Oil-saturated Porous Media Research Articles

Estimation of gas diffusion coefficient for gas/oil-saturated porous media systems by use of early-time pressure-decay data: An experimental/numerical approach

This paper presented a novel numerical method for estimating the gas diffusion coefficient based on the early-time pressure-decay data. Experimentally, “flooding–soaking” procedures were developed to perform the gas diffusion in an oil-saturated tight core under different gas phase volume conditions. After flooding, the capillary bundle model was used to calculate the oil–gas contact area. The early-time pressure-decay data of the gas phase were monitored and recorded during the soaking process. Theoretically, a non-equilibrium inner boundary condition coupled with the characteristics of experimental early-time pressure had been incorporated to develop a diffusion model for a gas/oil-saturated tight core system. Based on gas-phase mass balance equations and gas equation of state, the diffusion coefficients were optimized once the discrepancy between experimental data and numerical solutions was minimized. According to the estimated results in this study, the CH4 diffusion coefficients were 3.74 × 10−11 and 3.86 × 10−11 m2/s in tight core saturated with crude oil, respectively. Moreover, the oil–gas contact area significantly impacts the diffusion flux in oil-saturated porous media. Specifically, an additional 10% contact area results in a 75% increase in CH4 diffusion mass. In addition, with the application of our proposed model to CH4/bitumen and CO2/bitumen systems, the diffusion coefficients were in close agreement with the results reported in previous literature, indicating that the proposed model was applicable to both gas/liquid and gas/liquid-saturated porous media systems.

Investigation of gravity influence on EOR and CO2 geological storage based on pore-scale simulation

Gravity assistance is a critical factor influencing CO2–oil mixing and miscible flow during EOR and CO2 geological storage. Based on the Navier–Stokes equation, component mass conservation equation, and fluid property–composition relationship, a mathematical model for pore-scale CO2 injection in oil-saturated porous media was developed in this study. The model can reflect the effects of gravity assistance, component diffusion, fluid density variation, and velocity change on EOR and CO2 storage. For non-homogeneous porous media, the gravity influence and large density difference help to minimize the velocity difference between the main flow path and the surrounding area, thus improving the oil recovery and CO2 storage. Large CO2 injection angles and oil–CO2 density differences can increase the oil recovery by 22.6% and 4.2%, respectively, and increase CO2 storage by 37.9% and 4.7%, respectively. Component diffusion facilitates the transportation of the oil components from the low-velocity region to the main flow path, thereby reducing the oil/CO2 concentration difference within the porous media. Component diffusion can increase oil recovery and CO2 storage by 5.7% and 6.9%, respectively. In addition, combined with the component diffusion, a low CO2 injection rate creates a more uniform spatial distribution of the oil/CO2 component, resulting in increases of 9.5% oil recovery and 15.7% CO2 storage, respectively. This study provides theoretical support for improving the geological CO2 storage and EOR processes.

Reduction of formation damage in horizontal wellbores by application of nano-enhanced drilling fluids: Experimental and modeling study

