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Upstream scale inhibition in carbonate reservoir--evaluation of a green chemistry.

Introduction

Inorganic scale formation in the wellbore, surface facilities and near wellbore formation is a common cause of production loss in most matured fields. Scales are formed either by mixing of two incompatible brines (normally sulfate scaling) or the sudden changes in produced fluid conditions, such CO2 partial pressure, temperature and pH (carbonate scaling). When the formation of sulphate or carbonate scale is a problem in producer wells, the most common methodology is scale inhibitor (SI) squeeze treatments, having properties of good adsorption/precipitation and slow and regulated release above MIC (minimum inhibitor concentration). Factors that control adsorption and release of SI are pH, [[Ca.sup.2+]], [[Mg.sup.2+]], temperature, rock mineralogy, porosity, permeability, fluid flow rate etc [1]. When carbonate scales are predominant and form in the tubular, they are dealt either by periodical cleaning (mechanical or chemical) or by continuous circulation of SI above MIC level near the wellbore fluid entry point.

Organic phosphonate type scale inhibitors are most widely used scale inhibitors. Phosphonate scale inhibitors are generally admixed with other additives to avoid corrosion or bacterial fouling. High levels of phosphonates are becoming increasingly restricted in terms of release to the environment. Environmental regulatory bodies in North Sea (UK, Norway, Denmark, The Netherlands) and the US Gulf Coast are encouraging operators to use greener chemicals and avoid pollution at the source itself. As a result phosphorous free and less toxic scale inhibitors such as polyacrylates and derivatives and various maleic and sulfonic acid homo- and co-polymers are gaining importance. Some new chemistries were also proposed including natural and biodegradable compounds such as Carboxy Methyl Inulin and polyaspartate [2].

In pursuance of going greener we investigated the scale inhibiting potential of Tannic acid which is a specific commercially available form of tannin. Its weak acidity is due to multiple phenol groups in the structure. The chemical formula for commercial tannic acid is often given as [C.sub.76][H.sub.52][O.sub.46], (figure 1) but in fact it contains a mixture of related compounds. Its structure is based mainly on glucose esters of gallic acid. It is a yellow to light brown amorphous powder which is highly soluble in water (one gram dissolves in 0.35 ml of water). Tannic acid found in almost any part of a plant: bark, wood, leaves, fruits and root [3].

Rationale behind selecting tannic acid as a potential scale inhibiting chemical are: Green chemistry--Tannic acid perfectly meets the criteria of green chemistry. Its source is natural and renewable. It has a high LD50 factor and does not cause either a lethal effect in mice (at its maximum tested dose of 845 [micro]g/mouse) or necrotic lesions in rats (at doses between 7.5 and 60 [micro]g/rat) [4]. Pesticide Action Network North America (PANNA) doesn't consider it to be listed in any of the toxic category such as acute toxicity, carcinogenic, endocrine disruption, reproductive and developmental toxicity or chemicals of special concern. Tannic acids are found to have high biodegradability under aerobic and anaerobic condition. Rate of biodegradability by aerobic microbes is found to be higher than that of anaerobic microbes [5] which makes it safe to dispose on surface.

[FIGURE 1 OMITTED]

Chelating action- Tannic acid exhibits good chelating property with [Fe.sup.2+] and [Fe.sup.3+] ions. We envisaged the same property could exist for other scale forming group-2 elements. The phenoxide ion sites in aqueous medium may act as binding sites for different soluble metal ions.

Corrosion prevention--Tannic acid is commercially used as rust converter. It passivate steel surface by converting the rust into iron-tannate thus preventing further corrosion [6,7]. Several other works successfully exhibited corrosion inhibition property of tannic acid in presence of additives, a notable one of which is calcium gluconate [8]. Reduction of rust particles (major nucleating sites for scale crystals) in the produced water system may help in reducing scale deposition.

