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THE EFFECT OF SAND SORTING ON GRAVEL PACK
CHAIRUL ABDI
SUPERVISOR: MR. ARIFF BIN OTHMAN
PETROLEUM ENGINEERING DEPARTMENT
FACULTY OF PETROLEUM AND RENEWABLE ENERGY ENGINEERING
UNIVERSITI TEKNOLOGI MALAYSIA
Abstract
A laboratory research study has been conducted to investigate the effect of sand sorting on gravel pack toward
permeability. Several experiments have been carried out by using a transparent cylinder shape Perspex material
sand holder with diameter 5cm and 25cm length. By using five samples of sand with different sorting and
median grain sizes, a model was designed to simulate a production zone. Fresh water and diesel with 3.57 cp of
viscosity were used as injection fluid at various flow rates and outlet pressure had been set at an atmospheric
pressure. The experimental result reveals that the permeability of sand greatly reduced when smaller size of the
sand present in formation because of smaller pore throat and greater resistance to flow. Permeability reduction
becomes more significant when the sand distribution is poorly sorted and the higher injection fluid flow rate
applied. In this study, it is also found that high viscosity of injection fluid will give higher permeability
reduction. In addition, injection fluid under the continuous flow conditions is always given the higher
permeability compare to discontinuous flow condition.
Introduction
Sand problem is one of oldest and critical
problem in the production wells faced by most of
the oil and gas production companies due to
instability of formation sand is the inflow of
formation sand with hydrocarbon, and it is one
issue that cannot be easily solved.
Reservoir can absorb and accommodate a
large volume of hydrocarbon, and permeable sands
permits oil and gas hydrocarbons to flow to
production wells easily. However, in addition to the
many things that are so beneficial, porous and
permeable sand is not good enough in the knots
(poorly cemented). Therefore, when the fluid has
started to flow into producing wells, thus releasing
the reservoir began to crush the grains of sand into
the production wells. When oil and/or gas
produced, then the grains of sand are also
produced.
Besides reducing the volume of oil and
gas during production, resulting from sand
production can also reduce the pressure. And if
uncontrolled sand production could reach to the
surface production facilities, then the problem of
sand production will cause new problems in the
next.
In addition, the installation of gravel
packing is one of the ways to overcome sand
production problem. There is much research was
conducted due to this particular problem such as
sand control; gravel packing. Expected by
installing gravel packing sand problem can be
solved. Otherwise, by installing a gravel packing
these other things are also very important, flow rate
and pressure drop is often overlooked.
As a result, to optimize oil and gas
production in the oil or gas field, especially in a
poorly consolidated formation, further studies are
required. This project was undertaken to study the
effect of sand sorting on gravel pack, which may
cause the sand production problem.
Methodology
The apparatus had been used in this
project consist of sand holder with several piping,
pump, and manometer tube. Before experiment
conducted, several preparations regarding to the
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experimental apparatus had been prepared as
follows:
1. Design and fabricate a sand holder.
2. Calibrate flow rate of pump.
3. Design and prepare formation sand.
4. Prepare injection fluid.
5. Conduct the main experiments.
Figure 1 shows the schematic diagram of
the experimental apparatus; Figure 2 reveals the
sand holder design configuration and for Figure 3
to 5 exposes the photographic view of experimental
flow system, sand holder and also U-tube
manometer.
Result and Discussion
Several experiments were conducted to
investigate the effect of sand sorting on gravel
pack. The experiment was done on unconsolidated
sand under continuous and discontinues flow
condition. The outcomes of concern in this
investigation are:
i. The effect of particle size
distribution on unconsolidated
sand
ii. The effect of flow rate change
due to time
iii. The effect of injection fluid
viscosity
iv. The effect of sand sorting
v. The effect of permeability
responds to flow condition.