One of the basic challenges during drilling horizontal wellbores is the damage induced by invasion of mud filtrate into the formation. Addition of nanoparticles to drilling fluids has been recognized as a measure of control and reduction of filtrate invasion, which is the primary mechanism of the aforementioned formation damage. Despite notable advances in composing Nano-enhanced drilling fluids, the role of nanoparticle hydrophobicity on performance of the fluids has not been well studied. This study is based on a combined experimental-numerical methodology. In the experimental section, a procedure to find the optimum composition of Nano-enhanced water-based samples, containing nanoparticles of hydrophilic/hydrophobic silica and lipophilic clay to minimize the formation damage associated with mud circulation is recommended. The main idea was to investigate the effect of hydrophobicity of nanoparticles on damage reduction and furthermore, discover how other properties such as types/concentration of nanoparticles/micro-sized additives, temperature and pressure would affect functionality and characteristics of drilling fluids. In order to more accurately and practically investigate any probable enhancements in performance of fluid samples a particular radial filtration setup was designed and used; then, functionality of mud samples was examined on grain packed porous media at radial flooding flow condition. Results revealed that samples containing 0.2 wt. % hydrophobic nanosilica had the best functionality with returned permeability of 68.4% and 51.1% for the cases of water saturated and oil saturated porous media, respectively. In the modeling section, mass balance and momentum equations were solved simultaneously by using elapsed time data. Permeability as well as thickness of mud cake formed on the wellbore wall were evaluated and compared for different Nano-enhanced mud samples. Results showed that nano-enhanced drilling fluid samples generated thinner mud cakes with lower permeability values during quite shorter period of time. Results of this work might be helpful to better understand the behavior of nano-enhanced drilling fluids in radial porous systems at different conditions of nanoparticles hydrophobicity and applicable to minimize the induced formation damage during drilling of the horizontal section of a production well. • A combined experimental-numerical methodology is used to evaluate any possible enhancement in characteristics of drilling fluid, mud cake and damaging potential. • The effects of hydrophobicity of nanoparticles along with types/concentration of nanoparticles/ micro-sized additives, temperature and pressure on characteristics of drilling fluids are investigated. • A particular radial filtration setup is designed and used to more precisely evaluate the damaging potential of mud samples and to measure permeability recovery after mud filtrate invasion. • Samples containing 0.2 wt. % hydrophobic nanosilica had the best functionality with return permeability of 68.4% and 51.1% in water-saturated and oil-saturated porous media, respectively. • In the modeling section, mass balance and momentum equations are solved simultaneously; permeability and thickness of filter cake are evaluated and compared for each of mud samples.

Finite element-based evaluation on the role of various shaped containers (oil saturated porous media) for entropy generation versus heating efficiency involving thermal convection with identical heating

Current work involves entropy generation studies within nine porous containers involving identical area for Prm = 155 (engine oil), at The entropy generation maps for lesser values of Dam depict that is dominant at the boundaries whereas is dominant at the central zone. On the other hand, for larger values of Dam , is found to be dominant throughout the porous containers. Zones corresponding to are found at the bottom corners of all porous containers due to the hot cold junction whereas zones corresponding to vary with the shapes of porous containers. An energy efficient process corresponds to larger larger and lesser Stotal . Containers with curved walls are found to be thermally efficient for thermal processing of engine oil saturated porous medium.

Instability of mixed convection in a differentially heated channel filled with porous medium: A finite amplitude analysis

This paper reports the instability mechanism of parallel mixed convection flow in a differentially heated vertical channel filled with a highly permeable porous medium. Linear and weakly nonlinear stability analysis involving the finite-amplitude expansion method is considered to investigate the instability mechanism of the flow. Darcy–Brinkman's model is considered. The results are presented for both water-saturated and oil-saturated porous medium flows. The linear stability results show that the stability of the flow decreases on increasing the Reynolds number as well as the Darcy number, and the contribution of viscous dissipation in the kinetic energy balance is not negligible for highly permeable porous medium flows. The results from the weakly nonlinear analysis show only supercritical bifurcation in the vicinity of the critical or bifurcation point for both the fluids; however, for water, the parallel flow may experience subcritical bifurcation away from the critical point, which depends on the value of the Darcy number. The variation of neutral stability curves of the parallel flow of water reveals that a bifurcation that is supercritical for some wavenumber may be subcritical at other nearby wavenumbers. The nonlinear interaction of different harmonics enhances the heat transfer rate as well as the friction coefficient in the linearly unstable regime. A comparison with the results using a model based on volume averaged Navier–Stokes equation reveals the possibility of subcritical bifurcation even in the vicinity of the critical point.

