Evaluation of the Physicochemical Properties of the Postsalt Crude Oil

Article pubs.acs.org/IECR

Evaluation of the Physicochemical Properties of the Postsalt Crude Oil for Low-Field NMR Vinícius G. Morgan, Lúcio L. Barbosa,* Valdemar Lacerda Jr., and Eustáquio Vinicius Ribeiro de Castro NMR Laboratory, Center of Competence in Petroleum Chemistry− NCQP, and Department of Chemistry, Federal University of Espı ́rito Santo−UFES, Av. Fernando Ferrari 514, CEP 29075-910 Goiabeiras, Vitória, ES Brazil S Supporting Information *

ABSTRACT: Low-field (LF) nuclear magnetic resonance (NMR) is a very versatile technique that has increasingly shown its value, especially in research involving crude oil. In the field of viscosity and American Petroleum Institute (API) gravity research, physicochemical properties are of great interest to the industry because the first parameter can be an obstacle for production, whereas the latter is used to assess and market the product. Thus, models of viscosity and API gravity were developed in the state of Espı ́rito Santo using the transverse relaxation time (T2) and relative hydrogen index (RHI) of postsalt crude oil. The models showed a good degree of reliability for 50 samples (R2 > 0.96) with viscosity ranging from 23.75 to 1801.09 mPa·s and API gravity from 16.8 to 30.6. A set of more than 15 “unknown” samples was used for validation, with values calculated by the API and viscosity compared to those obtained by the American Society for Testing and Materials (ASTM) 7042-04 standards. Finally, this study proposes a new way to classify oil through T2 and RHI with the possibility of simultaneously estimating the aforementioned physicochemical properties on the basis of a single quick and reliable measurement.

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INTRODUCTION According to the American Society for Testing and Materials (ASTM), oil may be defined as a naturally occurring mixture consisting predominantly of hydrocarbons and organic sulfur, nitrogen, and oxygen in the presence of metals such as nickel and vanadium in low concentrations that can be extracted from the earth in a liquid state.1,2 Water, gas, and solid materials can also be present during oil production, requiring several steps for the treatment before the petroleum is sent to refineries, where it is fractionated according to boiling temperature to meet product formulation standards of diverse industrial demands. Oil can be characterized on the basis of various methodologies and physicochemical parameters. One of the most important classifications is the chemical composition of saturates, aromatics, resins, and asphaltenes (SARA). However, the simplest and most commonly used form to classify oil into light, medium, heavy, and extra heavy is the API gravity that was developed by the American Petroleum Institute (API). The better the quality of oil, the greater its market value is, so the lightest oil has a higher API and market value. Equation 1 shows how API gravity is obtained, where ρ is the density of oil relative to water at 15.56 °C (60 °F).2 API =

141.5 − 131.5 ρ

Table 1. Classification of Oil by Different Organizations According to API Gravity agency Alberta government/ Canada3 U.S. Department of Energy4 OPEC5 ANP/Brazil6

medium

heavy

extra heavy

≥34

25−34

10−25

≤10

≥35.1 ≥32 ≥31.1

25−35.1 26−32 22.3−31.1

10−25 10.5−26 12−22.3

≤10 ≤10.5 ≤12

which the product is subjected, and organic matter.7 In Brazil, even with the advent of the presalt, the oils tend to be lighter and less viscous. Actually, the production distribution in this country is approximately 56% medium, 35% heavy, and 9% light.6 Viscosity and density are two of the most important physicochemical properties in fluid reservoirs. Viscosity is a parameter characterized as extremely important for production and refining, serving in the control and monitoring of transport systems and determining the energy required and cost of production, thus enabling or preventing exploitation. Density determines the behavior of the fluid during its flow in reservoirs and pipelines, providing an indication of the content fractions of light and heavy crude oils.2 Thus, the interest in rapid, accurate, and reliable methods to measure various properties of crude oils becomes evident. Accordingly, LF NMR is an interesting and advantageous alternative offering a quick (analysis time 1000