Flow rate changes under continuous flow
condition mean that the flow rate will be started at
20 cc/s for 60 minutes before being increased to 30
cc/s for 60 minutes and finally increased to 50 cc/s
also for 60 minutes. Under the discontinuous flow
conditions, the flow rate will be started with 20 cc/s
for 60 minutes before switching off the pump for
20 minutes and then increased the flow rate to 30
cc/s also for 60 minutes. And finally, flow rate 50
cc/s will be applied after the pump switch off for 20
minutes.
Figure 2. shows that the schematic
diagram of the sand holder. This sand holder is
divided into three phase zone for the pressure and
permeability measurement. K0 represents the sand
pack permeability for zone 1 of which
measurement starts from P0 to P1. K1 and K2 are
the measurement from zone 2 and zone 3
respectively. However, the primary concern of this
study is with the permeability measurement for
zone 2 and 3, which are K1 and K2. The result
presented in this report is collected from
permeability K1 and also K2 respectively.
The experiments were conducted with
water as injection fluid with 1.0 cp of viscosity and
three experiments were used more viscous fluid,
which was diesel as injection fluid with 3.57 cp of
viscosity. The objective was to determine the
relationship between the sand pack permeability
with the experiments flow time in certain
conditions.
In the experiment, the permeability was
determined by using Darcy’s equation. This
equation had been used since it is applicable in
laminar flow with the porous media is 100 percent
homogeneous with the following fluid and the fluid
is not reacted with the particles (glass beads).
Formation Sand Grain Size Distribution
The grain size of the unconsolidated sand
used in this study was measured using dried sieving
technique. Five types of sample with different grain
size distribution were labelled as sand A, B, C, D
and E were used to demonstrate that different size
distribution that may cause different permeability.
In Figure 6. shows that the formation sand
distribution that had been used in this experiment.
The graph on figure 7. shows the pattern
of particle size distribution for these five samples.
From this graph, the median particle size, which is
D50 for each sample was measured and uniformity
coefficient, C; which is D40/D90 can be calculated
for each sample. From this graph shows all five
samples have significantly differ in its sorting.
Sorting sample was a measure of deviation
both from the median diameter to given a normal
distribution of grain sizes, both larger and smaller
are present in the total population of sand pack. The
sand size distribution graph in Figure 4.2 indicates
that sorting, D40/D90 for all samples varies from
1.3 to 10.6. The median sand size for Sand A, D50
is 130 μm and D40/D90 is 1.6. San A, B, and C
consider as uniform regarding to the Uniformity
coefficient; C is less than three (C < 3). But, sand
size for Sand D is consider as non-uniform and
sand E is consider as very non-uniform / very non-
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sorting this is due to the value of D40/D90 both
sample was very large. Which was uniformity
coefficient for Sand D are 7.6 and 10.6 sand E.
Permeability is the ease with which fluids
flow through a rock or sediment. A rock is
permeable if fluids pass through it, and
impermeable if fluid flow through the rock is
negligible. Normally, permeability depends on;
Grain size (Coarser-grained sediments are more
permeable than fine-grained sediments because the
pores between the grains are larger), sorting, grain
shape, and packing (controls pore size).
Response of the Sand Pack to the Effect of Flow
Rate
Permeability is the ease with which fluids
flow through a rock or sediment. A rock is
permeable if fluids pass through it, and
impermeable if fluid flow through the rock is
negligible. Normally, permeability depends on;
Grain size (Coarser-grained sediments are more
permeable than fine-grained sediments because the
pores between the grains are larger), sorting, grain
shape, and packing (controls pore size).
According to Darcy’s law, the fluid flow is
proportionally to the pressure differential between
inlet pressure and outlet pressure at constant
permeability of the formation. This is only true for
the solid-cemented particles (normal sand) such as
core samples but not in a loose pack or
unconsolidated sands such as gravel packing
completion.
