How the hole is drilled
The hole for an oil well commence when the 20 inches or
plus diameter surface hole is drilled.
The bottom hole
assembly (made up of drill pipes and drill collars with the
bit) is attached to the kelly. The rotary table drives the
kelly and hence the drilling bit the bottom is rotated.
Surface hole is drilled until reaching the bedrock. Then the
conductor pipe or the first casing is installed within the
hole.
Casing is very expensive.
During running in casings, the
drilling operation has to be stopped. Careful planning has to
be done before running in the casings.
Before running in casings, the drilling mud should be
suitable in keeping in the bottom hole pressure.
The thickness of the steel for casing should be appropriate
so that certain logging operation can be carried out.
1
The casing is run in from the top of the hole. The purpose
of the casing is to isolated the production zones.
Casing and cementing
The general functions of all casing strings are:
1.
To furnish a permanent borehole of precisely known
diameter through which subsequent drilling, completion
and producing operations may be conducted.
2.
To allow segregation of formations behind the pipe,
which prevents interformational flow and permits
production from a specific zone.
3.
To afford a mean of attaching the necessary surface
valves and connections to control and handle the
produced fluids.
The main purpose of primary cementing is as in (2) above,
other purposes are:
1.
To afford additional support for the casing, either by
physical bracing or prevention of formation pressures
being imposed on the pipe.
2
2.
To retard corrosion by minimizing contact between the
pipe and corrosive formation waters.
Casing types and specifications
Surface casings
In hard rock areas, a single string of surface pipe set a few
hundred feet deep is usually adequate to:
1.
control caving and washing out of poorly consolidated
surface beds
2.
furnish a mean of handling the return follow of
drilling mud.
3.
protect
fresh
water
sands
from
possible
contamination by drilling mud, or oil, gas, and/or salt
water from lower zones.
4.
Allow attachment of blowout preventer.
In soft rock areas, it is often necessary to set both a
conductor string and a surface string to perform these
functions. In such instances, the conductor pipe may be two
3
or three hundred feet long with the surface pipe extending
to depth of two or three thousand feet.
Intermediate casings
The decision to use intermediate string is largely dependent
on well depth and geologic conditions in a specific area.
Wells of moderate depth may use no intermediate pipe,
while deep wells may require one or more protective strings.
The principal functions of the intermediate casins is to seal
off troublesome zones which
1.
contaminate the drilling fluid, making mud control
difficult and expensive (salt, gypsum, heaving shales
etc.)
2.
jeopardize drilling progress with possible pipe
sticking, excessive hole enlargement, or other
fishing hazards.
4
Oil Strings or Production casings
This is the last and deepest casing string run.
It is set
above, part way through, or completely through the
lowermost pay zone, depending on the type of completion to
be performed. It furnishes the means of segregating the
pay section from all other zones and is the work shaft
through which access to the producing zone is gained.
Normally,
the
produced
fluids
flow
through
tubing;
sometimes, however, the oil string may actually serve as the
flow conduit.
Cementing operation
The casing has to be cemented to the well bore.
The
technique used is by pumping the cement through the casing
into the annulus (the space between the casing and the wall
of the borehole).
The amount of cement to be calculated before the
cementing begins, based on the size of the hole and the size
5
of the casing.
Logs may be run prior to cementing to
determine if the hole is in gauge.
Also prior to the cementing operation, the cement is tested
to determine how much time is available for pumping before
the cement begins to set compressive strength.
In any operation the hole is always filled with drilling mud.
Even when casing is being run in.
When the casing is run in and set in place at the top of the
hole, the top of the casing is connected to the pump.
A bottom plug is inserted into the casing, then the pump will
pump cement behind the bottom plug. When enough cement
(has been calculated) has been pumped, the top plug is
inserted.
The top plug is being pushed into the casing by pumping
drilling mud.
6
When the bottom plug hit the float shoe of the casing,
pressure will increase due to the continuous pumping of the
drilling mud. It will come to a stage when the diaphragm of
the bottom plug will break and through the shoe, the
cement is pushed into the annular.
The top plug will hit the bottom plug when the pumping
pressure increases. Pumping will cease and the wellhead is
secured while waiting-on cement (WOC) to set.
7
Fishing Operation
The term fishing applies to all operation concerned with the
retrieving of equipment or other objects from the hole.
Portion of the drill string, bit, drill string accessories and
inadvertently dropped hand tools are typical items which
may require fishing.
Stuck pipe
Many fishing jobs start with the drill pipe becoming stuck
during a trip. Some of the causes of stuck pipe are
i.
foreign objects or junk in the hole
ii.
sloughing formations (heaving shales, etc)
iii.
bit and drill collar balling
iv.
pressure diffrential sticking
v.
cuttings settling above the bit or drill collars
All of the above are quite obvious except (iv) which requires
further clarification.
8
Fishing tools
i.
Overshot tools: slip over the outside of the object
and pull.
ii.
Tapered tap: the tap is run into the fish and rotated
until sufficient threads are cut for a firm hold.
iii.
Spears: these devices pass inside the fish, grasping
its inner wall with expanding slops, which can be set
or released by rotating the pipe.
iv.
Washover pipe:
this is a section of pipe having
sufficient inside diameter to telescope outside the
fish.
v.
Inside and outside cutters:
9