VIBRATING METHOD TO ENHANCE OIL RECOVERY

Abstract

A vibration impact method on to enhance oil and gas recovery from productive geological formations by controlling phase surfaces of seismic waves to focus elastic energy on localized areas.

Description

BACKGROUND OF THE INVENTION

The present invention is in the technical field of oil and gas production. More particularly, the present invention is provided a method of enhanced oil recovery through the vibration impact on the oil contained formation.

Oil and gas companies consistently look for advanced technologies to increase recovery and intensify fluid inflow from wells in the hydrocarbon production process—particularly with deposits in complex reservoir conditions such as compacted oil reservoir rocks, high water cuts, and irregularly distributed permeability zones created by hydraulic fracturing. The interest of companies is essentially independent of the oil market since recovery and intensification technologies increase production volumes without significantly raising costs, as oilfield infrastructures need not be otherwise modernized or improved.

One of the methods of oil recovery enhancement is the use of seismic-acoustic reservoir stimulation methods [1-6]. Elastic (vibrational) influence on productive formations has emerged as a promising solution with the demonstrated influence of wave processes on fluid kinetics in multicomponent (oil, gas, water) and multiphase (gas, gas condensate, gas hydrate) flows.

Among these enhanced oil recovery technologies are those based on the vibrations excitation from the earth surface. The effect of seismic elastic waves on oil recovery was discovered in 1985 by the Institute of Earth Physics, USSR Academy of Sciences on the Abuzy oil field located in the Krasnodar region [2] of Russia. The method was carried out using a 20-ton vibrator on the earth's surface in the frequency range of 10 to 30 Hz. The energy of density flux in the oil reservoir did not exceed 10⁻³-10⁻⁴W/m2. As a result, an increase in oil production of 5-10% was achieved by decreasing the water cut from 90-95% to 85-90% in producing wells. Repeated experiments showed that this effect persisted for at least one month. Following these results, the theoretical, methodological and technological aspects of the specified research direction were intensively developed [7-13].

For the last several decades, new principles and devices for vibratory stimulation [14-17] of oil and gas reservoirs have been developed, while extensive theoretical studies [18-26], as well as modeling and field experiments, have also been conducted.

Currently, the interest in vibratory stimulation of oil reservoirs is growing again. Uses of ultrasonic equipment and technology for oilfield stimulation in Russia and in the United States [27-30] have confirmed that acoustic impact can improve the oil recovery ratio. This issue is the subject of international discussions [31].

Since the mid-20th century, numerous methods and devices have been developed for elastic impact on geological objects containing oil and gas in order to increase the hydrocarbon recovery rate, including:

• • global reallocation of the mechanical properties of the rock massif through a powerful explosion or hydraulic impact [32-36];
  • excitation of acoustic oscillations in the borehole and/or in the wells group [37-53];
  • ultrasonic and electromagnetic waves emission, including selective action on a porous environment (rock) and on the fluids within the pores (oil, gas, water) [54-63];
  • low-frequency impact through sequences of depressions and repressions on a zone saturated with hydrocarbons [64-73].

Yet solutions of vibrational impact from the Earth surface have not been developed to address the oil and gas services market's need, as without expensive, special equipment and other costs, explorers have been unable to create the density of elastic energy needed to use this method within the defined yet extensive areas of the hydrocarbon reservoir. Despite numerous theoretical and experimental studies, a constructive solution to this problem has not yet been found.

At the same time, techniques of adjustable wave front focusing have been applied and developed throughout the entire existence of seismic mineral resource prospecting. This is for the most part in reference to so-called “laboratory” variants, in which wave focusing is synthesized for processing recorded data. There are also various solutions for physically realizing group directed (focused) sources of seismic waves, for example [74-75]. But these solutions are aimed at increasing the signal-to-interference ratio in the process of recording and processing data.

Thus, the present invention proposes a solution to the problem of intensifying oil and gas production using the methodology and equipment demonstrated in another (geophysical) industry where they are using to improve the accuracy and reliability of seismic research results.

SUMMARY OF THE INVENTION

The present invention is a new method of enhancing oil recovery through seismic vibration, in which oscillations are excited on the surface of the earth. The proposed solutions differ from existing ones because they provide control to phase surfaces of seismic waves in order to focus elastic energy in the localized area of an oil or gas reservoir.

