XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
MULTIFUNCTIONAL CHEMICAL TREATMENT IN OFFSHORE FACILITIES
*María Carrasquero, Elluz Torín, Luis Castillo, Rosa Nadales, María Llamedo y
Alfredo Viloria. Gas Technical Management (EPMG), PDVSA Intevep,
Phone: 0212-3307680, Fax: 0212-3308730, Apdo 76343, Caracas 1070-A,
Venezuela. e-mail: carrasquema@pdvsa.com
ABSTRACT
Being Venezuela one of the first eight countries with the largest natural gas reserves,
and as part of plans to exploit oil and gas recovery that are underway in the country, it
is expected the development of important reserves located offshore by the Mariscal
Sucre, Rafael Urdaneta and Plataforma Deltana projects. These projects are intended
to meet the gas needs in the domestic market and export the surplus. To ensure their
success, it is necessary to make the transport of fluids on a continuous basis,
economic and efficient. However, the presence of certain compounds such as CO2,
H2S, CaCO3, MgCO3, H2O and certain conditions of pressure and temperature, can
generate phenomena such as corrosion, scale, hydrates, among others. These
problems cause the shorter life of the facility, which translates into increased
operating costs in the industry. The phenomena mentioned above, typically, are
mitigated with specific inhibitors for each case, which are directly disposed to the
environment. The vast majority of these inhibitors are compounds imported from
chemical synthesis, thus generating technological dependence and loss of foreign
exchange. For this reason, and in view of the new global trend that is focused toward
minimizing pollution at source through designed green chemical products and
processes, PDVSA Intevep is developing a new multifunctional chemical treatment, to
cushion the phenomena of scaling, corrosion and hydrates economically and
efficiently. Among the results are high compatibility between natural products from
plants, and high efficiency in each of the systems evaluated individually.
Keywords: scaling, corrosion, hydrates, inhibition, multifunctional treatment, green
chemistry, Aloe vera.
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
INTRODUCTION
Considering the huge natural gas reserves existing in Venezuela, there are currently
being developed various plans and projects for the exploitation and recovery of this
resource, which include an increase in production from 6,900 million standard cubic
feet per day (MMPCED), up to 11,500 MMPCED in 2012 (Plan Siembra Petrolera
2006). To fulfill this goal it is planned to incorporate additional volumes of gas from
both mature fields located on the mainland (Anaco, Occidente and Yucal Placer), as
future developments in the offshore region of Venezuela (Mariscal Sucre Plataforma
Deltana).
Given this increase in national natural gas production, it is expected the need to
develop clean technologies and products, sustainable and economically feasible to
allow the flow assurance from production facilities to the packaging and processing.
Among the various phenomena that impede the proper transport of fluids from the
reservoir to surface facilities there are mainly hydrate formation, scaling and
corrosion. Hydrates are solids formed in multiphase flow streams where the water
molecules crystallize around low molecular weight molecules (methane, ethane,
carbon dioxide, hydrogen sulfide, among others), under conditions of high pressure
and low temperatures (GPSA 1998). For its part, the scaling occurs due to
precipitation of insoluble inorganic compounds owing to changes in pressure,
temperature, pH, among others, which promote the saturation of calcium ions (Ca+2)
mainly present in formation water (Guo 2005). Finally, corrosion is a degradation of
metallic materials as a result of an electrochemical reaction with the surrounding
environment, which involves the deterioration of the physical properties of the
material (Min 2008).
The presence of these phenomena on transmission and natural gas distribution lines
causes many inconveniences, among which include obstruction or clogging of pipes
due to the formation of solid deposits, leading to decreased productivity, high costs
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
operations associated with replacement of worn parts from corrosion and safety
hazards of personnel and facilities during normal operations and maintenance.
One of the most used alternative technologies in mitigating this problem is the
addition of treatments, which work by inhibiting the reactions associated with hydrate
formation, scaling and corrosion, among others (Kelland 2006, Pickering et al. 2001).
