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Aramco Scale Monitoring Guide

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0% found this document useful (0 votes)
17 views23 pages

Aramco Scale Monitoring Guide

Uploaded by

hamdy nabawy
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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ENGINEERING ENCYCLOPEDIA

Saudi Aramco Desk Top Standards

SAUDI ARAMCO

SCALE MONITORING FIELD MEASUREMENTS

NOTE: THE SOURCE OF THE TECHNICAL MATERIAL IN THIS VOLUME IS THE PROFESSIONAL
ENGINEERING DEVELOPMENT PROGRAM (PEDP) OF ENGINEERING SERVICES.

WARNING. THE MATERIALS CONTAINED IN THIS MANUAL WERE DEVELOPED FOR THE SAUDI
ARABIAN OIL COMPANY (SAUDI ARAMCO) AND ARE INTENDED FOR THE EXCLUSIVE USE OF
SAUDI ARAMCO EMPLOYEES. ANY MATERIAL CONTAINED IN THIS MANUAL WHICH IS NOT
ALREADY IN THE PUBLIC DOMAIN MAY NOT BE COPIED, REPRODUCED, SOLD, GIVEN, OR
DISCLOSED TO THIRD PARTIES, OR OTHERWISE USED, IN WHOLE OR IN PART, WITHOUT THE
PRIOR WRITTEN PERMISSION OF VICE PRESIDENT, ENGINEERING SERVICES, SAUDI
ARAMCO.
ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

CONTENTS PAGES

GAUGE RINGS.....................................................................................................1
CALIPERS.............................................................................................................3
COUPONS.............................................................................................................8
X-RAYS...............................................................................................................12
TURBIDITY........................................................................................................14
MISCELLANEOUS TECHNIQUES...................................................................17
GLOSSARY ........................................................................................................19

SCALE MONITORING FIELD MEASUREMENTS


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

GAUGE RINGS
Gauge rings (Figure 1) are easy to use but provide limited information about the extent of
scaling. They do indicate that scaling is occurring downhole. For this reason, gauge rings are
often deferred in preference to running a downhole caliper survey, which will be discussed
later.

FIGURE 1. Gauge Ring

SCALE MONITORING FIELD MEASUREMENTS 1


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

Running gauge rings periodically in every well in a field is neither practical nor desirable.
We have already seen that the use of scale-prediction programs with injected and produced
water analyses can determine which producing wells are the most likely to scale. Therefore,
produced water analyses can be obtained for potentially troublesome wells. When these
analyses indicate a scaling condition exists, gauge ring surveys can verify whether scale is
indeed being deposited.

The use of gauge rings is limited because they provide very little information. Typically
gauge rings indicate

• An obstruction is growing in the tubing

• The minimum distance from the surface at which growth is occurring

Gauge rings give no information on the interval over which scaling in the tubing is occurring
nor the amount of scale deposited (Figure 2). When a gauge ring finds the first deposit of
scale that has decreased the open diameter of the tubing to less than the diameter of the gauge
ring, the survey stops. No additional information is learned.

FIGURE 2. Gauge Ring Encounter With a Scale Deposit in Tubing


SCALE MONITORING FIELD MEASUREMENTS 2
ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

As we have already seen, scale does not always occur even though the chemical conditions
exist in the produced water. Therefore, treatment to prevent scale without verification that
scaling is occurring is a wasteful practice. Gauge rings offer a simple, inexpensive method
for determining if scale deposition has begun.

CALIPERS
Wireline calipers provide a complete survey of tubing ID from surface to total depth. A
caliper survey provides information on the depth at which scale is being deposited and
measurements from which the total volume of scale present can be calculated.

Typically, a wireline caliper tool consists of a cylinder equipped with moveable arms that
contact the walls of the tubing (Figure 3). The diameter to which the arms extend is the ID of
the tubing, and this measurement is transmitted continuously to the surface through an
electrical cable. The contact surface between the arms and the tubing inside surface varies
among different devices. Some contact the tubing with cutter-like blades that rotate on
bearings at the ends of the arms. Others slide on the surface. The cutter-like blades are
preferred when measuring metal loss due to corrosion. The blades are able to cut through
corrosion deposits to contact the steel wall of the tubing. For measuring scale or other
deposits, a sliding contact is preferred so the true extent of scaling is determined (Figure 4).