Using a large number of seismic vibrators with controllable delay will enable accumulation of signals energy in-phase. As a result, the high density of elastic energy will be concentrated in a local area.

Wherein, the surface vibration in the area of the production facilities will not damage the infrastructure, because the process is similar to seismic studies with vibrators that are common in oil and gas field regions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Simplified scheme of a focused elastic exposure on a geological formation.

FIG. 2. General initial conditions of a model of an oilfield elastic stimulation system.

FIG. 3. Main regularities of the energy flux density distribution (a process of elastic energy focusing in a digital model).

FIG. 4. Energy flux density dependence by the number of vibration sources (a digital model of the elastic energy focusing process).

FIG. 5. Controlling the position of the area of focus in the lower half-space (a digital model of the elastic energy focusing process).

FIG. 6. Generalized dependence of the energy flux density on the position and depth of focus and the number of vibration sources (a digital model of the elastic energy focusing process).

FIG. 7. An interaction of the main components of the elastic stimulation system in an oilfield.

FIG. 8. Technology flowchart of the implementation of the vibrating method for enhancing oil recovery.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the invention in more detail, FIG. 1 shows the simplified scheme of a focused elastic impact on the geological formation. The numbers shown in parentheses refer to location points on the attached Figures.

Numerous seismic sources (node 10) are placed on the surface. The number of the sources (10) and their relative positions on the surface are chosen based upon the technical requirements of depth location and energy distribution within the area of focus for elastic stimulation. For clarity, FIG. 1 shows action for only three sources (10). In practice, 10, 15 or more of the sources (10) are used. The start time of each source (10) is set so that the fronts of elastic waves or the lines of equal propagation times of the waves (11) are crossed in the center of the elastic energy focus area. For homogeneous velocity models, the time of elastic wave propagation between the focusing point x_f and the oscillation source position x_i is

$t\left({\stackrel{\_}{x}}_f,{\stackrel{\_}{x}}_i\right)=\frac{\sqrt{〈\left({\stackrel{\_}{x}}_f-{\stackrel{\_}{x}}_i\right)|\left({\stackrel{\_}{x}}_f-{\stackrel{\_}{x}}_i\right)〉}}{V}=T_i.$

where <|>—denotes the operation of the vectors' scalar multiplication.

If the oscillations are generated by the sources (10) at each point x_i with a time delay in accordance with equation

ΔT_i=(T_o-T_i),

the corresponding spherical waves fronts (11) at time T_o (of V*T_i radius) are intersecting at the x_f point.

All other fronts positions, located before and after T_o, will not intersect but will concentrate in regions (12) and (13). So, the dimensions of the area will be determined by the difference between the isochrones value and T_o.

Thus, an in-phase interaction of waves from different sources (focusing) will occur only in the proximity of x_f. At other points, to a greater or lesser extent, the accumulable signal will be weaker due to the antiphase interference effect. The power-stream will change according to the signal amplitudes as it shows by (14). Therefore the density of elastic energy will reach the maximum value in the focusing zone near x_f.

There are three types of elastic wave sources in the practice of seismic exploration of mineral deposits: 1) explosive (explosives placed in special wells); 2) non-explosive pulsed (mechanical, pneumatic, electro-dynamic impact or explosion in an isolated space); and 3) vibrational (a vibrational platform on a heavy truck chassis). There are no significant restrictions on the use of the above-mentioned sources of elastic waves to stimulate the reservoir, but some important factors should be considered. Different source types have different energy characteristics, as well as different risks of environmental damage. Presently the most common method for excitation of elastic waves is the vibration method. Apart from the degree of environmental safety they afford, such sources have wide control ranges in frequency and duration of elastic impacts as well as advanced automatic controls for equipment operation.

In order to demonstrate the feasibility of the invention under consideration, and the possibility of achieving the main objective, regulated energy levels and the duration of the elastic effect are used in a digital model of the propagation of elastic oscillations in an infinite continuous medium. This type of simulation is an alternative approach to a difficult, costly field test.

