Experience in the Application of Hydrocarbon Optical Studies in Oil Field Development

Abstract

This article reviews the results of measurement of optical properties of oil, such as polarimetry, refractometric, luminescent-bituminological research, IR-spectrometry and UV-visible-NIR spectrometry used to solve geo-bituminology development of hydrocarbon deposits. The authors pay special attention to optical research in the field of UV-visible-NIR electromagnetic radiation, the results of which allow us to estimate the residual oil reserves, separate production for each formation during the operation of multi-layer objects, determine the producing gas-oil ratio, density and content of hydrocarbons, efficiency of hydraulic fracturing, flow-reducing technologies, and injection of solvents of heavy oil sediments, etc. The published approaches to methods of optical research, which are carried out by laboratories or in-well devices, have been analyzed. This article analyzes the main advantages and disadvantages of current technologies for determining the optical properties of oil. The authors propose wellhead devices for determining the optical properties of oil in UV-visible-NIR radiation (190–1100 nm) and their functional schemes, with a description of the operating principle.

1. Introduction

Deterioration of the mineral base of the mineral resources sector in case of a complex economic situation requires implementation of modern resource-saving and energy-efficient technologies in the oil and gas industry [1,2]. Directions of oil production development are connected with development of the near edge zone of the Jurassic–Cretaceous sediment basin and increase of efficiency of Jurassic sediments development in Western Siberia [3], the development of shale complexes, such as the Domanic deposits of the Volga-Ural oil and gas bearing province [4]. The rational and efficient development of oil and gas fields is accompanied by the construction of close-to-real conditions in the digital model of reservoirs [5,6]. In turn, geological and hydrodynamic models are based on theoretical knowledge of the conditions under which hydrocarbons accumulate and migrate in oil and gas traps [7] and on the filtration of stratal fluid in the pore space [8]. When studying filtration theory, it is necessary to understand the influence of the geological features of the hydrocarbon deposit and the characteristics of the fluid itself on the movement of water–oil or gas–liquid mixtures [9], including structural and mechanical properties [10].

Hydrocarbon field development monitoring and control allows for obtaining and interpreting information about processes occurring during the operation of oil and gas fields [5]. Thus, monitoring the formation pressure changes during the development process allows quantification of the formation of asphaltene deposits in the reservoir [11] and in the bottom-hole formation zone [12], which leads to a decrease in the flow capacity of the formation and its filtration characteristics. There are ways to monitor the formation of organic sediments during oil field exploitation using a radioisotope meter [13] that allows you to quickly assess the thickness of the layer of the formed sediments. Monitoring the operation and development of heavy and supervising oil deposits is carried out with consideration of the study of the features of the oil movement process, which has structural and mechanical properties [14,15]. Both at the planning stage and at the geological and technical measures stage, they monitor the operations carried out. Therefore, hydraulic fracturing process planning can be done using the optical control method [16] and the effectiveness of the GTA (geological and technical actions) can be assessed by [17]. Operational control of drilling processes [18] and well operation [19] with the use of machine learning elements reduces both technological and economic risks. In the final analysis, data from the monitoring of the development of the oil field at various stages of its development are used to refine geological and hydrodynamic models of oil deposits [20] and with the possibility of identifying areas of unprocessed oil reserves [21], prediction of reservoir productivity with regard to its anisotropy [22]. Geophysical, hydrodynamic and geochemical studies are the main methods of hydrocarbon field development control [23].

The geochemical method for monitoring the development of oil fields relates to the photocolorimetry method (optical method), which, for example, determines the water and oil content of the extracted products and wastewater [20,24]. It is also possible to determine the hydrodynamic bond, saturation and depth of the fluid contacts by estimating the optical density of hydrocarbons in the formation conditions with the aid of the MDT system [25]. It is based on studying the optical properties of oil that are most sensitive to changes in its chemical composition during the operation of accumulation [26]. Asphaltenes and resins are the most optically sensitive components in oil; changes in optical density occur due to changes in the content of said components in oil [27]. To optimize the production and maximum recovery of oil, intelligent systems that record changes in data by well temperature, pressure, flow rate and composition [28].