Although tremendous amount of research has been done on scale prevention and mitigation, much remains to be done, particularly for scaling in carbonate reservoir as the produced water is almost always supersaturated with calcium ions in producing wellbore. Even more needs to be done to mitigate the scaling problem with minimum damage to the environment. In this paper, we investigated the scale inhibition efficiency of tannic acid for application in oil wells with produced water composition from an offshore field. The work began with prediction modeling of scaling tendency and type, when formation water (FW) and sea water (SW) are mixed at different proportion. Static jar tests at different temperature and with different FW/SW mix water were conducted for preliminary qualification and dynamic flow tests were conducted to determine MIC requirement at well intake temperature. The inhibited scale mass were further characterized through FT-IR, XRD and SEM to investigate scale crystal morphology. Coreflood experiments were carried out at reservoir conditions with carbonate core plugs from field to determine the absorbance and rate of release of tannic acid in order to investigate whether it could be applied as squeeze inhibitor. UV spectrometry was employed for quantification of tannic acid release rate. The results are finally considered in light of the corresponding implications for field applications.

Experimental

Materials: Tannic acid was received in powder form with 100% active ingredient. Synthetic sea water (SSW) and formation water (SFW), employed in solution preparation and flow studies, had the composition reported in table 1. They were prepared dissolving weighed amounts of the corresponding chloride and sulphate salts in de-ionized water. All the brines and inhibitor solutions were filtered through 0.2 um membrane and degassed under vacuum before use.

Scaling prediction: Commercially available Scalechem-3.2 software was used to predict scaling potential of produced water at different FW: SW ratio (fig-2). Composition of FW, SW and produced water (PW) at different mixing ratio are given in table 1. From figure 2, it is understood that the major scale expected is CaC[O.sub.3]. Scaling potential of SrSO4 is significantly low and on the borderline. Injection water (SW) breakthrough in wells of the field under study is varying from 30 to 70%, the region having strong CaCO3 precipitation tendency as per the prediction model.

[FIGURE 2 OMITTED]

Static jar test: Synthetic formation water (SFW) and synthetic sea water (SSW) were prepared and filtered. Three synthetic produced water (SPW) were prepared having FW to SW ratio of 70:30, 50:50 and 30:70. The mixed waters were prepared just before the test and pH was adjusted to 7.2 with C[O.sub.2] flow. Test samples with or without tannic acid (of different dosing) were equilibrated in a water-circulated thermostat at different aging temperatures (70 to 110 [degrees]C). After the incubation period (24 Hrs), aliquots were filtered through 0.45um filter paper and immediately analyzed for residual calcium (soluble calcium). The percent of calcium inhibition (% [I.sub.Ca]) was determined according to NACE Standard [9]. Soluble calcium concentration was determined by Systronics flame photometer (model no. 128).

%[I.sub.Ca] = [[[[Ca.sup.2+]].sub.in] - [[[Ca.sup.2+]].sub.non-in]/[[[Ca2.sup.+]].sub.i] - [[[Ca.sup.2+]].sub.non-in]] x 100

where [[[Ca.sup.2+]].sub.in]: soluble calcium concentration of the inhibited sample, [[[Ca.sup.2+]].sub.non-in]: soluble calcium concentration of the uninhibited sample [[[Ca.sup.2+]].sub.i]: initial soluble calcium concentration

Scale inhibition mechanism study: FT-IR spectroscopic analysis were performed on the precipitated scale products obtained in absence and presence of inhibitor (20 ppm) by KBr pellet technique. FTIR analysis of the soluble complex of calcium ions with TAN (after 24h exposure at 70 [degrees]C) was also carried out to understand the scale inhibition behavior. For the analysis of the soluble complex, the inhibited test solution was filtered through 0.22um filter paper and freeze-dried prior to FTIR study. For comparison purpose a freeze-dried aqueous solution of tannic acid was also analyzed following the same procedure. The same samples were also subjected to XRD and SEM study. X-ray diffraction patterns were recorded in the D-8 ADVANCE (Bruker-AXS, Germany) diffractometer with Bragg-Brentano Geometry using parallel beam Cu K[alpha] radiation from 5 to 75[degrees] 2[theta] with 0.020 step interval at 1 sec/step count-time and spinning the sample at 15 rpm. The minerals were identified with reference to the JCPDS power diffraction file. Scanning Electron Micro-imaging (SEM) was carried out using HITACHI SEM Model S-3400N.