log η = a − bHI 49

Bryan et al.22

⎛ 2200 + 470TE2 ⎞⎛ T ⎞ ⎟ ⎟⎟⎜ η = ⎜⎜ ⎝ T2,LM − (TE + 0.5) ⎠⎝ 298 ⎠ η=

1000−1,000,000

1−3,000,000

1150 (RHI)4.55 T2,gm

fractions such as refractive index, boiling point, total acid number (TAN), viscosity, and API gravity. The correlations allowed the simultaneous determination of all properties mentioned as one T2 measure. According to the authors, LF NMR constitutes an alternative to ASTM 7042-041 methods, which were traditionally used but are now considered laborious and time-consuming. One of the main applications of LFNMR in the oilfield is the development of viscosity models. Bryan et al.22 investigated the dependence of viscosity on the temperature of heavy oil and bitumen from different reservoirs in Canada. On the basis of the geometric mean transverse relaxation time (T2,gm) and RHI, the authors developed a viscosity model represented by eq 2 to estimate viscosity in the range from 1 to 3,000,000 mPa·s for temperatures between 25 and 85 °C: 1198 η= (RHI)7.98 T2,gm (2)

preparation. In addition, the LF NMR technique offers the possibility of performing measurements in situ, rendering this a more attractive technique.8−10 NMR is a spectroscopy technique based on the interaction of radiofrequency waves with a nucleus that has a spin quantum number (I) different from zero into a magnetic field. The application of radiofrequency (RF) pulses excites the nucleus, and the time that the spins take to return to the thermal equilibrium state after a RF pulse is represented by two relaxation mechanisms, the longitudinal and transversal, which are represented by T1 and T2 relaxation times, respectively. These two mechanisms act through interactions with the environment that surround other nuclei. The relaxation times depend on the molecular mobility of the system; therefore, the nucleus acts as a probe of physical and chemical properties (e.g., viscosity).11−14 LF NMR instruments have magnetic fields lower than 1 T (precession frequency of 42.5 MHz to 1H nucleus), to allow them estimation of physical and chemical properties based on T1, T2, and the diffusion coefficient (D). Taking into account recent advances made by some manufacturers of commercial spectrometers of 1 T, today it is already possible to do Fourier transformation (FT) and to generate NMR spectra with a chemical shift. Among the parameters cited before, T2 was used in this work to obtain the correlation with dynamic viscosity, API gravity, and relative hydrogen index (RHI). LF NMR has been used to evaluate the ripening of bananas and the injuries suffered by the fruit during maturation,15 to analyze oilseeds by different pulse sequences16 with the potential of measuring the oil content in 20000 different seeds by the hour,17 and to analyze commercially produced18 butters, as well as to determine the velocity of fluid in flow.19 In use since the mid-1980s, the LF NMR technique is very well established in the oil sector due to its ability to determine physicochemical properties of the fluid and the rock formation of the reservoir.20 Several papers in the literature21−26 describe the use of the technique for estimating the viscosity based on the T2, characterizing the properties of the reservoirs (porosity, permeability, fluid saturation, the potential formation),27−32 studying emulsions and biphasic mixtures,33−38 determining the droplet size distribution of emulsions,38−40 and identifying and classifying crude oil from different reservoirs.41 Two-dimensional techniques that simultaneously measure T1 and T2 and the diffusion coefficient (D) are also used successfully for determining the molecular dynamics of petroleum reservoirs,42 the chemical composition of molecular groups,43 and the chain size distribution of oil.44 Fractions of oil have also been investigated by LF NMR. Recently, Barbosa et al.45 published the first paper establishing correlations between T2 and important properties of petroleum

RHI is an expression that associates the signal amplitude (AS) and the mass of the sample (mS) with the signal amplitude (AA) and the mass of water (mA). Through the RHI, it is possible to obtain quantitative information because this index is directly related to the amount of 1H nuclei in the sample. This ratio depends only on the sample and is readily calculated by eq 3 in the literature:20

RHI =

ASmA mSAA

(3)

