Currently, in order to maintain the level of liquid hydrocarbons production, oil reserves formed by fields with viscous, high-viscosity and extra-viscous oil (hereinafter referred to as “viscous oil”) are included in development. Most of these fields are located in regions with developed infrastructure, so their development today becomes a real task and a necessary condition for the development of oil business.
The reserves of fields with oil viscosity over 10 centipoise have been depleted in our country by only 10-30 per cent (depending on the degree of viscosity). According to GKZ RF estimates, reserves of such oil by ABC1+C2 categories in Russia amount to about 1.7 billion tonnes, i.e. about 10 per cent of the country’s total black gold reserves. And initial reserves exceed 5 billion tonnes.
The challenges in developing fields with viscous oil are associated with extracting it from the reservoir and transporting it from the reservoir to the refinery. To solve them, special methods of stimulation are used. These include thermal (steam displacement, vapour gas, combination of horizontal drilling and vapour gravity (SAGD), in-situ burning), physical (hydraulic fracturing), chemical, special waterflooding methods, and microbiological. Implementation of the above methods involves significant energy costs, difficulties in technical implementation, which leads to lower efficiency and higher production costs.
Acoustic stimulation of the reservoir can be used to increase the efficiency and reduce the energy consumption of the methods used. Acoustic stimulation can also be used as a stand-alone reservoir treatment.
The proposed method is based on a gentle and prolonged impact on the formation by acoustic vibrations, which are accompanied by significant alternating loads, which gives:
- Increased fluid withdrawal. Due to the “piston” effect, the filtration volume of the mobile fluid is increased;
- Intensification of oil extraction. By overcoming the viscoplastic forces holding the fluid in the filtration process, the still fluid is involved;
- Reducing oil viscosity and reducing water cut of products. Due to depolarisation of molecules and weakening of intermolecular bonds, the rheological structure of oil is destroyed, as a result of which its phase permeability increases, whereas for water it remains unchanged;
- Increase of oil displacement coefficient with water. By reducing the wetting angle between water and oil, surface tension forces are overcome;
- Redistribution of oil saturation and more complete oil recovery. Due to acceleration of gravitational separation of phases of different densities in the acoustic field, segregation (separation) of oil and water in highly watered formations takes place;
- Increased reservoir permeability and oil recovery rate. Due to the seismoelectric effect, which destroys the walled immobile layers of fluid (oil) that are electrostatic in nature, the effective cross-sections of pore and perforation channels are increased. Thus, they are cleaned from mechanical impurities, viscous deposits and disruption of surface fluid layers, as well as involvement of stagnant reservoir zones in the filtration process.
Pilot tests on acoustic influence on the formation with simultaneous induction of inflow by a jet pump in order to clean the bottom-hole zone of the formation gave positive results. The work was carried out at Rosneft, Lukoil and Gazprom Neft divisions. The acoustic transmitter was lowered on a geophysical cable and the bottomhole formation zone was treated (1 hour per 1 metre interval, f= 10-11 kHz) while the jet pump was operating at a stable mode; in parallel, the change in inflow from acoustic treatment was assessed. The average effect was an additional 4 tonnes per day, with effect duration ranging from 4 to 18 months.
Despite the positive results of acoustic treatment subsoil users are wary of acoustic impact. This is due to the peculiarities of the technology:
- The technology is science-intensive and requires coordinated work of various specialists (scientists, engineers, geologists);
- the technology is “capricious” and requires a detailed study of the geological structure of the sites for compliance of the applied processing parameters with the real geological conditions;
- few industrial trials (lack of statistical evaluation of application experience);
- high cost, which today is due to sporadic work.
The systematic use of acoustic processing technology on the basis of a geophysical enterprise will significantly reduce the cost of these works. Using a geological and geophysical approach to the candidate wells to be selected will address the “capriciousness” of the technology.
The conducted works on cleaning of bottomhole formation zone gave an impetus to the possibility of using acoustic impact in a constant mode in the production of hydrocarbons, including viscous oil. Both independently and in combination with existing methods of viscous oil production, acoustic impact will increase efficiency by reducing energy consumption, preventing formation bottom-hole zone colmatisation, increasing the overhaul period of downhole pumping equipment, and increasing oil recovery. In summary, acoustic stimulation becomes one of the steps towards reducing the production cost of viscous oil, which is an important factor in today’s world.
To select the required optimum operating mode of the well, an acoustic complex and a jet pump can be used. This will allow hydrodynamic studies to be carried out:
- at different operating modes of the acoustic complex by regulating the time and power of acoustic treatment of the formation;
- over a wide range of downhole pressures by adjusting the injection pressure of the working fluid at the jet pump inlet.
Hydrodynamic studies will allow selection of deepwater pumping equipment, necessary operational parameters of reservoir operation and its acoustic treatment, which is very important when working in multiphase media.
The acoustic complex will improve the efficiency of existing methods to optimise energy consumption for transporting viscous oil from the reservoir to the place of its processing.
Our laboratory tests on viscosity reduction were carried out with oil emulsion (30% water cut). The oil emulsion was acoustically treated at 17 kHz for 60 and 300 seconds. The treatment resulted in a 30% reduction in viscosity and a 38% reduction in the load on the electric motor of the transfer pump, while maintaining the temperature regime. Recovery to initial viscosity parameters was 5 hours. Moreover, the processing time did not affect the result. After acoustic treatment a homogenised oil emulsion with reduced viscosity was obtained with a time of gravitational separation into oil and water within 8 hours (the original oil emulsion has 1-2 hours). In the laboratory tests, the effect of temperature was excluded in order to evaluate the effect of acoustic exposure. The energy expended to operate the acoustic radiator is 30-40% of the energy consumed and 60-70% is spent on heating. It is advisable to use the processed oil emulsion for cooling, which will additionally lead to reduction of its viscosity due to heat exchange.
The viscosity reduction and homogenisation is due to the cavitation phenomenon. In fact, it is the formation and collapse of gas bubbles in a liquid medium. This results in the decomposition of high-melting high-molecular-weight paraffins during high-intensity treatment, which changes the physical and chemical (operational) properties of the oil. Also, cavitation effects, which occur during acoustic treatment of oil, prevent polarised associates from combining into large structures, dispersing them into smaller groups of molecules.
The results of laboratory works show the possibility of using acoustic treatment in a constant mode during the transport of viscous oil from the reservoir to the place of its processing as an independent method of influence and in combination with existing ones. For example, when chemical reagents are added, the reaction will occur faster and with less reagent consumption, better homogenising the mixture.
Today there are projects of development of equipment for acoustic influence on pumped viscous oil emulsions, which require industrial execution and testing. Feedback from the developers will improve the technology and reduce the cost of equipment production through optimal selection of operating parameters and mass production.
Mass application of the acoustic complex in the production and transport of hydrocarbons will allow us to talk about a new turn in the development of fields with viscous oil.
