(21) Mechanical Sand Control Techniques 16988266235639974256542097fb5c69

Mechanical Sand Control Techniques
Mechanical sand control provides a physical barrier to sand movement while allowing fluid to
flow across passages. In the rock, physical barrier is provided either by
•
•
a screen,
a combination of a screen and gravel pack
The flow passages through screen or gravel or gravel pack and screen must be small enough to
stop the formation sand but large enough to achieve adequate well productivity. The flow
passages are reduced with time due to plugging by clays, asphaltenes, wax and scales.
Screens
Screens are effective in controlling sand production from formations which are composed of
clean large grained sands with very narrow grain size distribution. These are primarily water
wells. Oil and gas wells are much deeper and formation sands are smaller grained, poorly sorted
and often contain clay sized particles. This type of sand plugs the screens. Use of screens as a
sand control technique has a number of inherent problems. They include:
•
•
•
•
slots or openings are eroded before sand control is achieved,
well productivity is reduced due to sand plugging,
because screens have always annular gap (between screen and formation) formation
collapses and fills the annular gap which leads to sand movement and causes
intermixing of sands and
intermixing of sands results in reduction in near wellbore permeability(please refer to
chapter 2).
Design consideration of screen/liner
The screen/liner must be designed to effectively trap formation sand while retaining maximum
productivity. This is achieved by selecting appropriate size (slot width), geometry and density
of slots to trap the larger grains which inturn stop smaller grains. Smaller grains are trapped in
the interstices of larger grains.
Hence, important design considerations for screens are as follows:
•
•
•
slot opening and slot geometry,
ratio of screen outside diameter to well bore inside diameter and
slot spacing, orientation and density.
Slot Opening and Slot Geometry:
Once the formation has collapsed behind the screen, the largest sand grains tend to form a trap
fro th smaller grains entering the screen. These large grained sands must be stopped by the
screen slot opening. Common slot openings used by the industry are:
• Parallel face opening and
• V-shape opening
Parallel Face Vrs. V-Shaped Opening:
In Figures 1 and 2 describe the bridging mechanism of sand grains at the slot opening. In the
case of parallel slot openings sand grains are likely to bridge across the opening, thus forming
a plug, whereas in a V-shape opening sand grains are stopped at the entry and form bridges at
the face of the slot opening. Because of the V-shape, sand grains entering the slot can easily
pass through the slot and prevent formation of bridges across the opening. This V-shape is
achieved in wire-wrapped screens by using trapezoidal cross section wire (see Fig. 7.9).
Large formation grains
Screen
/Liner
Small grains
bridging at
openings
Fig 1: Parallel face screen slot opening and sand bridging across the slot opening.
Screen
/Liner
Production
Fluids
Large grains
bridging at
the face of
the openings
Fig 2: V-shaped screen slot opening and sand bridging at the face of the slot opening.
Fig. 3: Wire Rap Screen with continuous trapezoidal wire.
Slot Size (width):
Common industry practice of selecting slot size is based on correlations. These correlations are
derived from laboratory experiments and field experience. Most widely used correlations are
the work of Coberly,1930 and Wilson, 1938. Coberly’s correlation is based on average sorting
of formation sand where slot width, w is determined from the correlation:
W=2𝑑10 (𝑑10 is the 10 percentile of formation sand)
The above correlation is based on the understanding that the sand grains form stable bridges
on slots that are twice the size of 10 percentile of formation sand. Following Coberly’s work
Wilson derived a correlation for the Gulf Coast where the sand grains tends to be more uniform
as:
W=2d10
This means that the width of the screen should be selected based on 10 percentile of the
formation sand. Gill, 1937 suggested a more conservative correlation to select slot width as:
W=𝑑10
A general rule was provided by DePriester,1972 in relation to selecting slot size for formation
for which very little information is available. To avoid plugging the minimum slot width should
be 0.05 inch. If the 20 percentile of sand is less than 0.05 inch then an alternative approach
should be adopted.
0.05𝑖𝑛𝑐ℎ ≤ 𝑤 ≤ 𝑑20
Slot Spacing and Slot Orientation:
The two basic types of screens which are widely used by the industry are slotted pipe and wire
wrapped. On tubing slots of various patterns are milled to produce screen. These patterns
include multiple vertical and horizontal patterns (see Fig.4)
Horizontal slotted
pipe
Staggered vertical
slotted pipe
Multiple vertical slotted pipe
Fig. 4: Horizontal, staggered and multiple vertical patterns (clockwise).