FIG. 2 shows the initial conditions of modeling. The position of the oscillation sources on the day surface (21) is assumed to be symmetric with respect to the central point of the coordinates system and is determined by the number of such sources Nx, Ny and the steps dX, dY of their placement along the X and Y axes, respectively. For a given velocity of elastic waves propagation V and the focusing point in the lower half-space x_f the distribution of elastic energy density in the bounded region—the “cube” (22)—is calculated given the position of the center at point x_o and by dimensions along the corresponding coordinate axes. The sweep-signal (23) is determined by the values of the maximum and minimum boundary frequencies f_max, f_min at the time interval 0-t_max, at a which the maximum amplitude=1. The power flux density is calculated in Joules per second (watts) per unit area (square meters)−[J/(m²*s)].

The main factors that affect the distribution of energy of elastic waves propagating in a continuous medium are geometric divergence and power absorption. The first factor expresses the law of the energy conservation. In the given case, the consequence of such a law is a decrease in energy density in proportion to the increase in the area of the wave front surface which has been moved continuously away from the source. The second factor is determined by the properties of the rock massif as a porous environment saturated with liquid or gas.

The elastic medium could be described for the ray-approximation in the generalized form as

$\sum _{n=0}^{2N}{\vartheta }_n\frac{{\partial }^{n+2}}{\partial t^{n+2}}\left[U\left(\stackrel{\_}{x},t\right)\right]=\sum _{n=0}^{2N}{\mu }_n\Delta \left\{\frac{{\partial }^n}{\partial t^n}\left[U\left(\stackrel{\_}{x},t\right)\right]\right\},$

where U(.)—the amplitude of elastic wave; Δ is the Laplace operator; ϑ_n=ρν_n; ρ—the density; ν_n,μ_n—rheological modules of the medium that slowly change in spatial coordinates and do not depend on time .

This form of the equation describes all dissipation mechanisms of elastic energy, including ones most used in similar considerations:

• • Hooke's body, characterized by the fact that there are only purely elastic stresses in the environment;
  • Kelvin-Voigt body, in which along with elastic forces there are additional stresses due to viscosity, proportional to the velocity of deformation;
  • Maxwell's body, in which the tensions arise during deformation and gradually relax, such that when the load is lifted, the body does not return to the previous state; there are always residual deformations;
  • Standard linear body, which takes into account both of these absorption mechanisms.

To solve the problem of estimating the distribution of elastic energy in a medium, the account of the wave dissipation (partially inelastic scattering) effect can be reduced to describing the wave eikonal as a frequency dependent on

$\tau \left(\stackrel{\_}{x},\omega \right)=\underset{0}{\overset{M\left(\stackrel{\_}{x}\right)}{\int }}\frac{\mathrm{dS}}{V_e\left(\stackrel{\_}{x},\omega \right)},\mathrm{with}\frac{\partial \tau }{\partial \xi }=\frac{\partial \tau }{\partial S}·\frac{\partial S}{\partial \xi },{\mathrm{and}\left(\nabla \tau \right)}^2=\frac{1}{V_e^2}\left[{\left(\frac{\partial S}{\partial x}\right)}^2+{\left(\frac{\partial S}{\partial y}\right)}^2+{\left(\frac{\partial S}{\partial z}\right)}^2\right]=\frac{1}{V_{\mathrm{ee}}^2}$

The delay function of the signal's components in the medium τ(x,ω), is similar to a time function. It is smooth, it can be differentiated and determined through effective parameters—such as effective velocity or the timing of the arrival of waves—used in the kinematic problems of the seismic survey.

Nevertheless, in this case there is a ray-transformation in accordance with which both geometric and absorption factors can be taken into account.

The problem consists of constructing signal parameter distributions (energy density and spectral characteristics) for various combinations of medium properties, source placement, and signal focusing zone location per coordinates.

It is worth noting the theoretically described and experimentally confirmed priority of surface waves in the energy balance of the elastic (vibrational) impact on the ground-air interface. Most of the energy of the source (65-70%) is expended on the formation of surface waves, and only 30-35%—on that of volumetric waves. In addition, it is essential to take into account the properties of the near-surface (loose) zone as well as the influence of the contact between ground and vibrator.

Virtually all real geological environments are much more complicated than every velocity model considered above. However, modern seismic surveys systems have all the necessary means for correctly predicting the conditions for focusing the wave field in any complex conditions. To substantiate the technical possibility and advisability of focusing elastic energy in a geological environment, it is sufficient to approximate the main factors that influence this process. With ray-transformation both geometric and dissipation factors can be taken into account. So the problem consists of constructing signal parameter distributions—energy density and spectral characteristics, for various combinations of medium properties, source placement, and signal focusing zone location.