The optical method allows high reliability of data received controlling processes, such as:

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In addressing geological and production tasks: refine the geological structure and geological and physical parameters of production facility, evaluate residual oil [29], and evaluate physical and chemical properties of produced products [30].

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In the case of oil field development control: determination of direction of filtration flow lines in the deposit [31], efficiency of geological and technical measures (well interventions) [32,33], and accounting for produced products during development of multi-zone reservoirs [34].

Laboratory devices for measuring geological and physical reservoir characteristics, including the optical properties of oil, are currently being developed and are widely used in field practice [35,36]. This entails sampling at the wellhead, transporting them to the laboratory and long-term measurement procedures using these devices and interpreting the laboratory data obtained. However, during sampling, transportation and processing of samples, significant changes occur in their properties related to the adsorption of surface-active substances and volatilization of light fractions, resulting in distortion of laboratory measurements and interpretation. Monitoring the oil and gas field development process will become more reliable if measurements and analysis of the optical properties of produced fluids are performed automatically at the wellhead (in situ). In this case, the device can become a reliable tool of the field geologist or process technology for the operational control of hydrocarbon field development.

To increase the efficiency of oil and gas development in the context of rapidly changing geological, physical and technological parameters, it is important to obtain timely and reliable information on the properties of hydrocarbons. The authors’ review of optical research indicates that this method is widely used to solve geological and field problems. The proposed mobile device for the automated measurement of optical properties at the well head will simplify and reduce the time for the entire cycle of optical research, starting with sampling and ending with data analysis.

2. Theoretical Foundations of Optical Oil Research

Optical investigation includes photocolorimetric, polarimetric, refractometric, infrared bands (in the visible-NIR spectrum), long-wave zones of the spectrum (UV-spectrometry), and other types of research.

2.1. Polarimetric Exploring

In the case of polarimetric studies, optical rotation α is measured, which characterizes the ability of oil to rotate the plane of polarization of the passing light. The mode of polarization plane rotation, specific rotation vs external factors, and relationship of the polarimetric properties with molecular composition of the investigated medium are defined [37]. As a result of a large number of experiments since the beginning of the twentieth century, it has been established that almost all oil, with very few exceptions, has the property α. Note that for oil products, this feature was discovered in 1835 by Biot [38]. Oil and most of its fractions rotate the polarization plane to the right. The absolute polarization angle is different for oil in different fields and productive strata but varies within a very wide range. In 1925, L.G. Gurevich explained petroleum hydrocarbon α by highly polycyclic naphthenes’ presence of cholesterol breakdown products. It should be noted that light overheads (gasoline, kerosene) do not possess α. However, the rotary capacity of the distillate fractions is maximized in the field of lubricating oils and is more dependent on the geological age of the oil, despite about the same molecular masses and boiling temperatures as the fractions (Fenske et al., 1942) [39]. The research of G.A. Amosov showed that the most optical were the highest-boiling oil fractions containing condensed polyaromatic HC (1955) [40]. Rosenfield (1967) [41] discovered that the optical activity of oil gradually decreases as it is produced. Thus, when comparing oil extracted from older rocks by geological age, the tendency to decrease their rotational capacity was established. Later in research, V.V. Devlikamov, I.L. Markhasin, and G.A. Babalian (1970) [42] showed that with the increase in the geological age of oil, there is a tendency to decrease α. However, there were no changes in α for oil in the Bobriko reservoirs in the Mancharov zone, even with high absolute values of the right equal to the right of equal values 0.28. Later, the experimental studies of G. Philippi showed that the α of oil requires a more thorough study (1977) since the same sample might simultaneously include left-hand circular (LHC) and right-hand circular (RHC) or not rotating inert components [43].