Production and physicochemical characterization of bacterial poly gamma- (glutamic acid) to investigate its performance on enhanced oil recovery

Bacillus licheniformis LMG 7559, which is capable of producing extracellular poly gamma- (glutamic acid) (PGA), was provided for the biopolymer synthesis. Using a modified PGA medium for PGA production, the isolated biopolymer, undergone dialysis process mainly for desalination and removal of other impurities. The bacteria produced high molecular weight biopolymers with a weight average molecular weight (M̅n) of 1.6 × 105 g/mole identified by gel permeation chromatography (GPC). Furthermore, GPC analysis was utilized to determine the poly-dispersity of PGA as well as molecular weight variation by cultivation time. The heavy weight fraction of 1.85 × 105 g/mole with poly-dispersity index of 7.42 was distinguished. For the extracted and dialyzed biopolymer, thermal properties were studied using DSC/TGA by which a mass loss of 36 percent was observed. Eventually, the biopolymer solution was injected into the oil saturated heterogeneous porous medium to evaluate the recovery factor enhancement by PGA flooding. It was found that 31.45% of oil in place was recovered by biopolymer flooding, whereas only 16.6% of oil in place was obtained by water flooding.

Study on Diffusivity of CO2 in Oil-Saturated Porous Media under High Pressure and Temperature

To figure out the influence of different factors on the diffusion of carbon dioxide (CO2) in oil-saturated porous media, an experimental method using a multifunctional core displacement instrument ...

Mathematical modeling of high-viscosity index oil formation with a sonic field at various frequencies

The paper describes the process of acoustic wave absorption in the oil-saturated porous medium in the case of cylindrical wave propagation. Terms of thermal and viscous-inertial absorption are introduced; the total attenuation ratio of the acoustic waves is calculated for various sonic field frequencies. The expression for sonic field thermal sources previously developed by the author is used to present the graphs of heat source distribution through the deposit in the cylindrical system of coordinates. The paper also considers a combination of the sonic field with a high-frequency electromagnetic field and some cross effects accompanying such a combination. Thus, it has been discovered that the energy of the sonic field is converted into heat in a closer near field of the deposit as compared to the electromagnetic field. At that, increasing frequency of the field leads to increased absorption zone in the radial direction.

A pore-scale study on improving CTAB foam stability in heavy crude oil−water system using TiO2 nanoparticles

The main challenge of foam injection for enhanced oil recovery (EOR) is foam stability at harsh reservoir conditions. With regard to this matter, this research aims to investigate the effect of TiO2 nanoparticles (NPs) in improving the stability of foam bubbles in porous media. The conductivity of hexadecyltrimethylammonium bromide (CTAB) solutions was measured to determine critical micelle concentration (CMC). Thereafter, TiO2 NPs were added into the foam solution and pH, contact angle, and interfacial tension (IFT) in the presence and absence of CTAB were measured. The stability of foam bubbles was evaluated through bulk foam tests and absorbance measurements. Finally, different nanofluids (at the concentrations of 0.00, 0.01, 0.03, 0.06, and 0.10 wt%), nano-CTAB solutions (at the concentrations of 0.00, 0.01, 0.03, 0.06, and 0.1 wt% NPs at the CMC of CTAB) and foams (at the concentrations of 0.00, 0.01, 0.03, 0.06, and 0.1 wt% NPs and at the CMC of CTAB) were injected into a heavy oil saturated heterogeneous porous medium. The results of the conductivity measurements showed that the highest adsorption of CTAB molecules onto the NPs surfaces occurs at 0.03 wt% NPs and 0.03 wt% CTAB. The results of micro- and macroscopic images from bulk foam tests revealed improved foam stability for this nano-CTAB foam. Moreover, contact angle and IFT results showed further improvement in interfacial properties for the mixture of NPs and CTAB solutions. Ultimately, in terms of oil recovery, the highest oil recovery factor (54%) was observed upon injection of foam with 0.03 wt% TiO2 NPs and 0.03 wt% CTAB, compared to the 22% recovery factor of DW injection. The visualization experiments showed that 32% enhancement in the recovery factor was mainly due to oil production from dead−end pores. This was because of different functions of NPs including stabilization of foam bubbles, wettability alteration, and improved foam generation mechanisms.