Therefore, the mass and chemical compositions of oil are directly associated with the RHI in terms of hydrogen content. With regard to specifically the case of oil, there is a clear trend in the reduction of this index due to a reduction in the percentage of hydrogen in heavy oils, whereas the opposite is observed with lighter oils. According to eq 3, the RHI does not vary with temperature, unless there is a loss of hydrogenated compounds. Moreover, heavier oils present large amounts of asphaltenes and resins, which are complex chains of high molecular weight with branches, rings, functional groups, and high aromaticity.46 These compounds contribute effectively to the reduction of hydrogen content and increased viscosity, making clear the inverse relationship between the RHI and the fluid viscosity. In 2008, Burcaw et al.26 showed a correlation between the logarithmic mean of the transverse relaxation time (T2,LM) and the hydrogen index (HI), which is nothing more than the ratio of the signal and sample mass (HI = AS/mS). According to the authors, the viscosity model represented by eq 4 shows that the T2,LM depends strongly on viscosity, whereas the HI is insensitive to changes in small viscous crude oils. With empirical constants α and β, the exponential fit of the HI 8882

dx.doi.org/10.1021/ie500761v | Ind. Eng. Chem. Res. 2014, 53, 8881−8889

Industrial & Engineering Chemistry Research

Article

Samples. Samples To Develop API Gravity, RHI, and Viscosity Models. In this work, 50 crude oils from different reservoirs in the state of Espı ́rito Santo were investigated. The samples had the following characteristics: viscosity between 23.75 and 1801.09 mPa·s at 27.5 °C and API gravity changing from 16.8 to 30.6 (Supporting Information Table S1). From this characterization, correlations with T2 and RHI were established. The characterization of the crude oils was previously performed as indicated by ASTM 7042-041 to obtain the required correlation. Samples for Validation. Besides the 50 oils analyzed, 15 more crude oils were used to validate the models and develop the correlation. The results of the physical and chemical characterization (viscosity and API gravity) and NMR data (T2 and RHI) are listed in Table 3, where it is possible to observe the dynamic viscosity varying from 24.85 to 1597.35 mPa·s, T2 from 3.98 to 126.40 ms, and API gravity from 17.3 to 29.9.

makes the equation suitable for oils with maximum viscosity of 107 cP, which is very important because of the large amount of heavy oil reservoirs in the world. ln(T2,LMμ) = α − β HI

(4)

Several other viscosity models use T2 to obtain basic information about oils. Table 2 presents some models and the viscosity range in which they are applicable. In all expressions, T2,LM is the logarithmic mean and the T2,gm is the geometric mean of T2, η is the dynamic viscosity, RHI is the relative hydrogen index, HI is the hydrogen index, α and β are constant variables that depend on the echo time (TE; time required to refocalization of spins), and T is the temperature in Kelvin. It is possible to conclude from Table 2 that each viscosity model has an ideal range of viscosity and temperature. In addition, the efficiency of the models to predict viscosity basically depends on the chemical composition of petroleum. Thus, it is important to emphasize that there is hardly a universal viscosity model, because reservoirs in different regions produce crude oil with distinct physical and chemical characteristics. For example, the paramagnetic ion content of each crude oil strongly influences the transverse relaxation time and, consequently, can lead to discrepancies in the viscosity models developed. Given the scenario described above, it is clear that there is a relentless pursuit by researchers to develop models that cover wide ranges of viscosity that can be accurately applied to crude oils produced in different regions. Differences in viscosities calculated by models developed at the same temperature indicate a strong variation of petroleum properties in the reservoir formation conditions, generating fluids with very different dynamic behaviors. Information about the viscosity and other physical and chemical properties of oil is extremely important for the industry. Therefore, a method that is capable of measuring various properties quickly, economically, and simultaneously with only a single experiment becomes advantageous. With that aim, this paper presents some correlations between the data obtained by LF NMR with physical and chemical properties as dynamic viscosity, API gravity, and RHI of postsalt oil in the state of Espı ́rito Santo. Besides, it was developed in this work a viscosity model that takes into account the simultaneous relationship between viscosity with relative hydrogen index and T2, increasing the amount of information obtained. This work has as additional contributions the possibility of classification of oil based on T2 and RHI with the possibility of simultaneously estimating the physicochemical properties mentioned on the basis of a single quick and reliable measurement. The results show correlation coefficients (R2 ≥ 0.97) larger than those present in the literature for correlation involving dynamic viscosity, suggesting that this work may be applied in the Brazilian petroleum postsalt oils.