2.2. Refractometric Analysis

Refractometry is based on measuring the conformability properties of oil, determined by its group composition and density. Changes in the chemical composition of oil in bed strike and thickness change the index of oil refraction (n_D). The first works on refractometry in the oil industry were associated with oil processing technologies. As early as the 1990s, Lengfeld [44], O’Neill [45], Rogers [46], Doolittle and Peterson [47], Chambers and Ubbelohde [48], and others also succeeded in establishing connections between the density, refractive index and boiling point of individual oil fractions. Currently, a number of oil refineries are using flow refractometers for process control, product quality and composition. Researchers under the leadership of I.L. Markhasina in the early 1960s started developing filtration annexes directly to problems of oil field development. Studies performed at the Mancharov field showed that there is a correlation between the n_D, the difference between the plane of the initial water–oil contact and the point where oil is taken. Increased n_D oil with approaching water–oil contact reflects an increase of oil content in the oil of aromatic compounds (resin and asphaltenes) [42].

2.3. Infrared Spectrometry (IR)

The development of near infrared (NIR) spectroscopy of individual HC and a number of petroleum products only happened about 50 years ago, when the first IR spectrometers appeared. However, NIR spectroscopy has proved to be more precise than photometric and refractometric surveys. Formerly by Epstein and others (1956) in the work [49] it was suggested that of infrared and mass spectrometry was suggested as a method of identifying new organic compounds in oil. Later, Seifert and others (1969) [50] a new and comprehensive approach to identify carboxylic acids in crude oil using ultraviolet, infrared and mass spectrometry. In the work [51] Mckay and others (1978) used IR spectrometry to determine the molecular mass of asphaltenes, which are a complex mixture of the most polar and high molecular compounds of oil. For example, a multivariate express method for evaluation of oil content in the gas condensate of Urengoyskoye and Yen-Yakhiskoye fields was developed. Calibration models were analyzed for the determination of oil content in the gas condensate, which was obtained with refractometric and photometric methods. Measurements were made on the visible range of the spectrum at the wavelength of 600 nm, which allowed for reducing the deviation of the method to 3%, increasing the sensitivity of the method with the exception of additional sampling and conducting measurements for the greater length of the optical path. The existing photometric method of measurement was performed in the ultraviolet part of the spectrum at 365 and 390 nm. Efficiency of mathematical treatment of info saturated with multivariate data, accuracy and velocity of NIR-spectroscopy are higher than for one-dimensional refractometric and photometric methods [52].

Many scientists have dedicated their work to the study of natural oil by IR spectroscopy [53]. According to studies in the near IR range, it is possible to determine indicators of oil quality using mathematical methods of analysis of multidimensional data, including the presence of hydrocarbons in water [54]. In the average IR range, the composition and properties of individual hydrocarbons are determined [55]. However, the multicomponent composition of the oil composition, intra- and intermolecular structure of HC are dictated by the curves of overlapping and ingress of the absorption lane with distortion of their shape and intensity. For example, similar maximums for inhibitor absorption lanes have compounds such as acids, ketones, aldehydes, ethers, anhydrides and lactones. For this reason, the authors [56] consider that direct qualitative interpretation is not always possible and even more so—quantitative calculations linking the intensity of absorption in the IR zone with high-molecular weight compounds in oil. Compared to studies in the UV-visible range, oil IR-spectroscopy is still characterized by high labor intensity, duration and uncertainty of analysis results. It should also be kept in mind the relatively lower sensitivity of specific IR lanes to the individual features of oil. The studies of Crombie [57], Andrews [58], Aske [59], etc. (1998–2006) showed strong distortion of oil peak due to water and methane presence in the tested oil samples.

2.4. Luminescence-Bituminological Analysis (LBA)

LBA is used for the analysis of cores and cuttings during drilling of the wellbore [60]. This analysis is based on the ability of organic compounds to luminescence when using UV with a wavelength of 366 nm and is designed for determination of the composition and quantity of bitumoids content in the rock. The founders of the LBA (Luminescent-bituminological analysis) method in oil geology are N.A. Schlesinger [61] and B.H. Florovskaya and B.G. Melkov [62].