SCALE MONITORING FIELD MEASUREMENTS 3


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

FIGURE 3. A Wireline Caliper Tool

SCALE MONITORING FIELD MEASUREMENTS 4


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

FIGURE 4. Cutting and Sliding Contacts

SCALE MONITORING FIELD MEASUREMENTS 5


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

Calipers have been blamed for downhole corrosion problems by damaging applied coatings
and by disrupting protective films. In some gas and gas-condensate wells, the resulting
corrosion appears as vertical grooves in the tubing. These grooves are called “caliper tracks”
although not all corrosion grooves in tubing are the result of caliper surveys (Figure 5). These
grooves can penetrate the tubing wall.

FIGURE 5. “Caliper Tracks” in Tubing

The type of caliper used for measuring scale deposits must be able to obtain the information
needed without damaging the well. The caliper body should be the smallest diameter
available for the size of tubing to be calipered; otherwise, the tool may become stuck if
heavier deposits are encountered. A continuous recording of tubing ID is preferred so the
total volume of scale can be estimated with a planimeter or computer scanning probe (Figure
6).

SCALE MONITORING FIELD MEASUREMENTS 6


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

FIGURE 6. Caliper Record

Some loose or friable scales may be removed by a caliper tool. Plans should be made in case
the resulting debris causes a problem in the well or to downstream process equipment. These
problems can be overcome by trailing the caliper with a basket, by bailing, or by equipping
the well’s flowline with a temporary sieve.

SCALE MONITORING FIELD MEASUREMENTS 7


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

The change in tubing ID obtained from successive caliper runs can be used to estimate the
rate of deposition of scale. Usually, scale deposits too rapidly and becomes troublesome
before successive caliper runs are practical or economical. Remedial action generally should
be taken soon after scale is detected or as soon as it threatens production. In some cases,
however, successive caliper surveys are valuable in indicating that the danger of scaling is
past. For example, if the composition of produced water has changed to a marginal scaling
index, then further treatment of the well might not be necessary. The percentage of injected
water in a produced water always increases, but the potential for scaling may exist only over a
range of injected/formation water ratios. Therefore, monitoring of scale growth as the limit
for scaling approaches might avoid additional expenses for treating the well or surface
facilities.

COUPONS
Coupons are test specimens that can be retrieved from a production stream and examined for
scale. Coupons can be

• Small specimens mounted on an insertable apparatus for a flowline

• Specimens mounted in a side stream

• Tubing sections used in a sidestream flow loop

• Flanged sections (spools) in a flowline

• Downhole tubing itself that can be inspected when pulled

Care must be taken so the environment of the coupons duplicates the conditions in question in
regard to flow, temperature, and pressure. For example, where two-phase flow exists, the
coupons must be inserted in the water layer. Figure 7 shows a typical retrievable installation
of coupons in a flowline. Coupons can be inserted by hand if the pressure is less than about
50 psi. At higher pressures, mechanical means of insertion must be used. Typically, the
coupons are about 0.5 × 1 inch × 0.1 inch thick.

Coupons are drilled with several 1/8-inch diameter holes and are mounted in pairs
perpendicular to the flowing stream so the stream impacts the face of one of the coupons.
New coupons are polished with abrasive, degreased, washed with detergent and water, rinsed
with water, rinsed with a water-miscible solvent such asacetone, then dried under a heat lamp
or in an oven, and weighed. Coupons can be carried to the field in a cleaned condition, but
they should be protected from moisture and fingerprints. Coupons should be handled only
with clean tools or clean gloves.

SCALE MONITORING FIELD MEASUREMENTS 8


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

FIGURE 7. Retrievable Coupons in a Flowline

The weighed coupons are mounted in the flowing stream for a period of time, say a week,
before retrieval. If possible, several pairs should be inserted so they can be retrieved at
different time intervals. On removal, the coupons should be rinsed immediately with distilled
water then dried and weighed. In this condition, the scale is subject to visual examination,X-
ray diffraction analysis, and chemical analysis. The amount of scale deposited is determined
from the difference in dry weights of the coupons before and after exposure. If corrosion has
occurred, the first weight of the coupon itself will be less than when it was inserted. This can
be determined by removing the scale and reweighing the cleaned coupon. The amount of
scale deposited is then the difference in weights of the scaled coupon before and after
cleaning. The corrosion rate can be calculated from the dimensions of the coupons and their
weights before exposure and after removing the scale.

