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

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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

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Handbook; Completion Technology for

Unconsolidated Formations. Rev. 3. USA:

BJ Services Handbook

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Figure 1: Schematic of experimental apparatus

Figure 2: Sand holder design configuration.

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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

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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.

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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.

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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.

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Figure 16: Effect of sand sorting on sand permeability with 1.0 cp.