Table 3. Physical, Chemical, and NMR Parameters of 15 Crude Oils (in Order of Dynamic Viscosity) Used To Validate the Models crude oil

μ (mPa·s)

API gravity

T2 (ms)

RHI

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

24.85 27.62 27.95 29.02 30.81 31.09 45.36 94.38 110.77 112.23 121.08 134.70 1,177.87 1,316.68 1,597.35

29.9 29.4 29.6 29.4 29.3 29.2 28.4 22.7 22.2 22.1 22.0 22.0 17.7 17.3 17.3

126.40 125.18 119.83 111.42 109.01 111.42 100.65 40.52 35.71 34.94 34.19 30.84 4.82 4.33 3.98

1.162 1.169 1.157 1.155 1.155 1.154 1.158 1.065 1.069 1.059 1.062 1.078 0.945 0.941 0.938

To validate the viscosity models, the T2 values obtained by the Carr−Purcell−Meiboom−Gill (CPMG) sequence and the RHI calculated via free induction decay (FID) were correlated with dynamic viscosity determined by ASTM D 7042-04.1 In the case of the RHI model, such parameters were estimated using the T2 of crude oils. Then, the values were compared with the values calculated by eq 3. Finally, the API gravity was validated by calculating the API gravity by the RHI models for the oils in Table 3 and comparing with the values for the characterizations obtained using the standard ASTM D 7042-041 method. NMR Measurement. Approximately 25 g of distilled water was introduced into a tube of glass. When thermal stabilization of 27.5 °C was achieved after 10 min, the instrumental parameters were adjusted and the equipment was calibrated. In the next step, the crude oil was placed in the NMR spectrometer to determine the T2 and the signal amplitude by the CPMG sequence and FID, respectively. In the FID sequence (Figure 1) only one pulse of 90° was applied to transfer the resultant magnetization (M0) of the sample to the transverse plane (xy), where the acquisition signal was made. After some time, the magnetization disappears in the transverse plane and is recovered along the z axis.

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EXPERIMENTAL PROCEDURES The NMR experiments were performed on Maran 2 Ultra spectrometers from Oxford Instruments, operating at a frequency of 2.2 MHz for the 1H nucleus. The procedure and the main characteristics of the sample are presented in what follows. 8883



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transverse relaxation time (T2*). However, the utilization of some tau value (time between pulses) eliminates this effect and, thus, allows the determination of the real transverse relaxation time of the sample. CPMG data were exported and processed using the WinDXP software to produce the T2 distribution curves. The Origin8 program was also used to build the curves. The T2 peak average of the triplicate was obtained to develop the correlation and models. The main parameters of the NMR measurement via the CPMG and FID of the distilled water and crude oils can be seen in Table S2 in the Supporting Information. Among them, P90 is the duration time of the 90° pulse, P180 is the duration time of the 180° pulse, NS represents the number of scans, RD is the relaxation delay, that is, the waiting time before each scan, and NECH is the number of echoes acquired for each scan. The water relaxes more slowly than the oil, thus requiring higher RD and NECH. The maximum experiment times of the crude oils were about 16.1 s and 0.38 min by FID and CPMG, respectively. The FID sequence applied only 90° pulses, whereas the CPMG experiments applied 90° and 180° pulses. The duration of these two pulses was 8.2 and 16.4 μs, respectively. Calibrating the pulses with water is essential to ensure experiment reliability. The RD must be higher than or equal to 5T1 for the magnetization to completely recover before each acquisition. If not, the signal would be lower than the real value due to signal saturation, leading to an incorrect result. The NS improves the signal/noise ratio, and NECH provides the quantity of echoes acquired by the experiment. The value of the last parameter must be sufficiently greater to give a complete decay on the transverse plane. The SI has two different significations: on the FID, it is the number of points to give a complete decay curve, but on the CPMG, it is the quantity of points per echo. For example, using SI = 1 on the CPMG, only one point in the peak of the echo is taken to build the decay curve. In the case when a greater number of echoes is selected, the program will use their average. The NECH and TAU were not used on the FID experiment because no echo was formed in it. In contrast, the wait time between the pulses (TAU) on

Figure 1. Free induction decay (FID) of amplitude and time. The vector diagram illustrates that the application of the pulse produced a maximum magnetization in the xy plane, which reduced with time.50