Dynamic tube blocking test: Tube blocking test is an industry practice, to evaluate MIC in dynamic flow condition at required temperature. In our study the test was performed at 110 [degrees]C with SPW (50:50 ratio of SFW & SPW) following NACE guidance [10]. Cation solution contained Ca, Mg and Sr as their chloride salt and anion solution contained sodium salt of S[O.sub.4.sup.2-] and HC[O.sub.3.sup.-]. The solutions were filtered and pumped through separate coils at 1 ml/min each, through the test loop (1 m x 1 mm). The flow through the test loop took place upon immediate commingling of the two fractions. Tannic acid was dosed with anionic solution at various concentrations. Scale inhibitor concentration was reduced stepwise until differential pressure ([DELTA]P) across the flow tube started to rise (tube blocking). The concentration below which blocking observed was considered as minimum inhibitor concentration (MIC).

Core flood study: The objective of core flood study was to assess the potential of tannic acid as squeeze inhibitor. The necessary criteria of a potential squeeze scale inhibitor are (1) retention of a good percentage of squeezed chemical either through precipitation in the pore space or adsorption on the rock surface and (2) release at a steady rate above MIC for a long period. Core flood study at reservoir condition is probably the best method known to investigate these properties.

Core flood measurement was carried out in a HT-HP core flow set up (Temco CFS 830). Two carbonate core plugs from the offshore reservoir (parameters given in table 2) were collected, cored to 1.5 inches in diameter and approx. 2.7 inches in length, cleaned through two stage submerged cleaning process (1st stage to remove organic hydrocarbon and the 2nd stage to remove soluble ions), dried and their porosity-permeability measured. Initial saturation was carried out in vacuum saturator with 4% ammonium chloride (non damaging brine) and loaded into a Hassler core holder at 110oC (reservoir temperature) under confinement pressure of 700 psi and back pressure of 200 psi. Differential pressure across the core, during flow was monitored with pressure transducers and fed into data acquisition system for online permeability calculation. Saturations with (1st) brine (and measurement of Kwabs), (2nd) filtered reservoir crude oil (to measure Ko at Swirr) and (3rd) brine (to measure Kw at Sor) were carried out at this temp-pressure conditions to bring the core plug into residual oil saturated wellbore condition. 10 pore volume of 5% and 10% tannic acid N[H.sub.4]Cl brine was injected through core-1 and core-2 respectively in the reverse direction (mimicking well treatment) and shut-in overnight under back pressure. The core was then flowed back from the forward direction with brine at 1 ml/min flow rate.

Inhibitor adsorption-retention-flow back--On UV spectroscopic analysis, tannic acid was found to have [lambda]max at 210 nm. At concentration below 100 ppm absorption at [lambda]max was found to have a near linear relation with concentration. At higher concentration however the correlation was poor. The flow back fluids were collected on pore volume basis (each sample represented one pore volume). The initial effluents were diluted below 100 ppm before UV analysis to accommodate within the standard UV absorption plot obtained prior to analysis.

Results and Discussion

Effect of concentration of inhibitor on scale inhibition: Table 3 shows scale inhibition efficiency of tannic acid determined through jar test (at 70 [degrees]C, 90 [degrees]C and 110 [degrees]C in three FW/SW mixtures). The results show that at temperature 70oC which is the temperature at near wellhead region, 100% scale inhibition is possible with 30-40 ppm of tannic acid in produced water. The concentration required for the midsecion and well bottom where average produced water temperature is 90 and 110[degrees]C are in the range of 40-50 ppm.

These results indicate that higher temperature demands higher concentration to keep the scale forming ions into solution. Previously two common calcite scale inhibitors such as PPCA and BHPMP have been investigated and inferred that thermodynamic inhibitors decrease supersaturation by lowering the ionic activity product through either chelation of the metal ions or by decreasing solution pH11. In our case, solution pH lowered due to tannic acid addition to a very marginal extent to account for the antiscaling activity. Thus chelating phenomena could be considered as the main antiscaling mechanism.

