Transcript Underbalanced Perforating

Underbalanced Perforating
Underbalanced Perforating
Early tests by Exxon showed that flow
patterns and perforation geometry prevent
the cleaning out of an appreciable
percentage of mud-or silt-plugged
perforations by simple production from a
well
Published studies of the flow rate
necessary to remove damage observed
that serious perforation plugging occurred
whenever the pressure was higher in the
wellbore than in the formation
Underbalanced Perforating
Plugs that formed when perforating in heavy
mud were almost impossible to remove by
reversing pressure.
"Permanent plugging of a high percentage of
perforations may result from killing a well with
mud or dirty fluid during well completion,
servicing, or workover."
"When perforating in mud with a pressure
differential into the formation, perforations are
filled with mud solids, charge debris, and
formation particles." Not easily removed.
Underbalanced Perforating
Differential pressure required to initiate flow
through each plugged perforation, varies.
When a few perforations requiring low
differential pressures open up, flow into these
perforations makes it difficult to create the
higher pressure drawdown needed to open
additional perforations.
“Crushed and compacted rock around the
perforation has essentially zero permeability
and further reduces the probability of
perforation cleanout.”
Underbalanced Perforating
The post-shot flow into the wellbore (a
function of the formation-wellbore
pressure differential, the formation fluid
viscosity and the formation permeability)
helps remove the crushed formation from
the perforation and provides improved flow
channels.
High post-shot formation to wellbore flow
generally provides optimum perforation
cleanup and minimum skin.
Underbalanced Perforating
Underbalance perforating followed by
flow has been shown to be the best
method for cleaning perforations and
establishing high flow capacity from
natural completions in moderate to high
permeability core
Even when compared to surging and
washing, underbalance perforating
followed by flow can be superior
The Level of Underbalance
Must balance perforation cleanup and
well performance potential
enhancement against the downside
aspects of mechanical problems such as
perforators or wireline sticking in the
wellbore, near-wellbore rock formation
disintegration, downhole tubulars and
equipment damage, etc.
The Level of Underbalance
The pressure differentials necessary to achieve
the flow rates required to remove perforation
and/formation-skin damage are affected by:
Formation pressure
Reservoir permeability
Perhaps limited by formation integrity
Usually range from approximately 500 psi to
over 5000 psi
Have been established by trial and error in
many fields
Implementation
Downhole and surface mechanical equipment
to achieve desired underbalance and maintain
the integrity of the well
Control the well during deployment,
perforating and retrieval,
Deploying the perforating device(s) to the
proper downhole position,
Activating the perforating mechanisms,
Monitoring the downhole perforating process,
Retrieving the perforating system
Precautions!
It is more costly to employ than conventional
perforating.
 Safety and well control is a primary issue.
It requires the appropriate combination of
reservoir pressure, reservoir fluid properties
and formation permeability to achieve the
required underbalance for effective
application.
Prospects must be thoroughly screened, and
there are those that may not be, or are not,
appropriate candidates.
Jet Perforating
Formation Properties
Compressive strength, effective stress and
specific rock characteristics can have
significant impact
In unstressed rock, penetration decreases with
increasing compressive strength
Pore fluid compressibility affects performance.
Increasing liquid saturation improves
penetration
Stress reduces penetration (other factors
being kept constant)
Penetration Multiplier
Jet Perforating
Formation Properties
Effective Stress (ksi)
Design
Perforator Type, Charge Strength and
Gun Clearance
Conveyance Logistics, Surface and
Downhole Equipment/Apparatus
Existing Wellbore Tubular/CementSheath Limitations
Design
Formation Mechanical Rock Properties
and Characteristics
Reservoir Pressure and Fluid Flow
Characteristics
Perforating Downhole Environment
Conditions/Limitations
Design (King’s Method)
3.5
Hsia and Behrmann
Perforating Skin (dimensionless)
3.0
Gold Sandstone
Berea Sandstone
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
0
500
1000
1500
2000
Underbalance Pressure (psi)
2500
3000
3500
1.0E+04
Non-linear Laminar Flow
Darcy's Law Invalid
Drag Enough for Cleanup
Friction Factor (dimensionless)
1.0E+03
1.0E+02
Turbulent Flow
1.0E+01
1.0E+00
1.0E-01
1.0E-04
Laminar Flow
Darcy's Law Valid
Drag Not Enough
for Cleanup
1.0E-03
Tariq
1.0E-02
1.0E-01
Reynolds' Number (dimensionless)
1.0E+00
1.0E+01
Cleanup Criterion
Based on Reynolds Number
pi  pperf

rc
v
1 
7
2 2 1
 94815 rc ln
 1.244 10 o v rc



kc
rperf
r
r
c
perf


NRe  1.31735 1012
 1
7.1974 1016 2NRerc 
rc
1  

p 
ln
 NRerc

2



k c o
 rperf rc  
 rperf
kv

Design
Of the factors influencing the
determination of correct underbalance
for cleanup the fluid properties,
especially viscosity, are important but
the key factor is the formation
permeability
Efficiency
The major factors affecting the efficiency
of a perforation include shot density (spf),
penetration depth into the formation,
angular phasing, and diameter
Injectivity increases as shot density
increases?
Injectivity increases with increases in
perforation penetration?
The effect is greater at shallow depths.
Efficiency
Angular phasing other than 0°
increases injectivity by reducing the
interference with flow resulting from the
presence of the wellbore.
Perforation diameter plays a relatively
minor role in determining injectivity?
The strength, in-situ stress conditions
and lithology can effect the penetration
length, the extent and severity of the
damage zone around the perforation,
and the cleanup characteristics.
Modular Gun
System
Deployment systems for
multiple guns.
Guns are loaded at the
surface, deployed
downhole individually,
and stacked on each
other at the perforating
zone, with the lowermost gun module being
supported by the gun
hanger.
Modular Gun
System
 An entire interval can
be perforated over- or
underbalanced
Gun sizes from 2 to 7inch OD can be run for
casing from 3 ½ to 8 5/8
inches,
Zone can be perforated
and tested with no
downhole restrictions
below or above the
packer.