LBA allows us to determine the genetic type and formation of bitumen contained in hydrocarbon deposits and determine their migration for the purpose of forecasting oil and gas reserves, including in Western Siberia [63] and the Volga–Ural region [64]. Fluorescent analysis is used to determine individual UV absorption groups in the study of the fractional composition of oil [65].

2.5. Overview of Optical Oil Surveys in the Area of the UV-Visible-NIR Spectrum

Optical oil surveys took place from the 1960–1970s. Scientists, such as S.S. Kurtz, C.E. Headington, O.C. Mullins, K.O. Erikson, B. Dai, C.M. Jones, I.F. Glumov, A.F. Gilmanshin, V.V. Devlikamov, G.A. Babalyan, I.L. Markhasin, R.R. Ibatullin, M.K. Rogachev, I.A. Guskova, I.N. Evdokimov, and R.N. Burkhanov, studied the optical properties of oil-dispersion systems in the field of UV-visible-NIR spectrum in different periods of time.

Optical studies of dispersion systems in the area of the UV-visible-NIR spectrum are based on Beer–Lambert–Bouguer law (1729):

$$Cla=\frac{D}{0.4343\cdot C\cdot l},$$

(1)

where Cla is the coefficient of light absorbance, cm⁻¹;

• D—optical density of the surveyed solution, non-dimensional value;
• C—solution concentration, unit fraction;
• l—mud layer thickness, cm.

The first studies of the coefficient of light absorbance of oil and its solutions in organic solvents in the optical range of electromagnetic radiation aimed at studying the composition of oil and its transformation under external factors were observed in the 1950s. Methods of estimation of the oil saturation factor by the color of the benzene extract [66] and separation of oil from various horizons (e.g., oil of the Tyumazin field, produced from Ashiysk D1 and Mullinsk D2 horizons, intensity of oil solutions coloring in benzene) (Orlov, 1955) are developed. A method has been offered for the evaluation of asphaltene content in surface oil samples (Lyutin, 1958) [67]. According to studies by Abezgauz and Terzi [68], no new asphaltene structures were formed during the liberation of oil from the Romashkin field. I.L. Markhasin, I.M. Abezgauz and F.D. Blazjevic on higher-pressure equipment confirmed the results obtained by the authors in the study [69].

The optical properties of oil disperse systems are highly dependent on the thermobaric conditions of the study, and with reduced temperature and pressure, organic deposits such as asphaltenes, resins and paraffins can be formed [70,71]. There is a need to study the optical properties of oil, including and in the UV-visible-NIR parts of the spectrum, at high temperatures and pressures corresponding to the pressure-temperature conditions of wells and formations. To address this issue in the work [72] describe the measurement cells capable of conducting studies up to 30 MPa and 600 K.

In the 1960s, photocolorimetric oil tests of Romashkin and Bavlin oil fields were used to control the development of oil and gas fields by I.F. Glumov and A.F. Gilmanshin These scientists discovered, and it was confirmed later by other authors, that there is a correlation between distance to OWC and coefficient of light absorbance of oil, in the form of regression equation [73,74]. In addition, it was determined that a change in the coefficient of light absorbance of oil might occur due to fluid movement in the formation and change of the inflow profile in the well [75]. Based on the results of the performed studies, instructions on application of photocolorimetry of produced oil ratio for solving geological and field tasks and a method for prediction of coefficient of light absorbance of oil were developed [76,77].

A.S. Panteleev performed optical oil surveys of Orenburg region fields in order to separate oil production in the case of commingled development of Turney and Bobrikov pools, as well as to evaluate the efficiency of perforation works. Based on the results of these tests, a relationship between oil density and the coefficient of light absorbance of the oil was identified [78,79]. V.N. Bykov and T.G. Altyntseva performed photocolorimetric studies of oil in the Yarino-Kamennolozhskoye field to determine oil inflows from certain assets as per studies on changes in the coefficient of light absorbance during development [80].