Wire wrapped screens are manufactured in many forms which include ripped welded, grooved
and wrapped on pipe. They are made as continuous slot on outside of the pipe that has already
milled or machined holes or slots. The wrapping wire is usually made of 403 stainless steel and
the core pipe is usually grade S or K. The typical screen sizes used by the industry are presented
in Table 1
Table 1: Typical dimensions of slots for slotted and wire wrapped screens (dimensions vary
with different manufacturer)
Pipe
size
OD
1-1/2
3-3/8
2-7/8
3-1/2
4
4-1/2
5
5-1/2
Slotted pipe
0.010
0.4
0.5
0.7
0.9
1
1.1
1.3
1.4
0.020
0.8
1.1
1.3
1.8
2
2.3
2.5
2.8
0.030
1.3
1.6
2.0
2.7
3.1
3.4
3.8
4.1
Wire Wrapped
Ribbed(Channel)
0.010 0.020 0.030
4.9
9.3
13.0
5.7
11.4
17.0
6.9
13.8
20.7
8.4
16.8
25.3
9.4
18.8
28.2
10.6
21.2
30.6
11.8
23.6
35.6
13.0
26.1
37.8
All-welded
ribbed(Channels)
0.010 0.020 0.030
9.0
16.4
22.0
10.8
19.7
27.1
12.7
23.1
31.8
15.1
27.4
37.7
17
30.8
42.4
18.8
34.2
47.1
20.7
37.6
51.8
22.6
41.1
56.5
Special applications of screen liners include:
• highly deviated and horizontal wells where gravel placemtn become cumbersome and
• wells completed in semi-competent (not fully consolidated) reservoir formation where
moderate sand production is expected.
GRAVEL PACKS
FORMATION
SAND
GRAVEL
Oil flow up
Gravel Pack refers to uniform graded commercial sand placed between the wellbore and slotted
screen to retain formation sands from movement. Figure 5 describes a typical gravel pack. The
main advantages and disadvantages of a gravel pack are described in Table 2.
Fig. 5: Schematic of gravel packing.
Table 2: Advantages and disadvantages of gravel pack
Advantages
Disadvantages
1. Effective control of formation sand as sands
1. Effective flow diameter of the
are stopped in pores formed by gravel
wellbore is reduced
2. High production is achieved because the near
2. Zone isolation is not feasible
wellbore permeability remained mostly
3. Screens are susceptible to erosion
undamaged due to grain intermixing
3. Screen is subject to less erosion
4. No chemical reaction involved
5. Regulating acid wash is feasible
Key to the successful sand control using gravel pack includes:
•
•
•
•
•
selection of gravel size,
selection of screen type and slot operating,
selection of gravel pack interval,
gravel placement and
selection of gravel pack to minimise formation damage.
Selection of Gravel Size:
Selection of gravel is important as the gravels must form interstices which can effectively trap
formation sands and prevent sand movement. Similar to selection of screen slots gravel size is
determined based on well-established correlations. These correlations are summarised in Table
3.
Table 3: Summary of gravel to formation sand relationships (correlation) developed by the
industry:
Authors
Coberly and
Wagner 1937
Gumpertz.1940
Hill,1941
Department of US
Agriculture,1952
Pack sand
Narrow
Formation sand
Broad
Rule
𝐷 ≤ 10𝑑10
Narrow
Narrow
Narrow
Broad
Broad
Broad
𝐷 ≤ 10𝑑10
𝐷 ≤ 8𝑑10
6.5𝑑50 > 𝐷50
> 3.8𝑑50
Depriester,1957
Broad
Broad
𝐷50 ≤ 8𝑑50
𝐷90 ≤ 12𝑑90
𝐷10 ≤ 3𝑑90
𝐷85 ≤ 4𝑑15
𝐷50 ≤ 6𝑑50
Stien,1969
Broad
Broad
Soucier,1974
----------Note: D is the diameter of gravel and d is the diameter of formation sand.
From the table, it is apparent that the diameter of the gravel sands is selected by matching the
diameter of certain percentile formation sand. Rules for selection of gravel sands are based on
the correlation present in Table 3
which varies significantly and that the distribution of
sand size is described by a particular percentile on the distribution curve.
To overcome the above anomaly of the selection process, Schwartz,1969 came up with a
unique uniformly coefficient, C. The uniformly coefficient is determined by comparing 40
percentile formation sand (d40) with 90 percentile formation sand (d90) as:
𝑑40
𝐶=
𝑑90
C<3, sand is considered to be uniform
C>5, sand is non-uniform
C>10, Sand is very non-uniform.