However, chemicals used for this application, in most cases, are toxic and nonbiodegradable, and frequently pre-treatments before their disposal are required.
Additionally, for the case of Venezuela, the purchase of these products is a high
outlay for the oil industry because of its foreign nature.
In this context, there is a concern within the national scientific and technical
community, to develop its own natural products for chemical treatment of oil, framed
within the principles of green chemistry. Thus, in previous work has been shown
excellent results, the efficiency of polysaccharides from Aloe vera in inhibiting scale
formation of calcium carbonate (CaCO3) in hydrocarbon production streams (Mata
2007, Carrasquero 2008).
Based on the antiscaling character seen in the inhibitor developed by PDVSA Intevep
(INTAVTM), the opportunity to develop a multifunctional chemical treatment for
preventing simultaneous scaling, hydrate and corrosion phenomena has been
approached, initially using commercial inhibitors on last two phenomena, these being
considered the most common problem-causing phenomena for flow assurance on
regional routes for hydrocarbon production.
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
EXPERIMENTAL METHODOLOGY
To assess the efficiency of multifunctional chemical treatment is necessary to conduct
separate trials of inhibitors in their specific area (scaling, hydrates and corrosion) and
then evaluate the synergy between them in each of the systems.
Trials assessing the efficiency of scale inhibitors in a static state are made in glass
cells, following the procedure specified in the NACE TM0374 standard (NACE
International 2001). The tests use synthetic mixtures of water whose chemical
composition is established by the standard and inhibitors to evaluate. The test lasts
24 hours.
For studies of hydrate formation, the method relies on the ability of tetrahydrofuran
(THF) to form hydrates at low temperatures and atmospheric pressure (Mamun
2006). The method involves monitoring of parameters such as temperature and
conductivity of the system for the evaluation of the efficiency of hydrate inhibitors to
be used, taking into account the exothermic reaction and loss of mobility of ions
(formation of solids) associated with the formation of hydrates (Valberg 2006). The
test lasts 2 hours.
For the evaluation of the corrosiveness of the multifunctional treatment, corrosion
tests are performed by weight loss of selected metal (steel API 5L X65). Tests are
carried out in 2-liter autoclave, able to withstand 13.8 MPa and temperatures up to
360 °C. The system consists of a solution of sodium chloride (NaCl) at 3.5% as a
corrosive medium on which the coupons are evaluated. CO2 is injected and the
temperature, pressure and rotation speed (agitation) is fixed, in reference to the
operating conditions of the Dragon and Patao fields. The test has a duration of 7 days
(ASTM G1 2003, NACE RP-0775 2005).
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
RESULTS AND DISCUSSION
The selected scale inhibitor product was based on natural polysaccharide extracted
from Aloe vera and developed by PDVSA Intevep (INTAVTM) due to its high efficiency,
demonstrated at both laboratory and field scale.
A scaling test was performed (according to NACE Standard TM0374) using 2000 ppm
of INTAVTM, as this proved to be the optimal dose of the product according to
previous studies (Castillo 2008). Figure 1 shows the concentration of calcium present
in the sample without inhibitor (blank) and the sample with INTAVTM. The dotted line
represents the total amount of calcium supplied to the system initially, about 1620
ppm.
The calcium content defines the efficiency of chemical treatment evaluated, since the
higher amount of calcium in solution (not precipitated calcium), the greater the
efficiency of the inhibitor. The efficiency obtained for the scale inhibitor in the assay
was 79% (Figure 1), higher than commonly reported for commercial scale inhibitors to
Calcium concentration (ppm)
the same conditions of criticality which sets the standard (Carrasquero 2008).
1800
1600
1400
1200
1000
800
600
400
200
0
1432
706
Blank
Maximum Calcium
Concentration
INTAV
Evaluated system
Figure 1. Efficiency of INTAVTM in scale trials
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
Additionally, to prevent tetrahydrofuran (THF) hydrate formation, Mono Ethylene
Glycol (MEG) was selected as base inhibitor, because it is a thermodynamic inhibitor,
used commercially to inhibit this phenomenon (Brustad et al. 2005).