Measurement of CO2 diffusion coefficient in the oil-saturated porous media

Molecular diffusion is an important EOR mechanism in naturally fractured reservoirs. However, the laboratory-measured diffusion coefficient in the fractured porous media is still limited; and grid sensitivity analysis is missing in the literature when the single-porosity system is applied to history match the pressure decline curve. We aimed to fill the gaps using Radial Constant Volume Diffusion (RCVD) method experimentally to investigate diffusion coefficients at different pressures in hydrocarbon saturated porous media. A special in-house cell is designed to hold the core sample in the center with the annulus around simulating the fracture. The core is initially saturated with oil while the annulus is filled with CO2 at the same pressure. During the measurements, the system pressure declines as gas diffuses into the oil phase until it reaches chemical equilibrium. The pressure decline curve is history matched to determine the diffusion coefficient. The initial pressure is 597 psi, and the diffusion coefficient is determined in numerical models accordingly. Molecular diffusion coefficients are estimated at different experiment periods to reveal the pressure-dependency. A workflow is proposed to obtain effective diffusion coefficients in dual-porosity models that could be extended to multi-component systems. Besides, flow characteristics in the RCVD system are characterized and capillary pressure effect is investigated in this study.

Core Scale Modeling of Polymer Gel Dehydration by Spontaneous Imbibition

SummaryCapillary spontaneous imbibition (SI) of solvent (water bound in gel) from formed polymer gel into an adjacent, oil-saturated porous medium was recently observed in laboratory experiments. Loss of water from the gel by SI might influence the blocking capacity of the gel residing in a fracture, by decreasing its volume, and might contribute to gel failure, often observed in water-wet oil fields.This work presents an original modeling approach to simulate and interpret spontaneous imbibition of water from Cr(III)-acetate-hydrolyzed-polyacrylamide (HPAM) gel into adjacent oil-saturated rock matrix. Simulations were compared to experiments on the core scale, using two different boundary conditions: all faces open (AFO) and two-ends-open free spontaneous imbibition (TEOFSI). Capillary forces enable water (used as gel solvent) to enter the rock matrix. The gel particle network itself is, however, inhibited from entering because of its structure, and remains on the surface of the rock matrix. We developed a theory that describes the gel as a compressible porous medium and describes the flow of water through gel. The polymer structure of the gel is proposed to constitute a gel matrix of constant solid volume. Gel porosity, defined by the volume fraction of solvent, is modeled as a function of pore pressure and gel compressibility. Gel permeability is modeled as function of gel porosity using a Kozeny-Carman approach. The flow equations for the gel and core domains were solved simultaneously by implementing the proposed description into the core-scale simulator IORCoreSim. The gel surrounding the core was discretized and included as a part of the total grid.The simulated flow of water through and from the gel occurred in a transient manner, driven by the coupled gradients in gel fluid pressure and gel porosity. Gel porosity initially decreased in a layer close to the core surface because of reduced aqueous pressure, and continued to decrease in layers away from the core surface. The propagation rate was controlled by two main gel parameters: First, gel compressibility controlled the pressure gradient within the gel network, and the amount of water transported from the outer part of the gel toward the core surface to balance the pore pressure; and, second, gel permeability limited how fast water could flow within the gel at a given pressure gradient, thus increasing the time scale of the overall imbibition process.

Waterflooding of Surfactant and Polymer Solutions in a Porous Media Micromodel

In this study, we examine microscale waterflooding in a randomly close-packed porous medium. Three different porosities were prepared in a microfluidic platform and saturated with silicone oil. Optical video fluorescence microscopy was used to track the water front as it flowed through the porous packed bed. The degree of water saturation was compared to water containing two different types of chemical modifiers, sodium dodecyl sulfate (SDS) and polyvinylpyrrolidone (PVP), with water in the absence of a surfactant used as a control. Image analysis of our video data yielded saturation curves and calculated fractal dimension, which we used to identify how morphology changed the way in which an invading water phase moved through the porous media. An inverse analysis based on the implicit pressure explicit saturation (IMPES) simulation technique used mobility ratio as an adjustable parameter to fit our experimental saturation curves. The results from our inverse analysis combined with our image analysis show that this platform can be used to evaluate the effectiveness of surfactants or polymers as additives for enhancing the transport of water through an oil-saturated porous medium.

