Frequently asked questions

  • Q: What media conduce propagation of ultrasonic oscillations?
  • A: Ultrasonic oscillations can only propagate in elastic media. Transmission of oscillations in such an environment is caused by elastic connections between individual particles.
  • Q: How a wave propagates in elastic media?
  • A: Elastic medium subject to longitudinal acoustic wave propagation. In the longitudinal wave the particles oscillate along the line of propagation.
  • Q: What is an ultrasonic wave?
  • A: An ultrasonic wave is a process of sequential transfer of the oscillation state from one particle of elastic medium to another particle.
  • Q: What is a wave field?
  • A: A wave field is the space in which ultrasonic waves propagate.
  • Q: How ultrasonic oscillations influence an elastic medium?
  • A: The propagation of ultrasonic wave changes distances between particles and therefore the density and pressure.
    The basic relations that characterize the elastic medium are:
    1. Expansion
    By expansion we mean a relative change of elastic medium.
    2. Condensation
    The volume change is accompanied by a change in the density of a medium.
    3. Relationship between the expansion and condensation
    The mass of a medium remains constant during all changes.
    For small deformations condensation is numerically equal to the expansion and opposite by sign.
    Physically, it is clear that if there was an increase in volume, the density of the medium must decrease and vice versa.
    The medium pressure is a unique function of density.
    The total medium pressure, covered by the wave motion can be considered as consisting of two parts, namely, initial hydrostatic pressure and dynamic or excessive pressure caused by the action of ultrasonic waves.
    Under the influence of ultrasonic waves density changes occur in a medium that lead to excessive pressure.
    Excessive pressure in the medium can be defined as a quantitative increase in pressure caused by changes in density.
  • Q: What is connection between pressure waves and displacement waves?
  • A: Propagation of the displacement wave is accompanied by changes in distances between particles of the medium.
    Particles displacement is related to the corresponding volume deformations, resulting in its turn in media pressure changes. Therefore, simultaneously with the displacement wave there propagates a medium pressure wave.
    The displacement waves and pressure waves are characterized by different aspects of the same wave process.
    Under the influence of the displacement wave the particles of the medium begin to oscillate along the propagation line.
    Under the influence of the wave on the particles of areas ab and de of the medium approached each other and parted in the area bd.
    In places of approach of the particles there formed areas of compression (in areas ab and de). At the areas bd, where the particles are moved away from each other, a depression zone was formed.
    The density of the medium in areas of approach of the particles is increased and decreased in the area of depression.
    At points A and E, where the displacement of particles at the given period of time is equal to zero, the convergence of the particles, and hence the acoustic pressure reaches its maximum value. The minimum pressure occurs at the point where there is the greatest discrepancy between the particles.
    As the wave propagates the medium particles lying on x-axis away from the origin, one by one will be involved in the wave motion, repeating the particles oscillations, began to oscillate under the influence of the wave before that.
    Therefore, areas of compression and depression will move with the wave at a rate equal to the velocity of the wave propagation.
    A displacement wave, thus, is accompanied by a pressure wave. Evidently, the two waves are different aspects of one wave process.
  • Q: Are there thermal processes in the medium under the influence of ultrasonic waves?
  • A: When it is compressed the medium starts to be heated, and starts to be cooled under negative pressure. This circumstance is associated with changes in the propagation velocity of longitudinal sound waves in elastic media.
  • Q: Are ultrasonic oscillations capable to destroy a casing cement stone?
  • A: No, they are not. A cement stone is an elastic medium and is subject to all laws of wave propagation in elastic media. Ultrasonic oscillations do not have enough power to destroy the bonds between cement particles.
  • Q: What is the bottomhole formation zone in the process of development?
  • A: During the development of oil fields there is a gradual deterioration of reservoir properties of bottomhole zones. This is typical for low-permeability and heterogeneous in thickness beds. The deterioration of the natural permeability is already in the process of drilling, when the rock excavation in the annular zone near the well occur compressive stresses. At the same rock layers, interacting with the rock cutting tools and drilling fluids, thermodynamically activated, which subsequently may promote the formation of high surface clogging layers. The drilling fluid is filtered into the reservoir and forms on the walls of the borehole mud cake thickness of 2-3 mm. Many studies confirmed the possibility of a deeper penetration into the reservoir through the mud cracks that occur due to the high hydrodynamic pressure exceeding the pressure fracturing. Such pressure may occur when you restore the circulation of drilling fluids, or tripping operations. For this reason, the decrease of the absolute permeability of the rock samples in the 1,8-42 times, and in some cases, the absolute permeability is reduced to zero. Filtrates of drilling fluids can penetrate into reservoirs at even greater distances, reaching 0.2 - 3 m. This is accompanied by a marked deterioration of the natural properties of the reservoir bottomhole zone for the following reasons:
    • swelling of clay particles reservoir rock, the water leachate differs in composition from the saline formation water;
    • reduction of permeability in the aqueous phase and the appearance of blocking action of water in the formation of wall segments having a high viscosity and elastic shear resistance at the pore surface reservoir;
    • the emergence of capillary phenomena at the contact of water with the oil reservoir;
    • Generation in the bottomhole zone of persistent water-oil emulsions such as "water in oil," weakfiltering not only due to higher than that of pure oil viscosity, but and because of the pronounced thixotropic properties;
    • clogging of the reservoir’s pores by water- and oil insoluble and solid precipitates formed by chemical interaction between mud filtrate and washing fluids with formation fluids.
    The penetration of mud filtrate in the bottomhole formation zone, characterized by vertical heterogeneity, the distance of the order of only a few centimeters reduces the scope of flooding for the reservoir capacity by 30-40%. Studies of oil wells show that for the reasons stated in the vast majority of wells perforated a large part of the productive section is not working, and working width is 25-50%.
    Significant changes in bottomhole zone is exposed in the course of the overlap seam casing and cementing.
    As a result of physic-chemical and hydrodynamic interaction of cement slurry and formation fluids according to the thermodynamic conditions of occurrence and lithological composition the bottomhole zone can both give and take away the water, which leads to a change in its filtration characteristics.
    After completion of cementation, mechanical stresses acting in the annular area around the well, start to unload to the column with cement stone, which causes changes in capacitance-filtration properties of the bottomhole zone.
    Violations of the bottomhole zone reservoir properties occur also in the process of punching and formation tests.