In the 1970s, photocolorimetric oil surveys of the oil fields of the Bashkortostan Republic were performed by V.V. Devlikamov, I.L. Markhasin and G.A. Babalyan, and the results were summarized in a monograph [42]. The work provides a detailed description of experimental study methodology, results of adsorption of active oil components with water, and information about areal regularity of coefficient of light absorbance changes. The scientists proposed a photocolorimetric method for evaluating the redistribution of high-resin oil streams in the formation [81,82] when solving the problems of selecting the parameters of the oil exploration system with abnormal structural and mechanical properties [83,84], the interference of production and injection wells in the pilot area of the Arlan field, and developed a method for calculating inflow from individual formations into the wells that penetrated them with a single filter [85].

In the 1990s, Mullins and Groenzin [86] determined the diameter of asphaltene molecules and their molecular mass using UV-visible-NIR and fluorescent spectrophotometers for fluorescence depolarization. In the 2000s, R.R. Ibatullin studied regularity of change of coefficient of light absorbance and complex of spectral curves of coefficient of light absorbance of oil during its displacement with water, developed a method for identification and evaluation of producing intervals in two-reservoir production assets [87,88]. In the 2000s, scientists from Schlumberger Mullins and others studied the optical density of oil, gas and condensate in the field of NIR radiation to solve various production problems. Therefore, in work [89] it is proposed to use optical studies of hydrocarbons in the field of NIR radiation to estimate the gas content of oil. In other work [90] the linear dependence of optical density on the density of the hydrocarbon mixture is determined, and the spectrum of the hydrocarbon mixture in the NIR part is equal to the sum of the spectra of its individual components at different temperatures and pressures. Determination of the optical density in the NIR radiation domain allows estimation of the compressibility of oil, as well as the asphaltene sedimentation rate at the pressure drop [91]. Initially, small, weighted, unstable non-volatile flakes are formed due to resin deficiency. The following flakes contain more resin at lower pressures and are more stable and sticky than oil field equipment. The NIR analysis of gas and condensate spectra allows us to determine the quantitative content of methane, ethane, CO₂, C3−C5 hydrocarbon group and C6+ [92]. Schlumberger scientists have concluded that oil samples for laboratory optical research are susceptible to contamination and distortion of final results due to container depressurization, leakage during transfer to other tanks, pollution by other fluids, and a long process (up to 1 year) of the entire research cycle. To solve the listed possible problems of optical oil analysis, it is proposed to introduce a downhole fluid analyzer (DFA) in the well, developed by specialists of Schlumberger Mullins et al. [93]. In the monograph of I.N. Evdokimova and A.P. Loseva theoretical issues of application of photocolorimetric oil research are considered in the UV-visible spectrum interval and refractometry for hydrocarbon field development monitoring was considered in detail [94,95].

3. Current State of Optical Oil Research in the Area of UV-Visible-NIR Part of Spectrum

To date, optical properties have been used to evaluate the remaining recoverable oil reserves. The studies are based on regression analysis of field data (cumulative oil production) and comparison with the results of optical oil surveys (variation coefficient of coefficient of light absorbance) (Figure 1) [96]. As a result of the re-estimation of hydrocarbon reserves, additional oil production may amount to about 1.5 thousand tons [97].

Equations of dependence, based on the analysis of technological indicators of the development process (dynamic variation of oil production rate, watercut) and coefficient of light absorbance, are also used as evaluation of well-intervention efficiency. For example, to justify a refract application, the time-dependent trend of the coefficient of light absorbance was determined [22,98]. After hydraulic fracturing, previously undrained oil-saturated part of the formation is connected, resulting in a decrease and further stabilization of the coefficient of light absorbance. After some period of time (efficiency duration of hydraulic fracturing), the coefficient of light absorbance increases, which indicates that the previously connected section is depleted and refrac is needed once again (Figure 2). Due to the prediction of the optimal period for refrac, an additional 78 tons (for one month) were produced [98].