For the above uniformity coefficient Schwartz gave the following correlations:
• for uniform sand(C<5), 𝐷50 = 6𝑑10
• for a non-uniform sand (C>5), 𝐷40 = 6𝑑40
The flow velocity for all the range of C should not exceed a critical value and be calculated as:
Flow velocity = production rate (ft3/sec)/ 50% of the open area of slots, ft2.
Gravel Thickness:
Gravel thickness is also an important factor which affects productivity of the well. Through
laboratory experiments, Allen and Roberts, 1989 have shown that the gravel pack thickness of
3 to 4 grain diameter should be sufficient in order to stop sand movement. In practice, however,
a thicker gravel pack is needed for effective sand control. Sage and Lacey, 1942, based on their
work, provided a rule for gravel thickness. Based on their work it is clear that 3 inch or greater
gravel pack thickness is required to effectively control sand production.
Mixing Gravel with Sands:
The work of Sperlin, 1972 has shown that the mixing of gravel sand with formation sand
reduces pack permeability significantly. With 100% of gravel sand (no mixing of gravel sand
with formation sand) the pack sand has shown to have the highest permeability whereas with
50-50 mixing of gravel and formation sand a 20 fold reduction in permeability can be observed.
Based on this experiment it is recommended that well sorted gravel sand should be used as
pack sand.
Physical Properties of Gravel Sand:
In addition to gravel diameter, suitability of gravels for a sand control job depends on a number
of physical properties. They include:
•
•
•
•
•
•
roundness or sphericity,
grain strength,
acid solubility,
uniformity,
presence of clay and clay size materials and
wetness.
Gravels should have a uniform geometry with roundness or sphericity 0.6 in Krumbein scale
or better. Flat or angular geometry reduces the porosity and hence reduces trap capacity of
gravels. Gravels should have strength greater than 2000 psi so that they do not breakdown on
formation stress and produce clay size particles. Presence of clay size materials reduces gravel
pack permeability. Usually turbidity is used to determine the presence of clay and the turbidity
should be less than 1. Gravel should have a Uniformity Coefficient, C=1.5. Materials finer than
1.5 (C=1.5) would have little effect on the control of sand movement.
Gravel should also be resistant to acid as often acid treatments are employed to clean up of
sand pack to remove pore blockage (formation damage).
Finally, gravel should be water wet to increase the effective permeability. It has been shown
that when water wetness increases relative permeability to oil increases greatly (Williams et al,
1972).
Placement of gravel pack
Gravel pack can broadly be classified as:
1.
2.
3.
4.
inside pack between casing and screen
outside pack between formation and casing
combination-inside and outside casing packing
open hole gravel
Inside Gravel Pack:
A slotted or wire wrapped screen/liner is placed inside the casing as shown in Fig. 6. Various
methods are used to place the gravels between the casing-screen annular gap.
SCREEN
PERFORATIONS
CASING
GRAVEL
Fig. 6: Inside pack.
PERFORATIONS
Outside Casing Gravel Pack:
Perforations are cleaned and washed prior to the gravel placement. Then slurry (mixture of
gravel and viscous fluid) is circulated under pressure to squeeze the gravel behind the casing
as shown in Fig. 7. Under pressure the gravel transport fluid squeezed into the formation
leaving the gravel dehydrated. The gravel transport fluid must be carefully selected in order to
avoid formation damage due to fluid invasion.
SCREEN
CASING
GRAVEL
Fig.7: Outside pack.
Combination of Inside and Outside Casing Pack:
Combination packs involves 3 steps:
1. washing behind the casing,
2. placing the gravel behind casing,
3. finally, placing gravel between the casing and screen as shown in Fig.8.
The combination pack provides an effective sand control and is widely used in the industry.
SCREEN
CASING
PERFORATIONS
Fig. 8: Combination- washout technique.
Open Hole Gravel Pack:
Open hole gravel pack involves underreaming to enlarge the hole diameter and then placement
of gravel between the open hole section and screen/liner (see Fig.9). This technique provides
a thick gravel pack with large unrestricted flow area and is very effective in controlling sand
production and increasing oil/gas production. Since it is not possible to produce from multiple
zones simultaneously, open hole completions are often used to control sand production fro each
completion.
CASING
UNDERREAMED
AREA
SCREEN
GRAVEL
Fig. 9: Inside pack-open hole completion-under reamed.
Under Reaming of Open Hole Section:
A hole opener is used to under-ream the pay section as shown Fig.7.16. About 4 to 6 inch on
the diameter should be under-reamed to provide sufficient annular gap for the gravel pack.
