PETROLEUM WELL COMPLETION OVERVIEW
WELL COMPLETION
DEFINITION
Well
completion means preparing the well for production, of course well completion
is worded only when the well have sufficient amount of oil and gas to be
commercially viable. That is why data from all the evaluations and tests is so
important.
Now
with probable positive evaluations, we are ready to complete the open hole,
part of the well. These open holes, where we conducted our wire line tests and
coring can now be cased and cemented to become a closed Cased Hole. When we
talk about running casing in well completion, we are only referring to the
casing the zone or zones of interest in the open hole. These casing strings are
referred to as production casing it is sometimes easy to confuse the function
of production casing, with those of surface intermediate casing.
Well Completion - Well Casing
WELL CASING
It
can be explained that surface casing for example is the first casing string to
be run when the well is first drill, as the name imply the surface casing,
since it's the top part of the well and his attached to the well head, it's
primary function is to protect the groundwater formations from
contamination. In deeper well a second
type called an intermediate casing is running and cemented, intermediate casing
is required, its primary function is to protect and support the hole above the
zone or zones of interest. Surface casing and if needed intermediate casing
strings are install during drilling on all wells regardless of whether they are
commercially viable or not. The
production casing strings on the other hand are only run when the well shows
problems of becoming a producing oil well, running and cementing this casing
segment are part of the first steps in well completion.
Well Casing – Surface Casing Function
Well Casing – Intermediate Function
Well casing - Production Casing Function
Once
the production casing is run, cemented and attached to the surface, and the
intermediate casing string at the wellhead, these casing form what is known as
a close hole or cased hole. This hole from the top to the bottom is now sealed
off from the natural fluids and solids that exist in the sub surface.
. Well completion – Open Hole
Well completion – Cased Hole
Well Completion
Procedure
Well
completion however, involves more than running casing and sealing the hole, to
complete a well after the casing running and cemented, the following procedures
can be performed:
1.
Perforation
Once
the production casing in place in the zones of interest, the casing then
perforated, which means blast through the casing and the cement deep into the
formation.
2.
Stimulation
After
perforating the zone is stimulated, so that inadequate production or flow rate
of the hydrocarbons can be attained.
3.
Gravel
packed
If its
needed the zone can also be gravel packed to stop sand production
4.
Tubing,
packers, and a christmas tree
Finally
the well is equipped with tubing, packers and a christmas tree to control the
flow of fluids to the surface.
Well completion –
Perforation
Well completion –
Stimulation
Well completion –
Gravel Packed
Well completion – Tubing, Packers, X-mas tree
Before
we described each of the above procedures, to finalize well completion in more
detail, we take a minute to explain one action is taken when are well is not
commercially viable. When a well stated no zones of interest or has a little
commercial viability it is declared a dry hole. This well would be permanently
plug, then abandoned call P&A, for plugged in and abandoned, this procedure
requires that several cement plugs be placed in the hole to seal it.
Well completion – Plug & Abandoned (P&A)
As
we know that wildcat wells, wells in areas where no one has drill before are
more likely to be dry holes. The industry figures suggest that as many as
seventy five percent (75%) of all drilled wildcat wells are dry. P&A procedures
therefore, can become quite routine in wildcat areas, in this book then would
pertain to the procedures and their functions used in well completion. Although
already mentioned well again present the three types of casing string and
explain their functions, in addition we illustrate the different procedures and
the specific equipment needed to ready the well for production.
Specifically
we present a brief summary of casing design, casing equipment, cement
operation, Perforation, well stimulation, sand control along with the file
equipment needed to prepare a well for production.
Well completion – summary
Well Casing Function
Let’s
start with the four main functions of the surface, intermediate, and production
casing strings with their cement sheet or covers. First, all three of the
different types of casing protect the whole from the mud, that's preventing
soft formations of shale from growing water out of the mud which then can cause
the shale to swell and blocked or impede the drilling operations.
Well Casing Function
In
addition casings prevent surface sediments and other unconsolidated formations
from being eroded by the mud system. Second function relates all to surface
casing, this casing protect near surface freshwater zones from contamination by
deeper salt water zones. we sure people rely on these freshwater aquifer,
polluting these aquifer is usually
prohibited by law or can create serious environmental issues. Third all three
types of casing the surface, the intermediate, and the production casing
provide a smooth entryway and path for running tools in and out of the hole.