Generally, the overall behaviour of the
sand particulates migration process is critical; this
is due to the magnitude of the flow rate, since it
affects the gravel permeability and may cause
serious plugging problem during high flow rate of
fluid. If the flow is too low, no migration of sand
will occur, as the fluid flow is not strong enough to
carry particulates, then the permeability is obeying
the Darcy’s law. Moreover, at high flow rate, a
large amount of particulates is moving quickly, and
possibly causing the sand pack to self filtrate after
sometimes. The self filtration is due to the particles
build up within the pores and pores throats, thus,
causing the pores to block and the porosity of this
element is reducing. The possibility for particles to
migrate depends on the compaction forces caused
by the flowing liquid. Therefore, the permeability
in this layer will decrease, causing a large increase
in the pressure drop.
A study on the effect of sand sorting on
gravel pack was conducted by measuring its
pressure drop toward permeability, which was
converted to permeability data with varying flow
rates. In each test, the injection fluid was injected
under continuous and discontinuously circulated
for about 60 minutes at each constant flow rate.
The permeability was measured periodically, and
the circulation was continued until the flow rate is
stable.
In this research study, three different flow
rates were set for these experimental studies, which
are 20 cc/s, 30 cc/s and 50 cc/s. Fresh water with
1.0 cp was used as injection fluid for all five
samples. A comparison was made for the results of
the permeability against flow time with three
different flow rates. There were some fluctuation
profiles in the graph. This is due to the
rearrangement of the particles in the sand pack.
Flow rate plays an important aspect to
determine the movement of sands particles process.
Basically, when the flow rate is become higher, the
potential of the sand particles to move is higher as
well. These movements occur when the fluids flow
rate is unsteady until it reaches a level where the
progress of the particles stopped after it achieves a
steady state of the flow rate.
Effect of Flow Rate on Sand A Permeability
Figure 8. shows the effect of flow rate on
Sand A permeability as measured at K1 and K2.
The results show that with increasing flow rate of
the injection fluid it will reduce the permeability of
the sand pack until it became constant after 10
minutes of flow time. The reduction was
significantly evident by the different between the
permeability at flow rate 20 cc/s and 30 cc/s and 50
cc/s for both zones.
As the permeability for the sample A is
determined, an analytical study has been conducted
to discover the relationship between the particle
grain sizes and permeability over time. From figure
4.3, the lowest injection flow rate gives the highest
permeability. The permeability is estimated about
1200mD for flow rate 20 cc/s, 970 mD for 30 cc/s,
and 820 mD for injection 50 cc/s. Mostly, after 10
minutes. The curve shows constant. These are
where the flow is reach stabilized and stabilized
permeability is reached. A huge reduction for flow
rate 30 cc/s and 50 cc/s curve is due to the
instability of the flow. Ironically, for this samples
the tame taken for flow rate to reach its stability is
relatively short. This had happened because of the
particles in the sample reaching its packing
rearrangement in short time and smaller grain
particle migrate faster to the pore space between
bigger grain size particles.
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Effect of Flow Rate on Sand B Permeability
Figure 9. below shows the result for
sample B. from the curve below mention that the
permeability at flow rate 20 cc/s is about 1160 mD,
followed by 970 mD for injection 30 cc/s and end
up with 960 mD for 50 cc/s. From this result it
proves that the permeability of each sample, mainly
decrease through the time of different flow rates.
Generally, drop of the value of permeability is
happened in the beginning of the experiment of
each starting injection applied until the minutes of
5th. From the 5th minutes until 10th to 15th
minute, the permeability seems to vary / unstable
for a short period, and it seem stable afterwards.
This phenomenon is not always occurred at the
highest flow rate used where the permeability is
decreasing for a long period but the reducing
permeability is higher when the higher flow rate
was applied.
Effect of Flow Rate on Sand C Permeability
Figure 10. is shown the relationship of the
permeability ratio aver time for sample C. As
bigger particle grain size is being tested, a higher
permeability is obtained. For this sample, the
porosity is 29 percent. It can be seen that the
highest permeability is shown about 1920 mD for
injection rate 20 cc/s, 1460 mD for 30 cc/s and
1452 mD 50 cc/s fluid injection rate. This had
happened because of particle are stable in slow
flow rate. The rearrangement of packing are
difficult to occur and the small particles are not
migrating to the pore space of bigger particle grain
size. In contrast, the force induced in high flow
fluid injection are enough to migrate a small
particle and rearranged the packing of particles
hence reduced the existing porosity.
