered in the wellbore. This done by first calculating the
Buoyance Factor for the given mud density, and then multiplying
the weight in air with the Buoyancy Factor.
Buoyancy Factor = 1- (Mud density in ppg/65.5)
where, 65.5 is the density of steel in ppg
Exercise:
A drilling assembly weighs 340,000 lbs in air. How much will it
weigh when lowered in a well full of 13.0 ppg drilling fluid?
Answer _________

12. 12
•Pore Pressure is the pressure of the fluids within
the pores of the formation
•Normal Pressure is equivalent to the pressure
of a column of water with a pressure gradient
ranging from 0.433 psi/ft (8.3 ppg) for fresh water
to 0.52 psi/ft (10 ppg) for salt water. Typical
value is 0.465 psi/ft (8.9 ppg)
•Abnormal (overpressure) is any pressure above
normal
•Subnormal is any pressure below normal
Pore Pressure

13. 13
Under-compaction: Entrapment of pressure due to rapid
deposition of overlying seal not allowing water to escape
Uplifting: Formations compacted at great depth can be
uplifted to a shallower depth due to tectonic forces.
Centroid: Deeper fluid pressure in communication with
shallower part of a dipping formation
Artesian: Outcrop at a higher elevation
Diagenesis: Temperature and pressure causing alterations in
the composition of formations over time
Hydrocarbon effect: Large effect at shallow depths
Origin of Abnormal Pressures

14. 14
•Fracture gradient is the mud weight required to fracture, or
cause the formation to separate. It is the static mud weight,
or equivalent dynamic mud weight, at which lost circulation
occurs
• Lost circulation is a major cause of well control problems
•At a given depth Fracture Gradient depends on:
Lithostatic or overburden gradient
Pore pressure
Ratio of vertical and horizontal stresses in the matrix
Fracture Gradient

15. 15
During the planning
stage, the two profiles
are developed from
offset well data
analysis, basin models,
and seismic data
During drilling, real time
data is derived from
actual mud weight
requirements, downhole
logging tools, drilling
responses, and Leak Off
Tests. Adjustments in
the program may be
required for observed
differences between
prediction and actual
Mud Wt Equivalent (ppg)
D
e
p
t
h

(
f
e
e
t
)
PP FG
Pore Pressure & Fracture Gradient Profiles

16. 16
D
e
p
t
h

(
f
e
e
t
)
Mud Wt Equivalent (ppg)
•Trip Margin
Generally 0.3 - 0.5
ppg over Pore
Pressure
On Deep Wells may
switch to 150 - 200 psi
over Pore Pressure
•Safety Margin
Kick Tolerance
Generally 0.3 - 0.5
ppg under FG
Mud Weight
Window
The Mud Weight Window

17. 17
D
e
p
t
h

(
f
e
e
t
)
PP & FG, Mud Wt Equivalent (ppg)
•Bottom Up
Vertical and horizontal
lines are drawn,
starting from the
bottom, to determine
casing depth for each
string of casing
•Top Down
Vertical and horizontal
lines are drawn to
select casing seats
•Generally, the one that
results in fewer strings of
casing is selected
•Other Considerations
Differential pressure
Lithology
Kick Tolerance
Exploration well
Casing Seat Selection

18. 18
RKB 0 ft
MSL 150 ft
Mud Line 600 ft
Typical Well Schematic
30” Conductor
20” Surface Casing
13-3/8” Intermediate Casing
9-5/8” Production Casing
7” Production Liner
Pay Zone

19. 20
Grades of Casing – according to strength & properties
API General Service
Grade
H-40
J55
K55
L-80
C-95
T-95
N-80
C-95
P-110
Q-125
Min Yield Strength - psi
40000
55000
55000
80000
95000
95000
80000
95000
110000
125000
API High Strength
API Sour Service