Finally in production casing, its surroundings cement sheet isolates down the
hole zones, so that the different zones can more easily be produce separately.
Now
let's examined the standard procedures involve the closing and open hole. The
first step in closing and open hole is to drive the production casing string to
the bottom of the hole. The casing string is permanently set in the well by
pouring concrete into the annular space between the casing and the wall or the
hole. let me point out here, that
because the drilling string must be able to fish comfortably inside the casing
string, the casing string pipe must be large enough to allow the drilling
string pipe fit inside it. Like the
drilling string, the three types of casing strings consist the multiple joints
but of larger diameter pipe that is screwed together one join out of time as it
is inserted to the bottom of the hole at total depth (TD).
Well Casing for Closing Open Hole
Well Casing in
Drilling Process
Casing
strings are run and cemented into place as the well as drilled deeper to
protect the hole from further exposure to the circulating mud. The exact number
of strings depends on the depth of the well, the relative stability of the
formation penetrated, and the characteristic of the drilling mud. As discussed
earlier, as each casing string is run it is run inside the previous string that
was slightly larger. Conversely the diameter of each new string will be smaller
than the one it fits into as the casing is run deeper. Imagine those sets of
plastic balls that you can buy at any supermarket where the smaller bowl fits
inside a larger one which fits inside and even larger one until you get to the
biggest bowl with all the small bowl fitted neatly inside. there for, to be
sure that the casing string fits together properly, it is imperative that the
casing program preplanned before drilling begin so that the surface hole and
pipe can be large enough to accommodate all the strings that will be push
through it. Keep in mind that the final
string must be small enough to fit through all casing joints while still being
large enough to allow oil or gas to flow adequately for its capacity.
Number of Drilling & Casing String
When
inserting the initial casing string, the drilling operation, drills their large
diameter surface hole to unconsolidated surface material and protects sources
of fresh water in the nearby water table, called the surface casing string. It
is usually a few hundred feet and runs to the bottom of the whole at the
beginning when drilling first begin.
Surface Casing
Once
in place cement slurry is then pump down the inside of the pipe and circulated
up the backside annular space between the casing and hole.
Running Surface Casing
Once
the surface casing is in place, drilling resumed for shallow wells the surface
casing maybe all that is required before initializing the production casing.
For deeper wells or where the formation becomes unstable because of long
contact with the mud and intermediate casing string maybe necessary.
A very deep hole may
require two or three intermediate strings. To establish and intermediate casing
the surface casing must be drilled through using it smaller bit that fits
inside the surface casing pipe. The drillable shoe on the surface string and
the cement in the bottom of the hole is easily penetrated. Once the slopping
zone is penetrated the drill strings is pull and intermediate casing string is run
to the existing bottom and then cemented into place.
When
using an intermediate casing it is important that this cement rises high enough
in the annulus to reach and tie into the cement in the surface casing pipe. It
is essential to provide an unbroken cement sheet that covers the entire length
of the hole, these steps or repeated as each success of string of the
intermediate casing is hung from the wellhead at the surface. Finally the
wellhead access is sealed, to seal off the annulus at the end of the casing
string, both the surface and intermediate of casing strings are installed while
drilling continues to total depth.
Once
in the zone of interest that has been identified, it is not cased yet, this
section of the well is referred to as open hole. Basically it is the length of
the hole from the surface or intermediate casing strings to TD, as we know
about open hole, that open hole well logs and identify the lithology, measure
permeability porosity, the reservoir thickness, and fluid saturation are finalized.
Once the potentially commercial viable well is confirmed, this section of the
hole is then cased and completed.
Open Hole Logging
Cased and Perforated
Then
the production casing, long string or oil string, the string is set through the
producing zone, and cemented, ensuring that it is tied to the string above so
that there is a continuous sheet of pipe and cement from bottom to top. Later
this casing and cement will be perforated to get access to the producing zone.
For
further read, you may get the book from amazon.
PETROLEUM WELL COMPLETION OVERVIEW
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