A trial was conducted without inhibitor (blank) to know the behavior that the system
has in the absence of chemical treatments (Figure 2). Note that all tests were carried
out at atmospheric pressure and temperatures below 15 °C.
Figure 2 shows the behavior of the conductivity and temperature versus time for a
system in absence of inhibitors (blank). These variables can describe the formation of
hydrates, because the presence of these solids in solution substantially decreases
the conductivity of the system, related to ion mobility. Moreover, temperature is
another variable that shows the behavior of formation of hydrates because the
formation reaction of these solids is exothermic, so a temperature increase indicates
the formation of the hydrates (Sloan 1998).
Figure 2. Behavior of hydrate formation in the absence of chemical treatment
Subsequently, tests were performed using different doses of MEG (20 to 30%),
obtaining the results presented in Figure 3.
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
Figure 3. Behavior of hydrate formation in the presence of MEG at 20% v/v
As can be seen (Figure 3) there was no THF hydrate formation in the systems
evaluated, because the temperature decreases steadily to reach thermal equilibrium,
and in turn the conductivity also decreases slightly and then stabilized at relatively
constant values, because ions lose their mobility with decreasing temperature. In all
cases curves were obtained similar to Figure 3.
Comparing the behavior of the system in the presence of MEG, with that obtained in
the blank (Figure 2), it can be confirmed that MEG acts as a thermodynamic inhibitor,
since no evidence of a sharp drop in conductivity nor temperature increase, as a
consequence of the exothermic reaction resulting in the formation of hydrates, was
found. Because MEG is a thermodynamic inhibitor, its presence is expected to shift
the hydrate formation curve at lower temperatures, a fact which is evident because
the temperature of formation of such solids in the blank was -2 °C while in the
systems evaluated in the presence of MEG reached values of -13 °C and there was
no formation of said solids.
Having examined the efficiency of the scale inhibitor (INTAVTM) and the hydrate
inhibitor (MEG), an assessment of compatibility between the products, in both
scenarios, was carried out.
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
For the evaluation of the efficiency of INTAVTM in the presence of MEG, scaling tests
were done (NACE International 2001), using 2000 ppm INTAVTM and between 10 and
50% of MEG. Figure 4 presents the results obtained in these experiments. Between
the dotted lines are the threshold concentrations at which efficient chemical treatment
is assessed with respect to the sample without inhibitor (blank).
Calcium concentration (ppm)
1800
1600
1432
1400
1200
1000
867
769
659
612
535
MEG 10%
MEG 20%
MEG 30%
MEG 40%
MEG 50%
800
600
400
200
0
Maximum Calcium conc.
Calcium conc. (blank)
INTAV
Evaluated system
Figure 4. INTAVTM efficiency in the presence of Monoethylene Glycol
As can be seen, as the MEG concentration increases, the amount of calcium in
solution becomes smaller, indicating that it promotes the precipitation of calcium in
the system, drastically reducing the efficiency of INTAVTM. This behavior may be due
to increased pH that the system suffers as a result of the addition of MEG (pH equal
to 8), because it appears that the solubility of CaCO3 is affected by the change in pH,
for each increase of pH unit, the solubility of CaCO3 is reduced by two orders of
magnitude (Sandengen 2007).
Moreover, the evaluation of the efficiency of MEG in the presence of INTAVTM
determine that it does not affect the inhibition of hydrate formation, because the
behavior of the curves of conductivity and temperature had a similar pattern to that
seen for evaluation of MEG (Figure 3).
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
Based on the results, it appears that although each inhibitor is efficient in its
performance, when evaluated for compatibility, it was noted that there is not synergy
between them, as the MEG significantly affect performance of INTAVTM as antiscaling.