    During the operation there is a further deterioration of the bottom-hole zone formation. In oil wells, in addition to the reasons already discussed the deterioration of reservoir properties, is the deposition of tar and asphalt-paraffin fractions, accompanied by formation on the surface of the pore channels of the adsorption-solvate layers. This leads to the formation of boundary layers of oil with an anomalously high viscosity compared with the volume oil, whose thickness can be comparable with the radius of the pore channels. This creates additional filtration resistance, decreased permeability of the bottom zone and increases the volume heterogeneity.
    The deterioration of the permeability of bottom-hole zones occurs in the injection wells. Particularly intense this process occurs in the injection of water into the reservoir due to the gradual clogging of pores collector by suspended mechanical and oil particles contained in the water.
    Content of mechanical impurities in the injected water above certain limits is causing a rapid and significant reduction in pickup, and sometimes even a full stop wells. Injected into the formation water, as a rule, differs in chemical composition and temperature from the aqueous solutions in pores. This causes a disturbance of natural physical and chemical equilibrium in the formation and leads to swelling of clay components, and their destruction of the collector.

    At the same time may be a complete blockage of filtration channels and transport of the clay cement deep into layer, including the previously caught mud particles, with subsequent clogging low-permeability areas. In general, it can cause a reduction in throttle response and reduced sweep flooding. Violation of physical and chemical equilibrium can also lead to the formation of insoluble sediments, the deposition of salts and precipitation of wax crystals in the pore channels.

    Reduced water permeability and selective flow of water can be caused by adsorption of asphalt and oil substances on the surface of pore channels and to form the structured hydrocarbon bridging layers, with increased viscosity and reducing the effective cross section of the filtration channel. The interaction of injected water with the existing or falling with the water into the reservoir oil, at a sufficiently high rate of filtration in the bottom hole and the presence of natural oil emulsion stabilizers - asphaltenes and resins may cause the formation of persistent oil-water emulsions, which leads to a sharp increase in viscosity of the injected fluid.

    For these reasons, bottomhole formation zone is the main subject of the impact of different methods at all stages of field development. Our investigations have established that the activities associated with cleaning bottom zone, the improvement of its thermodynamic state, contributing to the restoration of the permeability, cause not only an increase in the current oil production, but also increase the oil recovery of the reservoir as a whole.
    The most promising in this respect are these methods of influence on the bottom zone, which does not lead to the formation of new irregularities.

