Intensification of high-viscosity oil production

According to a report by British Petroleum (BP Statistical Review of World Energy 2019), there is an increasing tendency of hydrocarbon production in the world. At the same time, a major advantage for mining companies will be the use of local infrastructure and personnel in well-equipped old oil and gas producing provinces to ensure efficient production, transportation and processing of products.

A promising area of ​​maintaining production levels in the oil and gas regions is the development of high-viscosity oil (HVO) fields. However, for the effective extraction of such raw materials, it is necessary to adapt classic and develop new technologies in response to a number of technological challenges [7,8]. First of all, the production of HVO is complicated by the high viscosity of the product, the abnormal rheological properties due to the high content of asphaltenes, resins and paraffins in the oil [2,5,6,9-11]. This complicates oil influx rate from the pay bed to the development wells, and its transportation through infield pipelines. HVO tends to form complex asphaltene-tar-paraffin deposits (heavy oil deposits) both in the formation pay zone and on downhole equipment as well as production tubing. The high content of resins and asphaltenes, which are natural stabilizers of oil-water emulsions, leads to complications in the processes of bringing high-viscosity oil to marketable quality. According to the results of the analysis of Samaraneftegas JSC's number of wells at HVO facilities from 2015 to 2019, there is an increase in the share of HVO wells containing heavy oil deposits (Figure 1).

Figure 1 – Increase in the share of wells containing heavy oil deposits in the total number of wells at HVO facilities in Samaraneftegaz JSC

The share of wells containing heavy oil deposits in the HVO wells	Доля скважин осложненных АСПО в фонде ВВН
HVO wells containing heavy oil deposits	Фонд скважин ВВН осложненных АСПО
Current HVO well share	Действующий фонд скважин ВВН
The share of wells containing heavy oil deposits in the total share of HVO wells	Доля скважин осложненных АСПО в общем фонде ВВН

Based on the analysis of data on the chemical composition of oil, heavy oil deposits and production conditions, it is proposed to use hydrocarbon solvent reagents to comprehensively increase a number of technological processes aimed at intensification of the production of HVO. The affinity of the chemical composition of oil and solvents, ease of production use, and the possibility of laboratory tests using a minimum set of equipment make it possible to recommend such reagents for implementation under conditions of economic profitability.

Currently, a large number of different technologies are used to intensify oil production. The most common geological and technical measure (GTM) is the traditional acid treatment (AT) of the formation pay zone. The low cost of conducting such a geological and technical measure is ensured by the possibility of using standard equipment, the simplicity of the operation, the predicted efficiency. Despite the apparent simplicity of implementation, ATs require a careful approach to the choice of composition and technology. For example, the effectiveness may depend on the duration of exposure of the acid composition in the formation pay zone until the subsequent development of the well [4]. When choosing acid treatments as a way to intensify production in wells operating facilities with HVO, a number of possible complications should be taken into account: the formation of persistent and viscous acidic emulsions of AC with oil, the risk of heavy oil deposition in the formation pay zone, etc. These processes can have a negative impact on the final result, complicate the development and subsequent operation of the producing well.

One of the easiest ways to increase the efficiency of AT in HVO facilities is to pre-inject the solvent bank into the formation pay zone before the acid treatment itself. By conducting laboratory tests for the objects of the Volga-Ural oil-and-gas province, it was found that the use of a pre-selected solvent reagent allows cleaning the formation pay zone rock surface from heavy oil deposits to ensure a more complete AT reaction, to increase the efficiency of well development after completion of the GTM due to the effective separation of the emulsion "acidic composition – high-viscosity oil." Evaluation of the compatibility of the acid composition and HVO can be carried out as follows: oil and acid composition are poured in equal volumes into a container, then mixed for half an hour to simulate the most unfavorable interaction of fluids in the formation pay zone. A similar process is carried out with a number of other samples, but with the addition of a solvent reagent in oil. Then the containers are closed with lids and placed in a heating cabinet with a set temperature equal to the formation temperature. Assessment of the quality of separation of mixtures is carried out after 5 and 30 minutes when the containers are in the heating cabinet at formation temperature, then the mixture is poured onto a sieve with holes of 0.152 mm. If the fluids are compatible with each other, then after they pass through the mixture there shouldn't remain any sediment or emulsions on the surface of the sieve. When adding an oil-compatible solvent-reagent, no precipitate should be observed on the surface of the sieve, it is also possible to note a clear separation of the phases of the oil and the acid composition, which indicates the absence of an emulsion (Figure 2).

Figure 2 — How the solvent affects the efficiency of "acid-oil" emulsion separation

with the addition of surfactant species	с добавлением ПАВ
with the addition of a solvent	с добавлением растворителя
with the addition of surfactant species	с добавлением ПАВ
without the addition of a solvent	без добавления растворителя

Despite the fact that a number of acidic compositions were tested in commodity form, with surfactant species already added to them, not one of them was compatible with a sample of high-viscosity oil. The addition of a hydrocarbon solvent reagent ensured the efficient passage of the emulsion through a sieve. Thus, conducting such test allows you to quickly evaluate options for improving the technology of acid treatment of a formation pay zone in fields with high-viscosity oil.

Figure 3 shows the distribution of the solvent and acidic composition in the formation pay zone during an operation in a production well.

Figure 3 — Scheme of the treatment of the formation pay zone by sequential injection of solvent and acidic composition

Well	Скважина
Acidic composition	Кислотный состав
Solvent	Растворитель

An important task in the operation of HVO deposits is to ensure the optimal operating mode of deep-well pumping equipment, as well as preventing the formation of heavy oil deposits, lowering the viscosity of HVO, etc. Practical experience shows that the use of solvent reagents makes it possible to efficiently control the rheological properties of high-viscosity oil throughout its production and transportation – from the producing well bottom to the time of delivery of the HVO for processing.