SCALE MONITORING FIELD MEASUREMENTS 9


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

Ms = Cs - Cn no corrosion

Ms = Cs - Cd corroded

C.R. = F(Cn - Cd)

Where
Ms = weight of scale deposited
Cs = weight of scaled coupon
Cn = weight of new coupon
Cd = weight of descaled coupon
C.R. = corrosion rate
F = corrosion conversion factor

FIGURE 8. Weight of Scale Deposited

Descaling the coupons offers an opportunity to observe the best method for cleaning the
scaled equipment. If the deposit is calcium carbonate, it will fizz and dissolve in corrosion-
inhibited acid. The deposit might be a mixture of calcium carbonate and other scale
compounds. For such cases, observe if a residue remains after the acid treatment and what
methods are required to remove it. To determine the amount of residue that remains, carefully
rinse, dry, and weigh the coupon after the acid treatment.

Two solutions are available to remove calcium sulfate from coupons: potassium glycolate
solution or a solution of Na4EDTA and KOH. After use of either solution, briefly immerse
the coupon in corrosion-inhibited acid. Again, observe which method best cleans the scaled
coupon.

Deposits of strontium sulfate and barium sulfate are best removed mechanically. Although a
proprietary solvent exists for these compounds , it is not yet readily available. Mechanical
methods include scraping and wire-brushing of the coupon. The mechanical method should
not remove any base metal so that the coupon’s corrosion loss is preserved. Wire-brushing
usually does not remove a significant amount of metal. However, scraping must be done with
care.

Spools in flowlines and sidestream flow loops simulate more closely the conditions existing
in flowlines than do inserted coupons. A typical spool installation is shown in Figure 9 and a
sidestream loop is illustrated in Figure 10.

SCALE MONITORING FIELD MEASUREMENTS 10


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

FIGURE 9. Test Spool in a By-pass

FIGURE 10. Instrumented Test Loop

The arrangement in Figure 9 allows the spool to be installed and removed without shutting
down the main stream. Two spools should be available for each installation so a fresh spool
can be installed when one is removed for inspection. Spools allow direct observation of the
type and extent of scaling that has occurred. If necessary, saw the spool into pieces for better
determination of the thickness and laminar structure of the scale.

SCALE MONITORING FIELD MEASUREMENTS 11


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

Sampling the scale in a spool and cleaning the spool should follow the same precautions as
already discussed for coupons. The approaches and chemicals are the same except for the
differences in magnitude because of the large size of the spool. Some scales can be removed
by hammering on the external surface of the pipe. The internal surface can then be cleaned
up with a wire brush or jet lance. Lances apply high pressure water at about 5,000 psi to
dislodge particulate residue. In extreme cases, abrasives such as sand can be added to the
water.

X-RAYS
The preferred method for measuring the amount of scale in flowlines and surface equipment
in Saudi Aramco is by radiological examination. In this method, penetrating X-rays are
passed through the specimen in question and the resulting “shadow graph” is displayed on
photographic film or a fluorescent screen. Results for either method of display are obtained
quickly and can be used to calculate the amount of scale that has been deposited. The method
has also been used to examine boiler tubes and other equipment for metallurgical flaws.

X-rays are very short wavelength electromagnetic emissions. They are generated by
bombarding a metal anode with electrons in a vacuum tube. The higher the accelerating
voltage between the target and the electron emitter, the more penetrating (shorter wavelength)
are the resulting X-rays. Gamma rays (γ-rays) emitted by radioisotopes, such as cobalt-60,
are electromagnetic and have properties similar to X-rays. Radioisotopes are more portable
and easily applied as penetrating radiation than are X-rays, which require electrical power and
high voltage generating equipment.

WARNING: γ-ray emission from radioisotopes is continuous and


cannot be turned off. The source material must be stored in shielded
containers that absorb the γ-rays down to a level that is safe for
human exposure. The shielded containers must be safe-guarded to
prevent tampering by unlicensed personnel.

X-rays, which can be generated on-site and only when needed, are preferred.

SCALE MONITORING FIELD MEASUREMENTS 12


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

Figure 11 shows a schematic of the equipment used for radiological examination of pipelines
in the field. X-rays are more strongly absorbed as metals increase in atomic weight. For
example, lead (atomic weight: 207.2) absorbs X-rays more strongly than does iron (atomic
weight: 55.8). When X-rays are passed through a specimen to photographic film on the other
side, light areas on the exposed film are where the X-rays were strongly absorbed while
darker areas are where more X-rays were able to penetrate. The resulting pictures are
shadows that indicate the relative ability of the X-rays to penetrate the specimen. A crack or
void in a pipe wall shows as a dark streak or spot indicating that more X-rays were able to
pass through that particular place. Scale shows as a lighter area because the pipe wall plus
scale absorbs more X-rays than a pipe wall alone. It is not necessary to empty the pipe of
fluids for radiological examination.