The FID experiments for the water and crude oil were performed in triplicate with approximately 25 g in each sample. The average signal amplitude of the 20 initial points of distilled water (AA) and the sample (AS) were obtained and used afterward to calculate the RHI by applying eq 3. The T2 measurement of the crude oil was made on the basis of the CPMG pulse sequence in Figure 2, where initially one

Figure 2. Block diagram for the CPMG pulse sequence.50

pulse of 90° was applied by transferring the magnetization (M0) to the xy plane. Then, a train pulse of 180° was applied. The refocalization of spins [echo time (TE)] appears for each 2τ. Because of the diffusional process and the fact that magnetic fields are inhomogeneous, the disappearance of transverse magnetization is accelerated. Indeed, under such circumstances, the measure provides a relaxation time termed as effective

Table 4. T2 and RHI Values for the 50 Crude Oils Used To Obtain the Correlations crude oil

T2 (ms)

RHI

crude oil

T2 (ms)

RHI

crude oil

T2 (ms)

RHI

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

82.71 186.64 115.48 135.93 127.95 122.13 164.65 96.55 123.62 103.89 103.81 86.74 95.77 77.06 97.36 68.47 130.73

1.151 1.165 1.159 1.158 1.174 1.158 1.158 1.156 1.159 1.159 1.161 1.152 1.142 1.158 1.149 1.145 1.152

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

89.57 79.33 90.16 89.57 84.85 99.04 77.06 91.03 73.54 37.45 53.25 41.60 30.96 31.03 17.33 17.99 14.56

1.166 1.159 1.154 1.161 1.149 1.159 1.157 1.147 1.145 1.055 1.077 1.069 1.055 1.050 1.025 1.020 0.997

35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

13.52 16.75 13.58 11.42 6.48 6.89 32.97 13.52 7.02 6.11 4.60 5.09 5.04 4.07 4.07 3.78

1.033 1.023 0.983 1.023 0.945 0.946 1.073 0.991 0.911 0.918 0.953 0.918 0.940 0.935 0.937 0.912

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Figure 3. (a) Distribution curves of five samples in order of increasing oil viscosity [32.36 (1); 56.30 (2); 112.38 (3); 259.07 (4), and 1416.32 mPa·s (5)] and (b) distribution curves of distilled water compared to the less viscous oil (curve 1 in panel a).

the CPMG experiments was 200 μs, too quick to avoid the diffusion effect in this experiment. The T2 and RHI values for the 50 crude oils can be seen in Table 4, where it is clear that oil 2 presents a higher T2 (186.64 ms) and RHI (1.165). On the other hand, crude oil 50 has a lower T2 and RHI, 3.78 ms and 0.912, respectively. Additionally, the T2 value is strongly affected by viscosity; for example, low-viscosity oils are constituted predominantly by saturated compounds, so T2 and RHI tend to be elevated as compared to the oil with high viscosity.

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RESULTS AND DISCUSSION The T2 distribution curves of the oils provide important qualitative20 and quantitative37 information, such as the presence or absence of water in crude oil, the masses of each phase, and the viscosity. The decrease of T2 in the oils is strong evidence of the presence of heavy fractions such as resins and asphaltenes, which have long chains with high molecular weight, elevated viscosity, and low API gravity.41−45 Figure 3a illustrates five typical distributions of oil curves, where there is a reduction in T2 from 186.64 ms (curve 1) to 5.09 ms (curve 5) with an increase in viscosity from 32.36 to 1416.32 mPa·s. This difference is even more evident in Figure 3b, where the T2 of the less viscous oil in Figure 3a (curve 1) is compared with that of distilled water. The appearance of a single peak for each oil relaxation in Figure 3a is attributed to the spin population of the hydrogen nucleus of the oil compounds. The hydrogen nuclei in distilled water (Figure3b) precess at different frequencies in the oil because of the different local chemical environments, causing the spins to relax more slowly (i.e., with higher relaxation time of 2.70 s). Besides the influence of the viscosity on the distribution of the T2 curves, it is also observed that the API gravity exerts a marked effect on the decay curves for the CPMG. It is possible to observe in Figure 4 that the curve with the lowest API gravity (17.0) has the fastest decay, that is, the lowest T2. In contrast, the oil with the highest gravity (API 29.6) has the highest T2 due to the higher mobility of the oil molecules. Thus, this result indicates the possibility of correlating the T2 with the API gravity, suggesting a new way to classify oil as heavy and medium by the T2 value. As previously discussed, the transverse relaxation time depends on the mobility of the medium, which is associated with the amount of hydrogen, that is, the RHI. Thus, Figure 5 presents the correlation between the RHI and ln T2. It is possible to observe a variation of the RHI from 0.94 to 1.16. An

Figure 4. Influence of API gravity on the CPMG decay curves for the five samples in Figure 3a. The lower API gravity exhibits a faster decay (curve 5), and the higher API has a lower decay (curve 1).

