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1.General
Communication
between the
formation and the
wellbore will
directly affect the
productivity of the
well. Factors such
as hydrocarbon
saturation,
porosity,
permeability, fluid
properties and
geometry can be
measured or inferred
from the
measurements but
they cannot usually
be controlled. By
contrast, completion
can be controlled
and thus affect well
performance. During
the drilling,
logging and testing
phase of the well,
valuable information
will have been
gained and the
relevant completion
technique chosen.
There are two main
categories of
completions to
consider with, of
course, many
variations;
- Open hole
Completions.
- Cased Hole Completions.
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1.1.Openhole
Completions
An open hole
completion
is when the
well is
drilled to
the top of
the target
formation
and the
casing is
cemented at
this stage.
Drilling is
continued
across the
target
formation
and then the
well is
completed
and
produced.
Open hole
completions
are only
possible in
“competent”
rocks that
will hold
their form
and not cave
in or
crumble – so
called hard
rock
environments.
This
technique is
generally
associated
with older,
cheaper
methods of
drilling and
completing
wells and
today would
only be used
in very low
profile
applications,
if at all.
Variations
on the
straight
forward open
hole
completion
include
gravel
packing with
slotted
liners used
to contain
the pack.
Whereas this
technique
offers the
least
restriction
to flow from
formation to
wellbore and
as mentioned
is an
economical
completion,
it has many
apparent
disadvantages;
- No
possibility
for
selectively
producing or
treating
different
zones.
- Limited control of water or gas encroachment.
These
two factors
alone can
play a
significant
part in the
future
management
of the well,
and this
coupled with
safety
issues has
lead the
industry
down the
road of
cased hole
and
perforated
completions. |
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1.2.Cased
Hole
Completions
A cased hole
completion
is when the
well has
been cased
and cemented
across the
target
formation
and requires
shaped
charge
perforation
to achieve
communication
between the
formation
and the
wellbore.
This is the
most common
form of
completing
wells today
and our
discussion
will center
around this
technique.
2.
Completion
Types
Completions
can be
broken down
into two
main
categories;
-
Natural
(perforated).
-
Stimulated. |
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In
all three the
objective is to
maximize production
through enhancement
of some aspect of
the near-wellbore
reservoir
performance. Of
particular
importance is the
change in flow
geometry near the
wellbore caused by
wellbore damage
(from drilling and
filtrate invasion),
perforations
(debris), flow
convergence due to
partial penetration
and deviation. This
damage is known as
skin (S) and will be
discussed in more
detail in section 5,
but for the time
being can be
considered as an
induced pressure
drop across the
completion which
effects
productivity.
Examples of
Different
Perforating
Techniques.
Pressure
Distribution in a
Reservoir with Skin.
The aim of the
completion design
engineer and
reservoir engineer
is to reduce the
influence of skin as
much as possible.
2.
1.Natural
Completions
(perforated)
The natural
completion is
usually chosen for
sandstone reservoirs
with permeabilities
above 10 md and
porosities above 9
p.u. These
reservoirs typically
have small damaged
zones and limited
skin, good
transmissibility and
stable rock
mechanics. They
generally do not
require stimulation
or sand control
during primary
completion. The
objectives of the
perforation in this
case would be depth
of penetration and
effective shot
density, the
perforation diameter
is generally
unimportant if it is
larger than 0.25”
(0.5 cm). The
deepest penetration
with the greatest
phase distribution
is desirable for
production
enhancement.
2..2
Stimulated
Completions
These fall into two
broad categories;
- Hydraulic
Fracturing.
- Acidizing.
Occasionally the two
are combined in an
“acid frac” job.
2.2.1 Hydraulic
Fracturing
Hydraulic fracturing
is performed to
enhance the
effective wellbore
radius rw and is
usually employed in
reservoirs with
small permeabilities
(k < 1 md). This is
accomplished by
injecting fluids and
propant at high
pressure, in order
to create a bi-wing,
symmetrical fracture
or crack in the
reservoir. Hydraulic
fracturing is
generally a
five-step process;
·
Pre-fracturing
treatment.
· Fracture
initiation and
breakdown.
· Fracture
extension.
· Proppant
injection.
· Cleanout and
production.
2.2.2 Acidizing
Acidizing is a
stimulation process
used to repair
formation damage
caused by the
drilling or
perforating
operation. This type
of damage is usually
associated with
plugging of the pore
throats around the
wellbore. Acidizing
removes this damage
from the matrix rock
by injecting acid
into the naturally
porous rock at
sub-fracturing
rates, allowing the
acid to dissolve the
plugs.
Hydraulic Fracturing
Process
Acid frac jobs are
used to etch the
surface of the
hydraulically
induced fracture.
After the fracture
closes, the etched
surface cannot form
a closed seal. Acid
frac jobs are
operationally less
complicated because
no proppant is used
thus eliminating the
potential for
premature frac
termination that may
be caused by
screenout or
problems of proppant
flowback. The
principle
disadvantage of this
technique is the
expense of the acid
fluids and
non-uniform leak-off
resulting in
“wormholes.” Acid
frac jobs are
usually performed on
carbonate reservoirs
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