In the first modeling phase, we confine ourselves to a narrow, 10-30 Hertz seismic signal range. Such oscillations are weakly absorbed by the real medium and contain the main share of volumetric seismic wave energy (if they are emitted from the surface). In addition, serial seismic vibrators are the most stable and reliable in this frequency range, producing power of up to 300 kilowatts.

FIG. 3 shows how the primary regularities of elastic energy focusing change in the model conditions described above. Parameter values are shown in the table as the numbers for each option. Model energy distributions are formed for a horizontal cross-section of the focusing domain. The result is normalized, since in this case the shape of the signal and its relative change in the focus area—rather than the absolute value—is important.

In accordance with general theoretical assumptions, an increase in the sweep frequency from 10-30 Hz (option 31) to 30-60 Hz (option 32) narrows the coherent imposition area of the signals from different oscillation sources. Accordingly, the focusing area along the X and Y axes is reduced in proportion to the increase in the original signal's frequency. At the same time, in the harmonic mode at 50-50 Hz (33) so-called “mirror” effects appear, associated with a quasi-coherent accumulation of oscillations in the point positions, where the distances to the sources is a multiple of the emitted wavelength.

Options (34-35) illustrate a “quality factor”: the nonlinear dependence of the signal focusing system on the distance between surface oscillation sources. When the sources are placed at distances commensurate with the wavelengths, in this case −0.2 km (34), the signal focusing area expands. If the removal of oscillation sources are commensurate with the focusing depth −1 km (35), it is much more effective with regard to in-phase signal accumulation and focus zone localization. At the same time, a further increase in deletions of up to 1.5 km does not significantly manifest in the model.

All things being equal, the energy flux density is determined by the number of elastic vibration sources in the group. In FIG. 4, this dependence is shown using the example of one source (option 41), nine sources (42) which are located symmetrically with respect to the center of the group [3×3], and twenty-one sources (43) which have grouped as [7×3]. The distribution of elastic energy flux density in the vicinity of the focusing point is shown in the horizontal section by the XOY plane and in the vertical section by the XOZ plane. In the case of a single vibrator (there is no focusing), the indicated energy parameter smoothly decreases along coordinates in accordance with geometric divergence and dissipation rules. The magnitude remains in area 10⁻² [J/ (m²*s)]. With an increase in the number of oscillation sources to 9 (42), a region of increased power density (44) appears, up to 1.0 [J/ (m²*s)] at 9 sources and up to 4.0 [J/ (m² *s)] at 21 sources, respectively. In accordance with elementary theory, the energy in the harmonic oscillator model should be in a quadratic dependence on the amplitude. So the amplification of the signal in with 9 oscillator sources should lead to increase in energy of 81×. However, since the geometric divergence and dissipation processes are operating constantly, this value (at a given focusing depth, frequency range, and wave velocity) decreases to about 50. Similarly, 21 vibration sources would yield a value not of 441, but 200. The reduction is significant, but not fundamental.

The symmetry breaking in the location of the source on the surface has been inherited in the structure of the signal focusing area. In the case of (43), the number of vibrators along the Y-axis is smaller than along the X-axis. Therefore, this area expands along Y. However, the intensity of the side maxima (45) decreases with an increase in the number of sources.

An important practical problem of elastic energy focusing technology implementation for enhancing oil recovery is the possibility of changing the focusing region position in the lower half-space without moving oscillation sources on the earth surface.

FIG. 5 shows the model representations for the three focus positions: 0 km, 0.5 km and 1.0 km in coordinate X and its three positions: 2.5 km, 3.0 km and 3.5 km in coordinate Z, with a fixed placement of 21 vibration sources on the surface. The initial modeling parameters and the results are related to each other by option numbers (51-59). With depth, the energy efficiency of focusing naturally decreases; the horizontal displacement increasingly affects the shape of the focus area of the signal and, to a lesser extent, its intensity. It should be noted that the intensity changing at the region of +/−1 km horizontally and 1 km vertically (with a fixed position of the source group), remains within 30% -35%, relative to the “average position” x_.=(0,0,3).