Fig 10 Under reaming for open hole gravel pack.
Washing Behind the Casing:
In order to provide thick gravel pack, perforations are required to be washed. A typical washing
technique (after Tausch and Corley, 1958) or a cup type (after Allen and Roberts, 1989) can be
employed to wash behind casing and perforation tunnel (see Fig.11)
Fig. 11.: Washing of perforations.
Fig.12 Cup type perforations.
According to Tausch and Corley, 1958 a wash pipe is run with a packer which is set at the half
way through the perforation interval. Brine is pumped through the annulus into the perforation
and back to the surface through the wash pipe as shown in Fig.11 . In a cup type washing
technique, a ball is dropped before circulation commences. A Selective selective circulating
sleeve is used to direct brine to the perforation tunnel via wash pipe. Brine and sand mixtures
are recovered back to the surface via the annular gap (casing and wash pipe, see Fig.12).
Telltale:
Telltale is a device to indicate the position that can not be easily seen, in this case a short section
of the screen/liner located below the screen. A seal sub is installed between the telltale and the
screen to seal the wash pipe therefore ensuring that the return is from the telltale only. The
objective is to direct the gravel to the bottom of the screen to achieve a tight pack.
Crossover Tool:
Crossover tool is used to divert down flowing slurry (mixture of sands and fluids) to the outside
of the liner and flowing fluid into the annulus to return to surface. The slurry is pumped down
the tubing, as shown in Fig. 13, through the crossover tool into the casing-screen annulus. This
prevents the gravel to travel back to surface.
Fluid returns up casing
annulus
Crossover
Fig. 13 :Crossover tool.
Gravel Blending Unit:
The surface equipment needed to run a gravel pack job is a truck mounted fluid tank, a
mechanical mixture/blender, a slurry tank (gravel and fluid mixture) with a positive mechanical
injection device shown in Fig.14. Fluid (viscous brine) is injected from the tank into a jet
which sucks the gravel from the mixing hopper and mixes the gravel with the viscous brine
(slurry). The slurry is then directed to the well head. Usual pumping ranges from 2 to 6 barrels
per minute.
Fig. 14: Gravel blending unit (Courtesy of Solum Oil Tool Corporation).
SLURRY CIRCULATION TECHNIQUE
Gravels are placed down hole by a number of techniques which include:
• gravity circulation,
• normal circulation
• reversed circulation and
• squeeze gravel pack
Each technique is unique for certain bottomhole conditions and has associated advantages and
disadvantages. Selection of appropriate circulation technique is important for effective sand
control.
Gravity Circulation:
In gravity circulation the slurry is dumped down the casing and allowed to settle (see Fig. 15).
To allow the gravel to settle, low viscous brine is used as a carrier fluid. This technique results
in a poor gravel pack as the different gravel size travel down the well at different velocities
leading to segregation of gravel. This results in poor compaction of the gravel and hence is
primarily used in shallow water wells. Gravity circulation would be used as a cost effective
solution for water wells.
Gravel dumped
downhole
Linear
perforations
Gravel
segregation
Fig 15: Gravitate placement technique.
In normal circulation, the slurry is pumped down the working string via a crossover tool and
the carrier fluid is returned through the annulus as shown Fig.7.21.
SLURRY DOWN
WORKSTRING
DEHYDRATED
GRAVEL
PACKER AND
CROSSOVER
CARRIER FLUID
RETURN UP
ANNULUS
Fig. 7.16: Normal Circulation Technique.
It is primarily used for the inside casing gravel pack. Due to the injection pressure, the same
gravel may flow through the perforation tunnel. In order to achieve a gravel pack a two step
procedure is used: Outside pack and inside pack. The circulation is described in the following
steps:
1. Gravel is pumped down the tubing and forced into the perforation as shown in Fig.17.
The carrier fluid passes through the liner and up the wash pipe leaving the gravel
behind.
2. The tubing and wash pipe are then pulled up to some distance and step1 is repeated
again allowing more gravel to settle (see Fig.18).
3. Figure 19 shows the final outcome of repeating stage one. Gravel is tightly packed
behind the casing and inside the casing.
4. The second stage is a wash down procedure. It consists of pumping fluid down the wash
pipe to displace the gravel, thus allowing the screen to be placed down hole. This
procedure is demonstrated in Fig.20.
Fig.17: Gravel placement behind casing.
Fig. 18: Gravel placement inside casing.
Fig. 19: Gravel packed inside and out casing.
Fig. 20 Mechanical sand control: The Wash pipe is pushed down with the screen to remove
gravel from inside.