Injectivity profile modification technology is aimed at changing filtration flows in the formation to connect oil-saturated areas previously uncovered by waterflooding into the filtration process [99]. After successful profile modification, the watercut level is reduced together with the coefficient of light absorbance variation (Figure 3) [100].

Raupov (2016) [101] established the dependences of light absorption factor from development process indicators: change in oil production rate and change in water cut (Figure 4). To increase oil production, the minimum Cla change is characteristic (Figure 4a). A similar pattern of Cla change is observed when the variation of watercut in Figure 4b increases.

At the same time, the optical properties of oil are well correlated with the physical characteristics of oil, such as density, viscosity and interfacial tension. As a result, linear and logarithmic relationships between the coefficient of light absorbance and the root mean square value (RMS) of the coefficient of light absorbance were obtained: from density (Figure 5a), from dynamic viscosity (Figure 5b) [102] and interfacial tension (Figure 6) [101].

The results of studies on the influence of oil content in oilfield wastewater on the coefficient of light absorbance are presented in the work [24]. The Xenonian pulse photometer is proposed for monitoring chemical reagent content in oilfield pipelines [103]. The instrument operates in the ultraviolet and visible ranges of the spectrum. It is proposed to use short-wave UV spectroscopy (in the range of 190–250 nm) to solve the problem of shutting in the ingress curves of simultaneously used various chemicals.

It is proposed [104] to use fluorescent spectroscopy (FSS) by creating radiation of xenon tube with wavelengths 350, 450 and 532 nm in the hood with a thickness of 10 mm. The results of the studies revealed the possibility of using transparent mineral oil (not requiring the use of volatile solvents) as an organic oil solvent to determine the optical properties of oil.

4. Application Prospects of Downhole Devices to Determine Optical Oil Properties

In recent years, there have been improvements in the existing and development of new automated instruments (wellhead and downhole) for oil surveys in a wide range of UV-visible-NIR radiation, including on the flow [105]. Nanospectrophotometers were created, allowing measurements starting from 0.5 μL. Promising are the development of oil testing devices comprising emulsion and gas–liquid mixtures. Double-beam and pseudo-double beam (“split beam” circuit) spectrophotometers are the more accurate models. The intensity of the passed light was determined simultaneously by two detectors for the tested and open-off solution. A promising process is considered to be the matrix technology of recording through the signal solution for all wavelengths for 1 s using a specialized analog integrated circuit (CCD-matrix).

There is a modular dynamic formation tester on cable MDT [57], which is a tool that allows for conducting high-quality sampling of formation fluids and mini-DST using a double packer module for reservoir properties determination and sampling (Figure 7). The tool has been used as a modular assembly consisting of MDT (Modular Formation Dynamics Tester), CHDT (Cased Hole Dynamics Tester), FMI (Fullbore Formation MicroImager), USI (UltraSonic Imager), OFA (Optical Fluid Analyzer), LFA (Life Fluid Analyzer) and GCA (Gas Condensate Analyzer). There is a possibility of integrated well studies in the well: pressure determination, pressure build-up, determination of hydrodynamic parameters of the formation, fluid sampling in situ, analysis of electrical resistivity and optical properties of formation fluid.

Mullins et al. (2006) [58] suggested oil analysis in the well (DFA) in the area of UV-visible-NIR electromagnetic radiation using high-resolution mass spectrometry for detailing the information received. The downhole conductor offered in the study conducted by Jones et al. (2015) [106], defines oil composition and the content of limit and aromatic hydrocarbons therein with the use of multivariate optical computers directly at the bottomhole in real time.