Scale inhibition mechanism study: FT-IR spectra of the dry mass obtained after freeze-drying the aqueous solution of the inhibitor (in absence of calcium ions, showed a band at 1710 [cm.sup.-1] that belongs to carboxyl groups (figure 3). The bands between 1611 and 1503 [cm.sup.-1] are related to aromatic -C=C- bonds. The peaks between 1330 and 1037 [cm.sup.-1] in the spectrum of inhibitor belong to phenol groups. In case of the dry mass obtained after freeze-drying the aqueous solution of the inhibitor (in presence of calcium ions), we find that the band intensities specific to tannic acid are reduced in the spectrum of the Ca-TAN system. In the FTIR spectra of the Ca-TAN system, a reduction in the 1330-1037 [cm.sup.-1] band indicates hydroxy-Ca-tannate complex formation [12]. A reduction in the intensity of the band belonging to the carboxylic acid group at 1710 [cm.sup.-1] may be attributed to the dissociation of carboxylate groups to which Ca ions may be bonded via electrovalent linkages. Reduction of spectral intensity pertaining to phenolic and carboxylic acid functional groups indicate formation of calcium-tannate and a secure metal- ligand soluble complex.

[FIGURE 3 OMITTED]

FT-IR spectra of the insoluble scale products (figure 4) in absence and presence of inhibitor exhibited peaks at 1430, 875 and 712 [cm.sup.-1] that are characteristic of vibration of C-O bond of calcite. The spectrum of the insoluble scale product deposited in presence of inhibitor did not exhibit any peak corresponding to carboxyl or phenolic-OH moieties of TAN which resembles the fact that calcium-tannate formed in the medium was water soluble and underwent no precipitation under experimental conditions. In other words, the inhibitor formed soluble complex with calcium ions.

[FIGURE 4 OMITTED]

X-ray Diffraction Studies: Figure 5 shows the XRD patterns of precipitated scale products in absence and presence of 20 ppm inhibitor. Using the reflection peaks at 29.40[degrees], 39.40[degrees], 43.14[degrees] and 35.96[degrees] for calcite, we find that there are merely sharp calcite reflections in figures 5A and 5B confirming the efficient formation of calcite in absence and presence of inhibitor. Similarly reflection peaks at 31.69[degrees], 45.50[degrees], and 56.47[degrees] confirm the presence of halite phase as the brine contains a high concentration of sodium chloride. The XRD patterns also revealed formation of soluble complex of the inhibitor with calcium ions as none other mineral phase except calcite and halite appeared in the XRD pattern of the scale deposits obtained in presence of inhibitor. The reduction in the peak height of halite and calcite phases in the XRD pattern in presence of the inhibitor may indicate that the concentration of both the phases has been reduced in presence of the inhibitor. Reduction in the peak height of halite phase in presence of inhibitor may be due to the combination of sodium ions with the inhibitor molecules forming easily soluble sodium salt of the inhibitor that remains in solution resulting in lesser precipitation of halite phase under the experimental conditions. Reduction in the peak height of calcite phase in presence of inhibitor may also indicate deviation from hexagonal crystal structure of calcite and decrease in adherence property which is further revealed from the SEM study.

[FIGURE 5A OMITTED]

[FIGURE 5B OMITTED]

Scanning Electron Microscopic (SEM) Examination: The morphological study of the scale crystals obtained after 4 and 24h exposure of the test solution at 70 C (in presence and absence of inhibitor) indicate that the above inhibitor acts as crystal distortion inhibitor at a low concentration (figure 6 & 7). The effect of inhibitor causes deformation of the crystal morphology of both the adhered and precipitated crystals thus inhibiting the adhesion process. The function of the tested inhibitor appears to interfere or block the growth process of the growing crystals. The mechanism of control of the scale formation may involve irreversible adsorption of inhibitor at the active growth site of crystals, resulting in their growth control.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Dynamic tube flow test: Dynamic tube flow test result is presented in graphical form in figure 8. Differential flow pressures are recorded every 10 sec and plotted to represent scale inhibiting performance of tannic acid in dynamic flow condition.