Figure 5. Correlation between RHI and natural T2 logarithm for the 50 crude oils in Table 4.

increase in the RHI means an elevation of the hydrogen content in the oil from 10.44 to 12.88%. Therefore, oils with a higher API have higher molecular mobility, leading to high T2 and RHI values. It would be advantageous to obtain a viscosity model based on the RHI because this parameter also provides the amount of hydrogen present in the sample and does not vary with temperature, thus decreasing the chances of the model showing 8885



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significant error. Thus, a linear correlation between the RHI and the transverse relaxation time was obtained by applying the natural logarithm of T2. On the basis of the results of Figure 5, eq 5 is proposed. ln T2 = 13.067RHI − 10.481

(5)

Equation 5 indicates a strong dependency between relaxation time and the RHI. The results provide a good correlation (R2 = 0.96) to determine the RHI (in other words, the percentage of hydrogen of oil) quickly and without destroying the sample. The advantage of applying eq 5 will become evident in subsequent correlations involving the RHI. Besides enabling the determination of the hydrogen content, the RHI can be correlated with the viscosity of oils. As previously mentioned, this index is a function of the sample only, and the probability of errors in the correlation due to temperature gradients is reduced. Equation 6 shows the viscosity model as a function of RHI and T2. This mathematical expression obtained by the results of Figure 6 is valid for the viscosity range between 23.75 and 1801.09 mPa·s and RHI values of 0.94 and 1.18 at 27.5 °C. RHI = 1.368 × 10−4μ + 0.011 T2

Figure 7. Correlation between the transverse relaxation time and API gravity at 27.5 °C.

that this correlation is applicable to 27.5 °C, because the variation of the temperature changes the T2 and viscosity (T2 = T/η).11 The results in Figure 7 allow the proposition of eq 7. T2 = 0.153 e(0.2283API) + 2.200

(6)

(7)

To estimate the API gravity with more reliability, this parameter was correlated with the RHI, as can be seen in Figure 8, where there is a new border region between medium and

Figure 6. RHI/T2 versus dynamic viscosity for the 50 oils at 27.5 °C.

In Figure 6 it is clear that most of the points of the curve lie in a narrow range of low viscosity used for larger amounts of medium crude oil, a category prevalent in Brazil. However, these results show that even for heavy oils, eq 6 is applicable. It is possible to check in Figure 6 that there is a point of intersection between the vertical and horizontal lines (119.30 mPa·s and 0.03 ms)−1, determining a border region between medium and heavy oils. In the worked bands, the transverse relaxation time is the dominant factor. In the range for medium oils with higher API gravity and hydrogen content there is a high T2 value leading to low RHI/T2 values. On the other hand, very viscous oils have lower relaxation times, leading to higher RHI/T2 values. Besides the viscosity, the exponential behavior of the API gravity obtained by ASTM 7042-041 and T2 shows the possibility of determining the API gravity of oils, as shown in Figure 7. The area near the intersection of the dotted lines corresponds to the limit of heavy and medium oils. Note that oils with times lower than 45 ms (horizontal dotted line) and 22.3 API gravity (vertical dotted line) are heavy, and oils with values above these are medium. It is important to emphasize

Figure 8. Relationship between RHI and the API gravity of 50 crude oils. The intersection of the lines at RHI = 1.05 and API = 22.3 presents the border region between medium and heavy oils. All oils with a RHI > 1.05 have a hydrogen percentage greater than that of deionized water and are classified as medium.