FIGURE 11. Making a Radiograph of a Pipe

SCALE MONITORING FIELD MEASUREMENTS 13


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

TURBIDITY
Scale formation downhole or in surface equipment might be accompanied by the formation of
suspended particulates in the stream. Recall that if the degree of supersaturation is in the
metastable range, then seeding is required for precipitation to occur. Therefore, in the
metastable range, scale formation could occur without suspended particulates being formed.
Also recall that the metastable region of supersaturation is smaller for compounds that are less
soluble, such as BaSO4 and CaCO3. Scaling by BaSO4 and CaCO3 will most likely be
accompanied by an increase in the level of particulates in the stream.

In cases where scaling is accompanied by increased suspended particulates, the appearance of


crystals of the scaling compound in the stream indicates the beginning of scaling. Under
certain conditions, such as in oil-wet tubing or pipe, particulates could occur without scaling.
Since these factors cannot be predicted, the only basis on which particulates can be used to
indicate scaling is through experience with the system.

Suspended particulates contribute to an increase in turbidity of the stream. The increase in


turbidity can be measured by light diffraction using a turbidimeter (Figure 12). The
turbidimeter measures the light reflected by particles at an angle, usually about 90°, to the
incident light. A photocell measures the reflected light. Results are expressed in turbidity
units. Two systems of units are National Turbidity Units (NTU) and Jackson Turbidity Units
(JTU). The difference in the two systems is in the basis for standardization. Most industrial
turbidimeters use a suspension of very small polystyrene balls for calibration to a system of
turbidimetric units.

FIGURE 12. Turbidimeter

SCALE MONITORING FIELD MEASUREMENTS 14


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

Some precautions must be observed in using turbidimeters on a flowing stream. The


turbidimeter measures all particulates without discrimination. This includes gas bubbles, oil
droplets, and all the suspended particulate inorganics that a well stream might contain, such as
iron sulfide, sand, and silt. Therefore, establishing a reference basis for some streams might
be difficult or impossible if there are large variations in the level of turbidity.

Another precaution is that some of the light passing through the stream is absorbed by the
liquid itself. If light absorption increases, it would cancel some or all of the increase in
diffraction caused by an increase in turbidity. It is not unusual for the light absorbing
capacity (color) of a produced water to change if its composition changes because of
increasing production of injected or formation water. Some turbidimeters are equipped with
dual light paths, one straight and one angled. This arrangement measures the light absorption
at the same time as the light diffracted. The absorption measurement corrects the diffracted
light internally in the instrument for the light absorbed (Figure13).

FIGURE 13. Turbidimeter Corrected for Absorption

SCALE MONITORING FIELD MEASUREMENTS 15


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

A more direct method for measuring particulates is by filtration. This method of


measurement has been standardized by the National Association of Corrosion Engineers
(NACE). Filtration is widely used in petroleum production for evaluating the quality of water
for injection. A description of filtration procedures is given in Ostroff, Introduction to
Oilfield Water Technology, Appendix I.

For our purposes, the pressure in the filter holder should be as close to the pressure in the
flowing stream as possible. Recall that a drop in pressure can release carbon dioxide from
solution, causing CaCO3 to precipitate. Therefore, the flow control valve should be
downstream of the filter holder, and the filter holder must be constructed to withstand the
pressure of the flowing well stream. Figure 14 illustrates a filter set-up for this purpose.

FIGURE 14. Membrane Filter Rig

SCALE MONITORING FIELD MEASUREMENTS 16


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

The filter holder is typically equipped with a weighed polyethylene, solvent-resistant filter
membrane, 47 mm in diameter, having a pore size of 0.45 µ or 1.5 µ. The membrane is
mounted in the holder and the apparatus is filled with distilled water before it is attached to
valve A in Figure 14. Valve A is then opened slowly until the filter apparatus is fully
pressurized. Valve B is opened so a flow of about 50 cc/min passes through the filter. The
filter holder is filled with distilled water prior to attaching it to the system so that pressurizing
the apparatus does not burst the filter membrane. Filtration is continued until about 1 to 4
liters of water have been filtered. Valve A in then closed and the filter holder is removed
when the pressure has dropped to atmospheric.