A downhole instrument capable of measuring the optical spectrum of liquid (up to 5 mL) in the area of UV-visible-NIR radiation at a temperature up to 422 K and pressure up to 138 MPa [107] is developed. Multivariate optical computer (MOC) (Figure 8) is compact (not more than 2 AAA batteries), allows continuous operation under high temperatures and pressures (up to 503 K and 138 MPa) for 20 years, and records the spectrum in the IR part [108].

Multivariate optical computing devices (MOC) use an optical component called the integrated computing element ICE CORE. The ICE CORE sensor is coded with a pre-developed vector of multivariate regression and can have a radiation response ranging from 400 to 5000 nm. The reliability of the sensor used is dictated by the wide range of wavelengths used and the high ratio of the signal to noise [109].

5. Substantiation of Devices for the Evaluation of Optical Oil Properties at the Wellhead

The final results of optical studies for solving various tasks of exploration and field development become known with significant time delay and lag behind the current operation process and may be distorted due to changes in the physical and chemical parameters of the samples during their preparation, transportation and storage [109,110]. Owing to the constant change in hydrocarbon properties in the formation, prolonged laboratory studies of the samples will not correspond to the current reservoir conditions.

The authors offered to install various designs for D and Cla of oil directly at the wellhead in real time. Such instruments may be made as stationary devices mounted on an x-mas tree or mobile, which are installed on an x-mas tree for the period of research [111].

The proposed devices have the following operational and technical properties:

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Fast simultaneous recording and processing of well-process parameters in real time and space with a resolution of 1 ms;

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Autonomy (continuous work for at least 1 year), compact (dimensions: base party—up to 15 cm, length—up to 40 cm), low weight (up to 2 kg), enabling operation by one specialist;

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Mobility can be used in any production conditions, which allows it to be used in several wells for a short time;

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There is no need to keep organized measuring point at the wellhead of oil producing well and study oil parameters at the facilities, e.g., wells commissioned in production and where stationary measurement equipment is disadvantageous;

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Wide range of UV-visible-NIR radiation: from 190 to 1100 nm;

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Thickness of the surveyed sample: up to 250 microns;

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Number of preserved measurements, at least 1 million;

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Operating range of pressures (in oil wells): at least 5 MPa;

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Operating temperature range: −40 °C to +100 °C;

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Green and meet all environmental protection conditions.

The devices for D and Cla of oil (Figure 9) include the following principal components: (1) power supply source; (2) emitter sensor; (3) prism; (4) cross-section with the probe; (5) hood with the fluid sample; (6) display; (7) reflection grid; (8) receiver sensor; (9) signal converter; (10) memory and data logger; and (11) data processing device.

5.1. Stationary Wellhead Device

A stationary wellhead device is designed to measure the optical properties of oil at the wellhead (Figure 10) and is connected to the well flowline [112]. At the same time, it can be used for studying the optical properties of any liquid media in other areas of the oil and gas industry.

The device includes a measurement photometric unit 2 with a source of light emission 7, a prism 11, a monochromator 8, a photometry sensor 10, a sample holder 9, an analog and digital converter (ADC) 4, an electronic data storage and transmission unit 5, and a thermosetting unit 6. The instrument is installed on the discharge line using pressure hoses 1 and 3. The sample holder is made of panaphobic (non-wettable) material (e.g., fluoride glass) capable of creating super thin oil film (up to 250 microns), especially in the case of studies with heavy and extra heavy oil. During flow testing without separation into phases, software will automatically clean data acquired when the gas, water or various compositions of the mixture are supplied to the sample holder. The source of light emission was performed with the possibility of obtaining a light beam of pre-set wavelength for oil testing in the given spectral range of UV-visible-NIR radiation. The software of the downhole device ensures selection of the most appropriate electromagnetic radiation range and automatic approximation of the measured oil properties with process parameters of development, physical and chemical properties of the products produced, etc.