Increase of pressure differential is the indication of scale deposition and blockage of capillary tube. Figure shows that scale deposition rate is greatly influenced by tannic acid concentration. At 40 ppm, there is no scale deposition and hence pressure increase even after 150 min of flow. This observation shows that MIC required for scale inhibition in at well bottom temperature (110oC) is 40 ppm. It also shows that concentration required for scale inhibition in dynamic condition is less than static condition.

[FIGURE 8 OMITTED]

Core flood studies: Scale Inhibitor Adsorption-retention: Line-1 of figure 9 shows the permeability profile of brine, which could be considered as maximum permeability of core plug-1 at residual oil saturation condition. Line-2 is permeability of brine postflush, on 5% tannic acid squeeze and line-3 represents permeability variation due to brine post flush on phosphonate based commercial scale inhibitor. Figure 10 shows corresponding results for core-2 which was treated with 10% tannic acid. It is evident form the figures that reduction of initial permeability which is an indication of amount of adsorbed material, is much lower for tannic acid compared to phosphonate SI. The tannic acid treated cores regained their original permeability within 7-8 pore volume of brine postflush, which is not the case for the commercial squeeze SI considered as standard.

Scale Inhibitor release: Figure 11 presents the UV absorption spectra of tannic acid. The absorption-concentration relation is linear upto 100 ppm TA concentration. At higher concentration deviation is observed. Released tannic acid concentration in the brine postflush samples, collected at every pore volume was measured through UV absorption and the plot is presented in figure 12. It shows that most of tannic acid is released form the core within 4 pore volume of brine injection and the reached below threshold limit 9 pore volume. This may be due to less adsorption time given after tannic acid squeeze or the clay chemistry was not favorable for its adsorption. A detail investigation is planned to be carried out with more authentic detection technique in our future investigation with tannic acid.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

Conclusion

* Tannic acid has proven properties to be in the category of a green chemical. It is easily bio-degradable, non-toxic and has rust removing property.

* In this study, tannic acid is proven to be an efficiency scale inhibitor for high temperature (70 to 110 [degrees]C) application in oil wells of carbonate reservoir.

* Minimum inhibitor concentration requirement for high calcium and bicarbonate containing produced water environment would be 40-50 ppm as seen through static and dynamic tests.

* FTIR, XRD and SEM studies indicate that the mechanism of scale inhibition is through formation of soluble complex and deformation of crystal morphology.

* Core flood studies revealed that adsorption/retention of tannic acid is poor in limestone formation and the release rate is high.

* From the findings of our study it could be safely recommended that tannic acid has all the required property for application in oil well with high calcite scaling potential. The recommendation is however restricted to continuous injection through capillary tube at the bottom of production tubing and not for squeeze inhibition. For squeeze inhibition detail study with field core plugs and produced brine should be used for core flood study and all reservoir parameters should be maintained during the flow.

Abbreviations

HP-HT--High pressure high temperature

Kwabs--Absolute permeability to water

Ko--Oil permeability

Swirr--Irreducible water saturation.

Kw at Sor--Permeability of water at residual oil saturation

References

[1] Baraka-Lokmane, S., and Sorbie, K.S., 2006, "Scale Inhibitor Core Floods in Carbonate Cores: Chemical Interactions and Modeling," SPE 100515, SPE Eighth International Symposium on Oilfield Scale, Aberdeen, UK.

[2] Baraka-Lokmane, S., Sorbie, K.S., Poisson, N., and Lecocq. P.,2008, " Application of Environmentally Friendly Scale Inhibitors in Carbonate Core Flooding Experiments," International Symposium of the Society of Core Analysts held in Abu Dhabi, UAE.

[3] Haslam, E., 1989, "Plant Polyphenols," Vegetable Tannins Revisited, Cambridge Univ. Press, U.K.

[4] Pithayanukul, P., Ruenraroengsak, P., Bavovada, R., Pakmanee, N., and Suttisri, R., 2007 "In Vitro. Investigation of the Protective Effects of Tannic Acid Against the Activities of Naja kaouthia Venom," Pharmaceutical Biology, 45, 2, pp. 94-97.