One of the most effective methods for introducing a solvent into the HVO is dosing at a pump suction through a capillary tube (Figure 4). To do this, reagent dosing units are placed on the day surface, usually in a block-complex design, and a capillary tube is lowered into the well to the pump suction basket. This method of feeding the reagent has several advantages. Firstly, the solvent mixes well with HVO, the viscosity of the well produce decreases, because of that the load on the pump drive is significantly reduced (usually, it's an electric motor in various models). Secondly, modern reagent dosing units allow precise control of the feeding volume. Additionally, the likelihood of the formation of heavy oil deposits on production tubing, fountain fittings and field communications on the surface is reduced, and pressure in the flow lines is reduced as well. Due to this, the overhaul period of the well is significantly increased, there are no periodic shutdowns of the well due to engine overload.

Figure 4 – Scheme of reagent feeding for electric centrifugal pump suction (1 – dosing unit; 2 – onshore pipeline; 3 – input device through the lateral branch of the fountain fittings; 4 – input device through the cable input of the fountain fittings; 5 – power cable of the ESP system; 6 – downhole capillary tube; 7 – spray valve)

Preliminary laboratory tests are carried out to select the most effective and solvent-compatible reagent solvent. One of the easiest ways to pre-evaluate a reagent is to conduct a compatibility test. The solvent and oil are mixed at a concentration of 80% and 20%, respectively, and then poured onto a filter installed in a funnel. An oil compatible solvent does not precipitate any heavy oil deposit sediments.

Figure 5 – An example of a test to assess the compatibility of solvents with oil (checking the presence of sediment on the filters) and a visual assessment of mixing properties.

As seen on the presented figure, solvent A showed poor mixing properties with HVO and left a sediment on the filter, solvent B passed through the filter without leaving a sediment on it. Thus, the solvent B is compatible with oil, which allows us to recommend it for further testing and application at the well. It should be noted that there are a number of technologies where the process of deasphalting of high-viscosity oil is initiated purposefully both in formation conditions and on the surface, however, they are not considered in the framework of this work.

Despite the similar chemical composition, different solvents have different effects on the rheological properties of high-viscosity oil. Figure 6 presents the results of laboratory studies evaluating the effectiveness of reducing the viscosity of an HVO sample with the addition of reagents in a volume concentration of 3%.

Figure 6 – An example of the results of testing reagent solvents to lower the viscosity of HVO (volume concentration of 3%)

Oil viscosity, mPa-s	Вязкость нефти, мПа с
Source oil	Исходная нефть

The graph shows that the solvents have different efficiencies of lowering the viscosity of HVO. In this case, solvent No.1 showed the lowest efficiency and cannot be recommended for use in production.

The use of hydrocarbon solvents in complex technologies for thermochemical treatments of the formation pay zone is of great interest (Figure 7). This approach allows you to increase the intake capacity and the injection rate due to the most effective dissolution of asphalt, resin, and paraffin materials and an additional decrease in oil viscosity in formation pay zone. Thus, the chemical effect of the solvent and the subsequent thermal effect of steam have a synergistic effect on the additional production of HVO [3].

Figure 7 – The process of treating the formation pay zone of a vertical well formation with solvent and steam at a high-viscosity oil facility

Solvent pumping
Solvent displacement and steam injection
Well testing

Figure 9 presents the results of hydrodynamic modeling of the use of complex thermochemical formation zone treatment with solvent and steam at one of the HVO fields of the Volga-Ural oil and gas province.

Figure 9 – Example of modeling results of the use of complex technology for treating the formation zone of a vertical well with solvent and steam at a high-viscosity oil facility (oil saturated thickness 14 m, oil viscosity under formation conditions 400 mPa∙s, pre-injection of aromatic solvent reagent bank)

Accumulated oil production	Закачка растворителя
Steam injection	Продавка растворителя и закачка пара
Steam injection + solvent	Освоение скважины

What follows is that the use of a hydrocarbon solvent in combination with heat treatment of the formation pay zone with steam will allow to take advantage of the chemical and thermal effects.

Based on the material on the above methods of using solvent reagents in the processes of intensification of production of high-viscosity oil, it is possible to draw the following conclusions:

• Selection and justification of the use of solvents are impossible without preliminary laboratory studies for each high viscosity oil field. It is necessary to carefully study the compatibility of a hydrocarbon solvent with high-viscosity oil, as well as with other reagents used, for example, to avoid lowering the permeability of the formation pay zone due to organic deposits appearing.

• The justified use of such reagents in classical technologies for the oil production intensification can improve the results and increase the potential of various geological and technical measures. The injection of the solvent bank before the acid treatment allows to increase the efficiency of the acidic composition and accelerate well development. The feeding of a reagent to deep pump suction allows to control the rheological properties of the produced HVO and, as a result, increase the flow rate of the well and reduce the load on the pump motor. The preliminary injection of reagent solvent bank during the complex thermochemical treatment of the well allows to increase oil production compared to the classical formation pay zone with steam.

• Hydrocarbon solvent reagents are produced by a large number of petrochemical enterprises and are readily available on the market. The simplicity of introducing solvent reagents into a number of technological processes to intensify the production of high viscosity oil using standard equipment makes it possible to consider options for their prompt implementation in production.

References

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Authors:

Roschin Pavel Valeryevich, Chief Project Engineer, Specialized Institute for Development of High-Viscosity Oil Fields

Kireev Ivan Ivanovich, Key Project Chief Manager, Specialized Institute for, Development of High-Viscosity Oil Fields

Demin Sergey Valeryevich, Deputy General Director for Geology and Development

SamaraNIPIneft OOO