In order to develop a multifunctional chemical treatment environmentally friendly,
based on the efficiency results obtained for the formation of hydrates in the presence
of INTAVTM and knowing that Aloe vera is a plant that withstands extreme
temperatures, it was decided to evaluate the biotechnology inhibitor in THF hydrate
inhibition.
Different concentrations were evaluated of INTAVTM in hydrate inhibition, obtaining
that from 15% v/v, the INTAVTM is capable of inhibiting the formation of hydrates in
the same way it does the MEG (thermodynamic inhibition), revealing the absence of
exothermic changes and changes in the conductivity of the system (Figure 5).
Figure 5. Behavior of hydrate formation in the presence of INTAVTM
Subsequently, the best INTAVTM concentration on the inhibition of hydrates was
evaluated in the scaling system, and was found that said concentration does not
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
affect the formation of precipitates CaCO3, but continues to restrain the formation of
solids.
For this reason, INTAVTM was selected as both scale and hydrate inhibitor, and the
corrosive potential of this formulation in steel cylinders API XL 65 was evaluated,
which is the same material used in pipes for offshore systems of reference. The
system consists of a solution of NaCl 3.5%, the pressure of the CO2 was 15 psig and
temperature 60°C, conditions that simulate the corrosiveness of the gas offshore,
specifically in the fields mentioned above, where corrosion is associated with the
presence of CO2. The results are presented in Figure 6.
Corrosion rate (mils per year, mpy)
80
68
70
60
50
40
30
20
24
18
23
10
0
Blank
INTAV
INTAV + 50ppm INTAV + 150ppm
Dodecylamine
Dodecylamine
Evaluated system
Figure 6. INTAVTM corrosion rate in absence and presence of dodecylamine
It can be seen (Figure 6) that the corrosion rate of INTAVTM is three times higher than
the blank, thereby increasing the corrosion rate of the system. This behavior can
occur because the INTAVTM has a slightly acidic pH (pH equal to 3.9).
In this sense, the dodecylamine was incorporated as a corrosion inhibitor, in seeking
to reduce the corrosive potential of multifunctional treatment. The dodecylamine acts
as a film inhibitor, adhering to the surface of metals and forming a film that protects
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
the environment (Viloria et al. 1994, Méndez 2001). Figure 6 shows the corrosion rate
of blank and INTAVTM obtained in previous trials, as well as those obtained by
INTAVTM in the presence of 50 and 150 ppm of dodecylamine, which were 24 mpy
and 23 mpy respectively.
The major decrease in corrosion rate with INTAVTM and dodecylamine is evident, so
the latter is verified as an inhibitor of this phenomenon. However, this amine was not
able to reduce corrosion to levels lower than those obtained in the blank (18 mpy).
Therefore, its action was not enough, because the system is still corrosive. It's worth
noting that the maximum corrosion rate allowed on the premises of the oil and gas
industry should be 5 mpy, which are classified by all formulations as corrosive (NACE
Engineers 1979). Therefore, it is advisable to optimize the dose of this inhibitor in
order to obtain a formulation that meets the requirements of prevention of the
phenomena studied.
According to the results obtained, it can be concluded that optimization of
dodecylamine dose or the addition of another corrosion inhibitor is a requisite to reach
the corrosion rate required for its application in oil installations, thus completing the
formulation of a multifunctional chemical treatment, which allows the alleviation of
scaling, hydrates and corrosion phenomena.
CONCLUSIONS
•
The presence of INTAVTM did not alter the efficiency of MEG as a hydrate
formation inhibitor but MEG decreases efficiency of INTAVTM as scale inhibitor,
because it facilitates the precipitation of CaCO3 by increasing pH in the system.
•
The INTAVTM at 15% v/v inhibits the formation of hydrates, acting as a
thermodynamic inhibitor and this concentration does not alter its behavior as scale
inhibitor.
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
•
The INTAVTM increase the corrosive potential of the system, increasing the
corrosion rate of 18 mpy to 68 mpy.