[5] Cunha-Santino, M. B., Bianchini, J. R. I., and Serrano, L. E. F., 2002, " Aerobic and Anaerobic Degradation of Tannic Acid on Water Samples From Monjolinho Reservoir," Brazillian Journal of Biology, 62, 4a, Sao Carlos.

[6] Kenji Kowata, K.K., and Takahashi, K., 1996, "Interaction of Corrosion Inhibitors with Corroded Steel Surface," CORROSION 96, Paper No-96219.

[7] Cermack, J., article available at http://www.britannica.com/bcom/magazine/article/0,5744,63368,00.html.

[8] Lahodny-Sarc, NN. O., Kapor, F., 2002, "Corrosion Inhibition of Carbon Steel in the Near Neutral Media by Blends of Tannin and Calcium Gluconate," Materials and Corrosion Volume 53 Issue 4, pp. 264-268.

[9] Amjad, Z., 1995, "Mineral Scale Formation and inhibition, Plenum Press," New York.

[10] NACE Standard Test Method, 2005, "Dynamic Scale Inhibitor Evaluation Apparatus and Procedures in Oil and Gas Production," Item No.-24225.

[11] Yean, S., Al Saiari, H., Kan, A. T., and Tomson, M. B., 2008, "Ferrous Carbonate Nucleation and Inhibition," SPE 114124.

[12] Sengil, I. A., and Ozacar, M., 2003, "Effect of Tannins on Phosphate Removal using Alum," Turkish J. Eng. Env. Sci, 27, pp. 227-236.

Bisweswar Ghosh, Shiv S Kundu *, Balasubramanian Senthilmurugan and Mohammed Haroun

Petroleum Engineering Department, Petroleum Institute, Abu Dhabi, UAE

* Corresponding author E-mail: skundu@pi.ac.ae
Table 1: Basic reservoir parameters of an
offshore field in Arabian Sea.

Formation lithology Limestone

Vertical depth 1850-1970
Static Reservoir Pressure 156 kg/[cm.sup.2]
Static Reservoir Temperature 110[degrees]C
Oil API Gravity 39[degrees]
Porosity 23-26%
Average Permeability 5-100 mD

Table 2: Chemical composition of test brine.

Parameters Formation Sea Water FW/SW
 Water (FW) (SW) 30/70

pH 7.20 7.23 7.22
[Cl.sup.-] mg/l I7750.0 21300 20235.0
S[O.sub.4.sup.2-] mg/l 150.0 2496 1792.2
HC[O.sub.3.sup.-] 1400.0 214 570
mg/l
[Ca.sup.2+] mg/l 350.0 482 442.4
[Mg.sup.2+] mg/l 60.0 1372 978.4
[Sr.sup.2+] mg/l 51.0 -- 15.3

Parameters FW/SW FW/SW
 50/50 70/30

pH 7.21 7.21
[Cl.sup.-] mg/l 19525 18815
S[O.sub.4.sup.2-] mg/l 1323 854
HC[O.sub.3.sup.-] 807 1044
mg/l
[Ca.sup.2+] mg/l 416 390
[Mg.sup.2+] mg/l 716 454
[Sr.sup.2+] mg/l 25.5 35.7

Table 3: Scale inhibtion efficiency of tannic acid in
different Produced waters at different temperatures.

Sl.No Temp, Dosage Efficiency, %
 [degrees]C level,ppm

 PW(30/70) PW(50/50) PW(70/30)

1 70 Blank -- -- --
2 10 87 83 78
3 20 94 92 83
4 30 100 97 89
5 90 40 100 100 94
6 50 100 100 100
 Blank -- -- --
1 10 80 76 73
2 110 20 88 84 80
3 30 95 92 87
4 40 100 98 93
5 50 100 100 100
6 Blank -- -- --
 10 79 72 72
1 20 84 79 75
2 30 90 86 82
3 40 98 94 93
4 50 100 100 100
5
6
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Author:Ghosh, Bisweswar; Kundu, Shiv S.; Senthilmurugan, Balasubramanian; Haroun, Mohammed
Publication:International Journal of Petroleum Science and Technology
Date:Jan 1, 2009
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