Effect of Flow Rate on Sand D Permeability
Figure 11. shows the experimental result
for sample D with 600 µm of Median Grain Size
Distribution. The result shows that the effect of
flow rate on the Sand D permeability as measured
at K1 and K2 respectively by using the sand holder.
Same as the previous trend, any increasing in the
flow rate will result in the decreasing of the
permeability. The differential value of permeability
is as follows 820 mD for 20 cc/s, 670 mD for 30
cc/s and 528 mD for injection fluid 50 cc/s. The
collected data show that a relatively higher
reduction of permeability curve trend than the other
flow rate, which may be due to the rearrangement
of the particle in the sand pack.
Hence, if we continue injecting fluid with
the same flow rate after 60 minutes, there will be
no effect on the graph line, it will be at the constant
rate. Because at that moment all the particles in the
sand pack had reached a dynamic rearrangements.
Effect of Flow Rate on Sand E Permeability
Figure 12. The effect of injection flowing
fluid rate toward permeability on the sample E. The
curve below shows that the permeability is varied
and very unstable in sometime at the starting point,
this is regarding to rearrangement of particle grain
size due to hydrodynamic force.
The Effect of Injection Fluid Viscosity
In Figure 13 through 15 it shows that the
effect of injection fluid viscosity. There were three
samples used in this experiment, which is “Sample
C” where represent of uniformity sample. Sample
D represented the non-uniformity and sample E is
represented of very non-uniformity.
In all cases, declining of permeability rate
is more significant with 1.0 cp viscosity of water as
injection fluid. When diesel is injected into the
sand pack with 3.57 cp of viscosity it shows that,
the permeability reduction significantly achieved
higher. This is due to the higher lifting power for
more viscous injection fluid, thereby more grains
and particles are invaded and plugged the pore
spaces. The increase in viscosity also affects the
mobility ratio.
The increasing in permeability of the sand
pack is due to the increasing in injection fluid
viscosity. With high viscosity was injected in the
sand pack, the grater the permeability reduction
was achieved. These phenomena occurred due to
the increase of flowing fluid viscosity will increase
the pressure differential too. The increase of
flowing fluid viscosity will increase the drag forces
as well. The increase of drag force will cause more
severe plugging on pores spaces and reduces the
flow path respectively. Because of higher drag
forces have the higher capability to carry particles
and will increase the pore plugging and minimize
the pore space simultaneously.
The Effect of Sand Sorting
The experiment has been conducted to
identify the effect of sand sorting on gravel pack
toward permeability. This experiment only used
water at 1.0 cp as injection fluid and at three
different flow rates as well as under both
conditions; continuous and discontinuously flow
condition. The experiments were conducted with
5
flow rate at 20 cc/s, and then followed by 30 cc/s
and finally end up with 50 cc/s. All five samples
(Sand A, B, C, D and E) had done the same
procedure. In Figure 16. it shows that, by
increasing the median grain size it will increase the
permeability value.
In addition, on Non-Uniformity sample
with 7.6 of Uniformity Coefficient; C and on Very
Non-Uniformity sample with 10.6 Coefficient; C in
sample E, presented the very high permeability
reduction. This is happened because of non-sorting
particle grain size. The small grain size particles
are migrating to the pore space of bigger particle
grain size. Thereafter, the force induced in high
flow fluid injection are enough to migrate the small
particle and rearranged the packing of particles
hence reduced the existing porosity.
At the beginning of the flow time Figure
16. shows that all the graph line has fluctuated,
these occur at 0 to 30 minutes of flow time. At
these moments, assumed that all the particles in the
sand pack are rearranging each other because of the
velocity of the flow rate. Nevertheless, after 30
minutes of flow time due to injection, the Figure
16. shows that all line at a relatively constant rate.