heavy oils, highlighted by points near the intersection. Clearly it is possible to observe that the oils with a RHI < 1.05 are classified as heavy because they have an API < 22.3. On the other hand, oils with hydrogen content >11.66% (RHI > 1.05) are classified as medium. The correlation presented in Figure 8 is valid for medium and heavy oils according to the ANP classification shown in Table 1, ANP being the predominant organization in the country. On the basis of the exponential relationship shown in Figure 8, the following mathematical expression is proposed. RHI = − 0.2866 e(16.9721 − API/6.84854) + 1.20911

(8)

By way of expression 8, it is possible to predict that heavy oils present a high content of resins and asphaltenes (composed of complex chains of high molecular weight branches)46 and, 8886



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Figure 9. Validation of the proposed models to RHI (a), dynamic viscosity (b), and API gravity (c) for the 15 samples in Table 3. The comparison is made on the basis of the values calculated by eqs 5, 6, and 7 and determined by standard ASTM 7042-041 for viscosity and API. In the case of the RHI, the comparison was made between the values calculated by eq 5 and the definition shown in eq 3.

aforementioned properties can be estimated with a high degree of reliability. It can be seen that the correlation coefficients are generally larger than those shown in other models in the literature, suggesting that this work may be applied to the Brazilian petroleum postsalt oils. The proposed correlations in Figure 9 are better performed (that is, show higher R2) in relation to physical and chemical properties in Figures 5, 6, and 8 due to the fact that the number of samples used in the validation is lower than those used to develop the correlations. Even so, the proposed models show excellent results and reliable adjustments, becausse R2 values in these figures are very close.

therefore, have low levels of hydrogen and API gravity. This equation also shows that the classification of oils actually depends on the amount of hydrogen in the sample. The resins and asphaltenes have not only a low hydrogen content (reducing RHI) but also other compounds, for example, aromatics. The last compounds presented are the cyclic rings and, consequently, have a low amount of hydrogen. In contrast, saturated compounds present high hydrogen content, contributing to the elevation of the RHI value.2 From eqs 5, 6, and 8, it is clear that with a single measurement of LF NMR it is possible to obtain various physicochemical properties of oil simultaneously using the RHI. This is an advantage over eq 3, where the RHI is obtained by FID and is limited to the intensity of the signal as the main source of information. Hence, the proposition of eq 5 is justified. To test the validity of the models proposed in this paper, the RHI and T2 of 15 crude oils were measured. On the basis of eq 5, the RHI was calculated and compared with the RHI obtained by eq 3, as can be seen in Figure 9a. Similarly, the dynamic viscosity and API gravity were calculated by eqs 6 and 7, respectively, using the RHI calculated in eq 5. Subsequently, the results were compared with the values obtained by the procedures of ASTM 7042-04,1 as shown in panels b and c, respectively, of Figure 9. On the basis of Figure 9, it is possible to verify in a clear way that the correlations are linear and show excellent agreement between the results obtained by the proposed models and ASTM 7042-04,1 thus validating eqs 5, 6, and 8. Therefore, the

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CONCLUSION The technique of LF NMR showed great versatility and reliability to simultaneously determine the physicochemical properties of interest to the oil industry, such as viscosity, API gravity, and RHI. The correlations showed good results, confirmed by high correlation coefficients (R2 > 0.96). The models were very applicable for the crude oils in the state of Espı ́rito Santo, allowing the determination of the properties mentioned in ranges from 23.75 to 1801.09 mPa·s and API gravity between 16.8 and 30.6. The models proposed here can also be applied to crude oil from other regions with similar properties and composition. Limitations can be found because of differences in the dynamic behavior of the fluid or in the case of measurements performed under different conditions. It can be concluded that the application of the developed models allowed the classification of oils and the determination of their 8887



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quality. The medium oils had T2 >45 ms and hydrogen content >11.66% (RHI > 1.05). LF NMR led to rapid measurements, dispensing with more expensive and laborious instruments. This study provides relevant information for engineers regarding the optimization of the production efficiency of crude oils with high or low viscosity. The viscosity model developed here can help to indicate the specific pressure required to pump the oil from the reservoir to the surface, improving the refining of crude oil.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(L.L.B.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Brazilian agencies PETROBRAS S/A, FINEP, CNPq, CAPES, FAPES, and LabPetro and the Federal University of Espı ́rito Santo for financial and technical support.

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REFERENCES

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Evaluation of the Physicochemical Properties of the Postsalt Crude Oil

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