The membrane filter is removed, rinsed carefully with distilled water, dried, and weighed. If
oil is present on the solids, it can be removed by rinsing with xylene. The rinsing sequence
should be water, methanol, then xylene. The methanol removes water from the filter because
water and xylene are not miscible. The difference between the weights of the membrane
before and after filtration is the weight of suspended solids collected. The weight of
suspended solids divided by the volume filtered in liters is the concentration of suspended
solids in mg/l.

The presence of scale compounds in the filtered solids can be verified by chemical analyses of
the solids. Unless you are interested in the total composition of the solids, only the
percentage of BaSO4 and CaCO3 need be requested from the analytical laboratory.

MISCELLANEOUS TECHNIQUES
Additional techniques that have been used to detect or measure scales with varying degrees of
success are ultrasonic thickness gauges and thermistor probes. Ultrasonic gauges are
normally used to measure wall thicknesses of pipes or vessels. An ultrasonic signal is
generated onto the surface of the metal by a transducer held in close contact with the surface.
The signal travels through the metal wall and is reflected by the inner surface back to the
surface where it is picked up by the transducer. The instrument displays the wall thickness
based on the difference in phase of the reflected and emitted signals. The signal is reflected
by all interfaces that it encounters such as impurities, voids, and cracks. If a scale is present,
part of the signal is reflected by the metal-scale interface and part by the scale-water interface.
The thickness of the scale can thus be measured under optimum circumstances. However,
most scales are porous and rough so accurate measurements cannot be made in most cases.
Ultrasonic inspection can be a valuable tool for scale measurement in some cases because it is
an inexpensive nondestructive method.

SCALE MONITORING FIELD MEASUREMENTS 17


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

Thermistors are electrical resistors whose resistance has a large increase with an increase in
temperature. They have been applied to scale detection by inserting the device (they can be
very small) in a drilled hole in a coupon or in a tube. A small electric current is passed
through the thermistor to generate a constant input of heat. When scale forms on the coupon
or tube, conduction of heat from the thermistor declines so its temperature rises. The
beginning of scaling is thus detected by an increase in resistance of the thermistor.
Thermistors work well in clean water where scale is the only foulant. Other foulants that
would decrease heat dissipation are oil, paraffin, biological fouling, or anything else that
would adhere to the specimen and decrease its thermal conductivity.

SCALE MONITORING FIELD MEASUREMENTS 18


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

GLOSSARY
acetone A volatile ketone solvent, C3H6O.

(to) acidize To treat with an acid.

caliper An instrument with hinged arms for measuring internal or


external diameters.

coupon A small piece of test material that represents a much


larger body of the same material for which information is
needed.

diffraction Modification of the behavior of light by an obstacle or


aperture.

friable Readily crumbled, brittle.

gamma rays ( -rays) A portion of the electromagnetic spectrum lying just


above X-rays in higher frequency; emitted by
radioisotopes.

gauge ring A downhole tool of known diameter for determining if


the hole is open to that diameter.

membrane filter A thin sheet of filter material having a known pore size,
usually circular, for filtering suspended solids from
fluids.

methanol Methane with an alcohol group attached, CH3OH; also


called wood alcohol.

mitigation The act of moderating in force or intensity.

particulate Consisting of individual particles.

perforations Holes that are shot, jetted, or blasted in casing that covers
the oil-producing zone in an oil well.

radiograph An image produced on a radiosensitive surface, such as


photographic film, by radiation other than visible light,
especially X-rays.

SCALE MONITORING FIELD MEASUREMENTS 19


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

remedial Supplying a remedy or cure.

SCALE MONITORING FIELD MEASUREMENTS 20


ENGINEERING ENCYCLOPEDIA
Saudi Aramco Desk Top Standards

spool A short, flanged section of pipe that can be inserted and


removed from a working pipeline for test purposes.

(to) squeeze To force chemicals into an underground formation.

thermistor A resistor made of semiconductors giving resistance that


varies rapidly and predictably with temperature.

transducer Any of various substances or devices that convert input


energy of one form into output energy of another.

turbidimeter An instrument for measuring the turbidity of water.

turbidity The existence of cloudiness in water caused by suspended


particles.

ultrasonic Acoustic waves having a higher frequency than the


human ear can hear (>20,000 cycles per second).

wireline High strength wire on a winch for lowering and retrieving


various devices into and out of oil wells.

X-ray A portion of the short wavelength (high frequency)


electromagnetic spectrum lying between ultraviolet
radiation (lower frequency) and gamma rays (higher
frequency).

xylene An aromatic compound consisting of a benzene ring with


two methyl groups attached.

SCALE MONITORING FIELD MEASUREMENTS 21

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