Wash Down Procedure:
The wash down procedure, described in Fig.21, is also used inside the gravel pack. In the wash
down method, the gravel is injected into the perforations before the screen is placed. Then the
screen is run into the hole. The assembly is then “washed down” into its final position by
circulating brine through the wash pipe and shoe. When the shoe reaches the bottom,circulation
is stopped and the gravel is allowed to settle around the screen and liner (see Fig.22).
Washpipe
Liner Screen
Sealing Mechanism
Fig. 7 22: Wash down technique.
Reverse Circulation:
In reverse circulation a conventional water/gravel mixture is circulated down the casing-tubing
annulus allowing the fluid to return up the tubing (see Fig.23). The slurry flows down the
annulus and the gravel is retained on the outside of the screen. The carrier fluid flows through
the screen and up to the surface through the tubing.
Assemblies for reverse circulation usually involves running in the hole, down hole functional
check, packing of gravel to a selected point in the casing, pack off after packing and
disengagement. A production packer with an overshot assembly is then run over the polished
bore nipple. A prepack should be used as discussed earlier in case of outside casing gravel
pack.
WATER/GRAVEL
DOWN ANNULUS
GRAVEL PACK
TELL TALE
SCREEN
Fig 23: Reverse Circulation Technique.
Squeeze Gravel Pack:
In a squeeze gravel pack, the gravel pack assembly is positioned opposite to the completed
interval, the packer is set and the crossover tool is opened. The gravel is pumped with a viscous
carrier fluid down the tubing via crossover tool into the casing-screen annulus and the
perforations under pressure (see Fig. 24). The viscous fluid is squeezed into the formation
leaving the gravel in the annulus as a dehydrated gravel pack. Pumping is continued until screen
out occurs. After the circulation of completed excess gravel above the screen is circulated back
as the wash pipe is pulled out. This technique is designed for only a short interval of 30 feet or
less. The major disadvantage fro the squeeze gravel pack is that the carrier fluid is “squeezed”
into the formation causing formation damage.
SLURRY DOWN
WORSTRING
DEHYDRATED GRAVEL
PACKER AND
CROSSOVER
SCREEN
CARRIER FLUID
LOST TO
FORMATION
Fig 24: Squeeze Technique.
GRAVEL TRANSPORTING FLUID CHARACTERISTICS
In order to obtain a successful gravel pack it is important to select fluids with the appropriate
properties for effective transportation. Essential characteristics of gravel transport include:
• Viscosity,
• fluid leak off control and
• density.
Viscosity:
Brine has been the common fluid used for transportation of gravel as it readily leaks off into
the formation, thus providing a tighter gravel pack. It must be a clean and contain minimum
amounts of any clay-like solids (clear fluid) and its water wettability should not be impaired.
Low viscosity has limited transport capability to 0.5-1 lbs gravel/ gallons of fluid with a pump
rate of 5 bbls/min.
High viscous fluid is needed to increase the carrying capacity characteristic of the fluid. This
is achieved by adding gelling agent such as the hydroxyethyl cellulose (HEC) or x-tham gum
polymers. For 8 to 9 lbs of HEC in 100 gallons of brine can yield a viscosity of 100-200 mPaS
at 100 S-1 and can transport upto 15 lbs of gravel /gallon of brine. By adding beaker fluids this
viscosity can be easily broken and recovered by producing fluids (oil and gas) without much
formation damage.
Fluid Leak off Control:
Formation damage by transport fluid leak off must be considered. Most viscosity building
agents provide good fluid leak off control, however additional fluid leak off control material
can be added. Common material used to control fluid leak off include: ground calcium
carbonate and oil soluble particles.
Ground calcium carbonate provides good fluid leak off by plugging pores of formation sands.
Calcium carbonate is also acid degradable such that with acid (HCl) treatment carbonate
particles can be removed. Finely graded oil soluble particles are used to control leak off.
Fluid Density:
Densities of up to 10 lbs/gallon can be achieved in common brines which are adequate to
control formation pressure in shallow and low pressure reservoirs. In deeper and high pressure
wells brine densities are often required to increase densities up to 19 lbs/gallon. Using calcium
chloride, brine density can be increased to 11.4 lb/gal. Calcium and zinc bromide can provide
densities between 12 and 19.2 lbs/gal. These brines are expensive and corrosive. In most cases,
ground calcium carbonate is added to the common brine in order to increase density between
12 and 14 lbs/gallon and polymers to suspend both gravel and carbonate particles, leading to a
cost effective solution,