The most important feature of the invention, however, is the limited number of vibrators −N, that can be used in a group composition for the signal focusing. Large seismic survey projects suggest the simultaneous use of dozens of elastic wave sources. At the same time, existing source management tools in the group provide a wide range of control over operating modes such as start time, duration of operation and sweep signal parameters.

FIG. 6 presents the results of modeling the dependence of energy density on the N and signal focusing depth. This is the actual change in energy density [J/(m̂2*s)] when N increases from 0 to 40 for a constant depth of focus (61) and energy density increment [J/(m²*s)] when N increases per unit (62). The focusing depth [km] for each curve is shown the callouts. Due to the influence of the above-mentioned processes of geometric divergence and dissipation, the greatest energy in the focusing effect takes place at shallow depths of 1-2 km. With increases in depth, this expression of energy is leveled, while absolute values of energy density correspondingly decrease. For a depth of more than 5 km, with the N=40, the density of elastic energy no longer reaches 1 [J/(m²*s)]. Nevertheless, for N=20-30 units, this value is two orders of magnitude higher in comparison with the case N=1.

In sum, the main conclusions of the results of simulation are:

• • Focusing of seismic signals from a plurality of vibration sources provides a concentration of elastic energy in a limited volume of the lower half-space, commensurate to the wavelength of the lower frequency sweep signal;
  • The location of the focusing area in the medium can be substantially changed through surface and depth coordinates without changing the vibrator system position, as well as without significant losses of energy efficiency;
  • An increase in the power flow in the focus area of the signal and the density of elastic energy from a number of sources in the group (N=10-30) is achieved approximately N^1,3-N² compared to a single vibrator use.

All the above arguments confirm the possibility of constructing an industrial technology for elastic impact on hydrocarbon reservoirs from the day surface to enhance oil recovery, using existing field equipment and seismic survey information systems.

FIG. 7 displays an interaction of the elastic stimulation system components in the reservoir. The sources of seismic waves (71) have been positioned on the day surface with the proper relative distance and distribution. In addition, the conditions are observed for safe removal of the powerful vibrating installations in the surrounding production facilities and constructions.

It is possible to use both variants of source positioning accepted in seismic prospecting with given coordinates (topographical pickets) as well as arbitrary landmarks, with subsequent measurement of coordinates by GPS. The sequence of switching on the vibrators is set by the system control station in accordance with the current position of the signal focusing point and the velocity distribution into the rock massif. The start time to actuate each source (71) is calculated so that the initial phases of elastic vibrations from all sources coincide in a predetermined focus point. At some point in time, the wave fronts (73) occupy the position shown in the figure. In the vicinity of the point (74), the fronts of elastic waves (lines of equal propagation times) from different sources intersect with each other. As a result, elastic waves from each source are added together in equal phases. The process can be repeated cyclically:

• • For accumulation of elastic energy at a given focus point;
  • To extend the impact zone within the reservoir, the focus point has been shifted but the location of the sources remains unchanged;
  • For moving to another object (seam) with a change in the relative location of the vibrators, as well as the depth, shape, and duration of the sweep signal.

A practical and significant adaptation of the present invention is an industrial technology for increasing enhanced oil recovery (EOR) by elastic action from a daytime surface.

Traditionally, options in the implementation of such technologies have been described in oilfield recovery projects, where for each local deposit (impact object) a particular program is defined that determines the modes and the execution of a sequence of the corresponding technological operations and events. FIG. 8 shows a technological scheme as a block diagram of the relevant basic procedures and cycles:

• • Procedure (81) provides for the systematization and storage, in the form of a project database, of all information necessary for project implementation in accordance with the spatial location of impact objects—including:
    • Coordinates of the source positions;
    • The model of velocity distribution in the geological environment and data for calculating the time delays in the vibrator operation process;
    • Parameters of the sweep-signal for a given sequence of signal focus points. All formats for the information digital descriptions, as well as the data transferring protocols, are determined by the technical regulations that are used to carry out the project.
  • Procedure (82) presupposes placing sources of elastic vibrations (71, FIG. 6) on the day surface, based on the project database section content which applies to a current local hydrocarbon object—including coordinates, size, and depth of the reservoir location.
  • Procedure (83) provides an initial setting of operation control parameters for the previously positioned sources of elastic vibrations. The specified program is executed through the station (72, FIG. 7) to set the given position and modes of the signal focusing process.
  • Procedure (84) generates a set of commands and control parameters for triggering a single impact in a given zone of a local hydrocarbon deposit. As part of the control function, sweep-signal parameters have been established in accordance with predicted characteristics of energy distribution in the focus area (74, FIG. 7) as well as the table of the start time for each elastic vibrations source (71, FIG. 7). Before the procedure ends, the control station (72, FIG. 7) checks the system to ensure it is ready to work and then sends the command to start it.
  • Procedure (85) is performed automatically by all elements of the system in accordance with the program of elastic impact through the station (72, FIG. 7).
  • Cycle (86) presupposes repetition of processing a given focus area to achieve the given elastic impact duration in accordance with the project.
  • Cycle (87) involves the moving the energy focusing area within the local target object.
  • Cycle (88) involves reinstallation of the focusing system to a new layer or another productive formation located above or below the current impact object.

There is significant potential in employing wave processes generated on the earth surface for oil and gas fields recovery. In this case, there is a possibility of volumetric concentration of elastic energy commensurate with the energy intensity of formation flooding under external pressure and other advanced oil recovery technologies. According to some experts, this is essential not only for both the recovery of depleted (marginal) deposits and the stratum-fluid condition optimization of objects in the initial field development phase.

Among the issues associated with the use of such elastic energy, three deserve special attention:

• • 1. The possibility of controlling the energy level in the localized area of a geological environment.
  • 2. The conformity of vibration energy with respect to mechanical properties of a reservoir system.
  • 3. Modern seismic vibration technologies do not harm oil or gas production infrastructures.

There are also significant prospects for the application of more complex schemes of elastic impact on formations both in combination with known seismo-acoustic (primarily borehole ultrasonic) and geomechanical (primarily depressive-repressive and hydro-shock) technologies.

Data from geophysical research has made it possible to construct a wave field model in a real environment with high precision. This model defines the kinematic characteristics of focused elastic waves emitted from the surface.

The usage of elastic wave focusing enables control of the signal level in the environment. Substantial changes in the relative energy of elastic impact near the focus point (in the wavelength range of about 30-80 meters) affect the physical characteristics of the fluid-filled pore space, but do not destroy the well's casing or the cement sheath around the casing.

All this creates favorable prerequisites for the production practical application of the present invention.

The usage of the elastic waves focusing provides the possibility to control the signal level in the environment. Substantial changes in the relative energy of the elastic impact near the focus point and insignificant distance from it (in the wavelength range of about 30-80 meters), affect the physical characteristics of the pore space filled with fluid but does not destroy the well's casing or the cement sheath around the casing.

All this creates favorable prerequisites for the production practical application of the present invention.

Claims

1. A method of enhanced oil recovery through elastic impact on productive formations based on the use of vibration sources and control units utilizing seismic surveys. The method is characterized by the concentration of elastic energy on local areas in the rock massif, with the dynamic, duration and intensity of the impacts regulated according to the distribution, shape, size, and location of the effect zone. The method utilizes numerous elastic wave sources located at varying distances from each other. In accordance with these distances and the distribution of the massif's elastic parameters of the massif, the start time, duration and parameters of the emitted signals are tuned for each oscillation source to focus waves in a given zone of impact. The energy efficiency necessary for enhanced oil recovery is achieved through cyclical repetition of focused elastic impact action. Damage to environment and to the production infrastructure are also minimal compared to other methods.

2. A method according to claim 1: To optimize the elastic impact on the porous collector and the hydrocarbon-fluid in the pore space, several groups of oscillation sources are used—each of which is tuned so that the wave can be focused with the different dynamic characteristics needed to influence the rock and fluid, respectively. As the elastic waves from vibrational sources are being focused, their spectral composition and the duration of elastic oscillations stream (sweep-signal parameters) can also be altered.

3. A method according to claims 1-2: In order to improve the efficiency of oil recovery enhancement, the focusing of elastic energy is consensual in time with other processes of the fluid inflow intensification, such as a depression and repression sequences, ultrasonic, electromagnetic and magnetostrictive stimulation methods in the wells. The dynamics of the elastic action—and, accordingly, the operating modes of the vibration sources (groups of sources)—are consistent with the dynamics of these processes.