So, it is predicted that the particle had reached the
dynamic rearrangements. From this plateau region,
it shows that the uniform sand distribution which is
Sand A, B and C have the highest value of
permeability compare with non-uniformity and
very non-uniform sand distribution, which is sand
D and E that had slightly lower value of
permeability.
The Effect of Permeability responds to Flow
Condition.
From Figure 16. Generally, permeability
under continuous flow conditions is always slightly
better than permeability under the discontinuous
flow conditions. These phenomena occurred due to
the packing already reach their stability while
flowing fluid flow at 20 cc/s. The particles only
face a small increase of hydrodynamics force
compare to unstable gravel packing, which had to
face a higher increment of hydrodynamic force at
the beginning of particles movement and
rearrangement before it reached their packing
stability. Therefore, it minimized the pore space
sizes and ability of the fluid to flow through the
gravel pack besides reduces the permeability
respectively. Whereas, for discontinuous flow
condition, the increasing of flow rate will increase
hydrodynamics force as well. Higher
hydrodynamic force will cause a faster movement
of particles and more sever of plugged at the pore
throat for an unstable/no-cemented gravel pack
which the packing had not been reach their stability
yet. Therefore, the increasing hydrodynamic force
will increase permeability reduction respectively.
Field Application of Experimental Results
Base on the experimental result, it shows
that good sorting will perform the good
permeability. Meaning, in field application, gravel
pack needed good sorting. We cannot control the
sorting on the formation. But, for gravel packing
placement, we can control the sorting. However, if
formation particles (has smaller size particles)
manage to invaded (not penetrate), there is no way
to control them (permeability impairment). If
smaller particles were invaded, the fine grain
particle will plugged the pore throat and it will
reduce the existing porosity as well. The bottom
line here is that we need to control the movement
of formation particles at the sand face.
Conclusions
The following conclusions can be made based on
the experiments conducted:
1. Higher injection fluid flow rate gave higher permeability reduction.
2. Large median grain size particles with
the uniformity coefficients; C, less
than three gave better permeability
compared to smaller grain size
particles with C value less than three.
3. The sand pack permeability reduction
is more severe when more viscous
injection fluid was used.
4. Good sorting with the uniformity
coefficients less than three performed
better permeability compare to poor
sorting with the uniformity
coefficients greater than 5.
5. Gravel packing under the continuous
flow conditions is always rewarded
the better permeability compared to
permeability under a discontinuous
flow conditions.
References
1. BJ Services (1996). BJ Services
Handbook; Completion Technology for
Unconsolidated Formations. Rev. 3. USA:
BJ Services Handbook
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10. D. L., Tiffin (1998). “New Criteria for
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Hendrick, J.O.Jr. (1977). “Design, Plan,
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for Maximum Productivity”. SPE 5709.
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7
Figure 1: Schematic of experimental apparatus
Figure 2: Sand holder design configuration.
8
Figure 3: Photograph of experimental flow system.
Figure 4: Photograph of sand holder. Figure 5: Photograph of monometer U.
9
Figure 6: Pressure and permeability measurement zone
Figure 7: Formation sand size distribution
10
Figure 8: Effect of flow rate on Sand A permeability with 1.0cp
Figure 9: Effect of flow rate on Sand B permeability with 1.0 cp.
11
Figure 10: Effect of flow rate on Sand C permeability with 1.0 cp.
Figure 11: Effect of flow rate on Sand D permeability with 1.0 cp.
12
Figure 12: Effect of flow rate on Sand E permeability with 1.0 cp.
Figure 13: Effect of injection fluid viscosity on Sand C permeability.
13
Figure 14: Effect of injection fluid viscosity on Sand D permeability.
Figure 15: Effect of injection fluid viscosity on Sand E permeability.
14
Figure 16: Effect of sand sorting on sand permeability with 1.0 cp.
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