  • Q: What are the main reasons for the decrease of oil and gas recovery?
  • A: - The appearance of capillary forces, which prevent the displacement of oil from the pores of microscopically porous medium;
    - Unfavorable mobility ratio of the displacing and displaced fluids;
    - Geological heterogeneity of structure of the productive reservoir.
    Geological heterogeneity typically leads to the fact that not in the all volume of the reservoir within the area covered by water flooding is an active process of exclusion oil by water. Therefore, in each section of the reservoir water flooding may be areas to which the displacement front has not approached or who have been left out of the displaced water.
    Most of the known methods of enhanced oil recovery involve use of funds, partially or completely eliminate the expression of any one of the three above-mentioned main reasons for the decline of oil displacement efficiency from the producing formations. However, the application of these methods in the later stages of development gives disappointing results. One of the main reasons for their poor performance is that they ignore the natural tendency of hydrocarbon liquids to move under the influence of gravity and capillary forces. In this connection, is the attention of oil and gas workers to vibration and acoustic technologies understandable that are able to take account of this trend.
  • Q: What is the essence of the wave (vibration) technology?
  • A: The bases of the wave (vibration) technologies are different ways of energy transfer from the borehole sources of oscillations in a bed trough the well fluid. Becauseof damping of oscillations in a fluid the transfer way of energy leads to damping of oscillations at distances up to 10 m from the borehole walls. However, this is quite enough for effective cleaning of the walls of the wells and the bottom zone of the dirt and clogging substances. In addition, under the influence of fluctuations is blocking effect of the residual phases of gas, oil and water eliminated, filtering fluids in low-permeability zones is initiated and it increases coverage of bed in the thickness and along.
  • Q: What is an indicator of the effectiveness of exposure?
  • Answer: An indicator of the effectiveness of exposure at a given frequency can serve radius of coverage, within the certain relations between the threshold values of the parameters of oscillations - vibrational displacement and vibrational accelerations are supported. These parameters are determined by the density of vibrational energy flow in this environment and at a given oscillation frequency.
  • Q: How does the frequency the useful work affect?
  • A: At sufficiently high frequency oscillations the particle surface remains almost stationary in space. The above pattern is manifested in the oscillatory motion of unconsolidated bed. There is a range of accelerations of the surface oscillations, in which the vibration penetrates into the reservoir more efficiently.
  • Q: What is the mechanism of the effect of fluctuations in bottomhole formation zone?
  • A: The low-frequency fluctuations in two - three orders of magnitude speed up the relaxation of mechanical stresses. When exposed to the bottom zone it may help to eliminate the adverse effects of drilling and wells development, associated with undesirable stresses of the borehole walls and perforations. Thus the natural state of equilibrium bottom-hole zone and the initial permeability of the reservoir are recovering. The experimental results indicate a change in absolute permeability of saturated porous media under the influence of high-amplitude low-frequency oscillations of the fluid pressure of 0.3 MPa, developed by hydraulic vibrator. Relative changes in the permeability of artificially cemented cores reach 30% and are associated with the formation of new filter channels in the porous medium, the change of porosity, cracks opening, repacking and orientation change of the grains composing the porous medium. With increasing clay content up to 35% these processes are enhanced.
    Other effects are directly related with the impact on colmatant and fluids during their interaction with the skeleton formation in the oscillations field. Experimentally observed there are changes in rheological behavior of non-Newtonian liquids, characterized by the presence of viscoelastic and visco-plastic properties.
    Studies of the shear viscosity of oil changes under the action of elastic waves intensity from 8 to 100 kW/m2 and a frequency of 20 Hz to 4.5 MHz have shown that after exposure the shear viscosity is reduced by 20 - 30%, and after a while restored (in the precovitation mode ) or not fully restored (after effects of developed cavitation regime). The greatest changes in the viscosity in the precovitation mode at low frequencies are observed with increasing content of asphalt-resinous and waxy components in oil. The recovery time of the viscosity after exposure is 5-6 hours or more.
    High-frequency vibrations affect mainly on the boundary layers of liquid with the solid phase, contributing to the destruction of the structure of surface layers and decrease adhesion of the liquid to the solid phase, and make no appreciable effect on the structure of non-Newtonian fluid.
    Low-frequency oscillations, conversely, deform the structure of the liquid mainly in its entirety.
  • Q: What are the effects observed by ultrasound exposure?
  • A: In addition to changes in permeability and viscosity experimentally observed:

    • an increase (up to 18 times) the relative velocity of filtration of water or model Newtonian oil through the sandstone cores;
    the intensity fluctuations at the same time amounted to several hundred kilowatts per 1 m2, the frequency - from 3 to 10 kHz;
    • an increase (up to 10 times) the rate of filtration of polar and nonpolar liquids, dielectrics and electrolytes, the intensity of vibration was 1.9 kW/m2 at a frequency of 17 kHz;
    • an increase (almost 100 times) the rate of filtration of distilled water and solutions of salts through natural cores under the influence of oscillations with a frequency of 26.5 kHz.

    The authors explain the experimental results of studies through the destruction of surface layers, which reduces the flow resistance of the liquid in the pore channels and increase the effective cross section.

    Another mechanism that explains the multiple increase in the rate of filtration in the field of the oscillations has been proposed to solve the problem of the directed motion of a viscous compressible fluid through a narrow capillary, the walls of which are deformed in the form of traveling waves. In the numerical solution is established that a wave of transverse motion excites inside the fluid the another one , the essential feature of which is that even at low amplitudes of the transverse displacements of the wall of the capillary pressure and the amplitude of the longitudinal velocity of the fluid flow in some areas may reach values ​​substantially exceeding the velocity amplitude and pressure, causing the acoustic flow. This phenomenon is an example showing that the velocity fluctuations and pressure with the scale of the order of the pores can cause one-sided directed flow at rates substantially greater than the rate of filtration.
  • Q: How do fluctuations affect the stability of the cement sheath around the well?
  • A: The experience demonstrates the reliability of mounting holes in the case of vibration exposure. Reasonable quantitative criteria governing permissible amplitude and duration of exposure are absent.
    Therefore of great practical interest are the studies of the conditions for destruction and origin of creeping of the cement sheath based on available data on standard tests of concrete under periodic loading.

    However, due to the fact that for a fixed power source the oscillation amplitude with increasing frequency is sharply reduced, the amplitudes, sufficient for the manifestation of creep, occur only at the resonant modes of oscillation excitation.
    For example, when the duration of the impact of fluctuations does not exceed 24 h, the visible expression of creep is observed at amplitudes of the pulsating pressure greater than 8 - 10 MPa.
    The amplitudes of 3-5 MPa, which the currently existing downhole vibrators develop, practically do not cause any symptoms cement sheath creeping, and therefore do not lead to violations of the well isolation.
  • The main problems of incomplete oil recovery
  • When considering the factors impeding the full recovery of oil from the reservoir, above all, it should be noted incomplete displacement of oil by water from the pore space by using water flooding processes.
    In cases where a heterogeneous porous medium is combined with the presence of capillary effects, the water cannot displace oil from the side of the pore space. It is believed that the oil is mainly concentrated in the largest pores.
    Completeness of oil displacement by water from the porous medium is usually characterized by the displacement, which is the ratio of the difference between the initial and residual oil saturation in the initial oil saturation.
    The coefficient of oil displacement by water in various conditions varies in the range 0.4 -0.75. In this case the residual oil is 15-40% of the total flooding of the pore space.
    Another factor in determining oil recovery by water flooding is the incomplete coverage of the formation process of repression.
    As you move the displacement front in the direction from the injection wells to the operational, turns out that not all the area of productive reservoir is covered by the development.
    Thus, the following main reasons reduce flooding in the recovery:
    • display of capillary forces, which prevent the displacement of oil from the pores of porous medium;
    • an unfavorable mobility ratio of the displacing and the displaced fluid;
    • inhomogeneous geological structure of the productive reservoir.

    In the specific geologic conditions of one of the factors often becomes the dominant value.
    Thus, when low-viscosity oil field development with productive reservoirs of moderate heterogeneity of the main reason for incomplete recovery of oil associated with the manifestation of capillary forces. Clearly, methods to improve oil recovery in such fields should lead to the exclusion of (partial or full) manifestation of capillary forces in the process of oil displacement.

    In developing of high viscosity oil, no removable, a significant proportion is associated with an unfavorable mobility ratio, and therefore, for enhanced oil recovery such deposits must first try to increase the viscosity of the displacing fluid and reduce the viscosity of the displaced oil.


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