•
The dodecylamine significantly reduces the corrosive potential of INTAVTM, but did
not manage to achieve the minimum values (5mpy), since the rate decreased from
68 mpy to 23 mpy.
REFERENCES
•
ASTM G1. Standard Practice for Preparing, Cleaning, and Evaluating Corrosion
Test Specimens. 2003.
•
BRUSTAD, S., LØKEN, P. y WAALMANN, J. Hydrate prevention using MEG
instead of MeOH: Impact of experience from major Norwegian developments on
technology selection for injection and recovery of MEG. OTC 17355. OTC
Offshore Technology Conference. Mayo 2005.
•
CARRASQUERO, M. Estudio de un inhibidor de incrustaciones a base de gel de
Aloe vera para el aseguramiento del flujo en la industria de los hidrocarburos.
Trabajo especial de grado. Universidad de Carabobo. Agosto 2008.
•
CASTILLO, L. Escalamiento tecnológico de un inhibidor de incrustaciones a base
de gel de Aloe vera para el aseguramiento de flujo en la industria del crudo y gas
natural. Tesis de maestría. Universidad Central de Venezuela. 2008.
•
GAS PROCESSORS SUPPLIERS ASSOCIATION (GPSA). Engineering Data
Book, 11th Edition, Tulsa, OK, USA. 1998.
•
GUO, B. et al. Offshore pipelines. 1ra ed. Oxford: Elsevier. 281 p. 2005.
•
KELLAND, M. History of the Development of Low Dosage Hydrate Inhibitors.
Energy Fuels, an American Chemical Society Journal. Volumen 20, Número 3.
Mayo/Junio 2006.
•
MAMUN, M. Surface Phenomena in Gas Hydrate Systems. Diploma Thesis.
NTNU. Junio 2006.
•
MATA, C. Polisacáridos Naturales como agentes anti-incrustantes. Trabajo
Especial de Grado. Universidad Simón Bolívar. 2007.
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
•
MÉNDEZ, M. Efecto de la velocidad del fluido sobre las propiedades protectoras
de crudos ante la corrosión por CO2 mediante la técnica de electrodo de Cilindro
Rotatorio. Tesis de grado. Universidad Simón Bolívar. 2001.
•
MIN, M. An introduction to Corrosion. CMM NDT Services. 2008.
•
NACE Engineers. Corrosion Control in Petroleum Production. TPC Publication 5,
p.p. 6 - 10. 1979.
•
NACE INTERNATIONAL. Standard Test Method: Laboratory Screening Tests to
Determine the Ability of Scale Inhibitors to Prevent the Precipitation of Calcium
Sulfate and Calcium Carbonate from Solution (for Oil and Gas Production
Systems). NACE Standard TM0374-2001. Item N° 21208. 2001.
•
NACE STANDARD RP-0775. Preparation, Installation, Analysis and Interpretation
of Corrosion Coupons in Oilflield Operations. 2005.
•
PLAN SIEMBRA PETROLERA 2006-2012. Siembra petrolera utiliza recursos de
crudo como palanca para crecimiento. Nota de prensa publicada el 10-03-2006.
Disponible en: http://www.soberania.org/Articulos/articulo_1995.htm. Consulta
01/02/2010.
•
PICKERING, P., y col. Evaluating new chemicals and alternatives for mitigating
hydrates in oil & gas production. IIR Conference. Aberdeen. Septiembre 2001.
•
SANDENGEN, K. Prediction of mineral scale formation in wet gas condensate
pipelines and in MEG (mono ethylene glycol) regeneration plants. NTNU.
Trondheim. 2006.
•
SLOAN, E. Clathrate Hydrates of Natural Gases. Segunda edición. Marcel Dekker
Inc. New York. 1998.
•
VALBERG, T. Efficiency of thermodynamic inhibitors for meeting gas hydrates.
Master Thesis. NTNU. Junio 2006.
•
VILORIA, A.; VERA, J. Inhibidores de Corrosión. CYTED. Río de Janeiro. 1994.
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