A stationary wellhead device to measure the optical properties of oil works as follows. The instrument is installed using pressure hoses on the sampling valve and pressure gauge. According to the scheme, the product is supplied to the flow line and from there through the delivery hose 1 to the measuring photometric unit 2. In certain cases, less than 1 microliter of oil is required for D and the light-emission factor. Measuring photometric unit 2 includes a source of light radiation 7, the light of which passes through monochromator 8 to receive a light beam of the given wavelength. The produced product flows into the sample holder 9. Monochromatic light passing through the studied fluid was partially reflected and partially absorbed. The intensity of the passed luminous beam is measured by a photometry sensor 10. The range of electromagnetic radiation is of principal importance (in laboratory conditions, oil solutions are studied in organic solvents). Light oil usually features lower light-emission factors and can therefore be studied without dilution of organic solvents in the spectral range of 190–780 nm. Heavy oil should be tested in visible, NIR and IR zones (780–1100 nm). The optimal condition is the presence of a source of light radiation 7 in a wide range of electromagnetic radiation and an automatic selection by well gauge of the most suitable spectrum of oil in both test and operating modes. Thermostat unit 6 is required to maintain the standard temperature (T) of measurements or evaluation of the current sample temperature to make proper adjustments. For matching purposes, valve 13 is provided for periodic oil sampling with the purpose of performing control laboratory studies of its optical properties in the test mode of equipment. ADC 4 is designed to translate analog results of oil properties into digital format for long-term storage and transmission by wire or wireless methods into an electronic storage and data transmission database 5, with constantly updated data on well performance and other field development indicators.

5.2. Mobile Wellhead Unit

Mobile devices for automatic measurement of optical properties of oil at the wellhead of the oil producing well include heat-resistant housing 1, intake 2, photometric 3 and discharge 4 blocks (Figure 11) [113].

Structural differences between mobile devices and stationary equipment are:

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Presence of valve-reducer mechanism (batcher) 6 and valve units 11, 12 to automatically control the pressure and flow rate of the fluid supplied to the device;

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Absence of a pressure hose.

A mobile device for automatic measurement of the optical properties of oil at the wellhead works as follows. The mobile device is mounted on the wellhead of the oil-producing well into sampler valve 1 on the flow line of the x-mas tree through connector 5. After finishing assembly and readiness for start-up, valve 6 of the mobile device will be opened. Subsequent operations to determine the optical properties of products produced are similar to those performed with stationary wellhead devices. After completion of a specified measurement and research cycle, the mobile device can be removed and transported to another production facility.

6. Conclusions

During the use of the optical method, the hydrocarbon analysis method succeeded in becoming an integral tool for engineers in the oil and gas industry. To date, optical oil studies in the field of the UV-visible-NIR part of the spectrum can solve tasks that are relevant to oil and gas companies: monitoring reservoir properties variation and physical and chemical properties of fluids, evaluation of remaining oil reserves, determination of direction of filtration lines in the reservoir, efficiency of well interventions, accounting of produced products for multiformation development, etc.

It is impossible to consider the implementation of the tasks accomplished without intellectual realizations and digitalization of hydrocarbon fields with the application of new devices. At the same time, the proposed devices are only an instrument for improving the efficiency of the optical monitoring method, provided by the automated registration in the field conditions of the optical parameters of the oil produced in the given interval of wavelength, pressure, temperature and their initial processing. As a result of obtaining a large amount of data by the optical oil properties in-situ mode, it will become crucial to clean them and construct correlations and regression equations with geological and physical properties and process indicators using machine learning elements (neural network).

7. Patents

• Mobile device for automatic measurement of optical properties of oil at wellhead of oil producing wells. Available online: https://new.fips.ru/Archive/PAT/2012FULL/2012.12.27/DOC/RUNWU1/000/000/000/123/455/DOCUMENT.PDF (accessed on 31 December 2021).
• Well device for measuring optical oil properties at wellhead. Available online: https://new.fips.ru/Archive/PAT/2012FULL/2012.06.10/DOC/RUNWU1/000/000/000/116/893/DOCUMENT.PDF (accessed on 31 December 2021).