Porous media acidizing simulation: New two-phase two-scale continuum modeling approach

In Recent years some experimental studies have been conducted to investigate the effects of presence of a non-aqueous phase during carbonate acidizing process. Narrower and longer wormholes were observed to be formed when acid is injected into an oil-saturated porous medium, instead of a water-saturated one; which can provide breakthrough earlier with less injected pore volume. However, the mechanisms responsible for these observations had not been comprehensively studied. In this research, a two-phase two-scale continuum (TPTSC) model for simulation of dissolution process during acid injection into carbonate rocks is presented. This model couples two-phase fluid flow equations in porous media and the previously developed two-scale continuum (TSC) model equations for carbonate acidizing. A synthetic case is simulated and the effect of the restricted flow due to various relative permeability values, viscous fingering effects on injected pore volumes to breakthrough; and dissolution patterns are investigated. Our simulation results demonstrate positive effects of two-phase flow and viscous fingering on formation of narrower and longer wormholes. It is observed that pore volumes to breakthrough for an initially high viscosity oil-saturated core is significantly less than pore volumes to breakthrough for an initially water-saturated core. Flow restriction around wormhole due to less relative permeability values and viscous fingering phenomenon can be responsible for gaining lower injected acid volumes to breakthrough. Thus, especially for high viscosity oil reservoir conditions, direct injection of acid to an oil-saturated medium, instead of injecting a pre-flush to push back the saturating oil, can be beneficial.

Experimental study and simulation of CO2 transfer processes in shale oil reservoir

Understanding the CO2 transfer processes in shale oil reservoir is essential for the development of shale oil reservoir by CO2 injection. Although many mathematical models have been proposed to evaluate CO2 diffusion in oil-saturated porous media, very few models consider the effect of kerogen on the CO2 transfer processes in shale oil reservoir, and research studies systematically combining a mathematical model history-matched to experimental data is also rare. To describe these processes involved in the measurements, a mathematical model and numerical solutions were derived. In solving the mathematical model, the adsorption and dissolution of CO2 in shale and oil-phase swelling caused by CO2 dissolution were considered. From the fitted experimental results of the numerical method, the effective diffusion coefficients of CO2 in shale and tight sandstone were estimated at different pressures and the effective diffusion coefficients in shale were larger than those in tight sandstone at high pressures (>9.1 MPa). The sensitivity analyses for the model reveal that the effective diffusion coefficient, oil-swelling factor, and the adsorption and dissolution quantity of CO2 determine both the pressure decay and equilibrium time of the gas transport processes. The analysis and comparison of the modeling results and experimental results reveal that the effect of CO2 adsorption and dissolution in shale was stronger than the oil swelling. Finally, quantities of the free CO2 and the adsorption and dissolution CO2 in shale sample were calculated by the given model. The effective diffusion coefficients measured through the proposed method already incorporated the characteristics of shale and thus can be used directly in modeling CO2 transport in shale oil reservoir. The study also indicates that the geological storage of CO2 in shale oil reservoir is one of the mitigation measures for the greenhouse effect.

A numerical study on the impact of thermal alterations in porous media during hot fluid injection process employing a modified Boussinesq model

The Oberbeck-Boussinesq (OB) approximation is widely employed as a simplifying assumption for density-dependent flow problems. It reduces the governing differential equations to simpler forms, which can be handled analytically or numerically. In this study, a modified OB model is formulated to account for the variation of rock permeability and porosity with temperature during the hot fluid injection process in an oil-saturated porous medium under the assumption of local thermal equilibrium (LTE). The mathematical model is solved numerically using a fully implicit control volume finite difference discretization with the successive over relaxation (SOR) method to handle the non-linearity. Subsequently, the numerical model is validated with the analytical solution of the simplified problem successfully. Through detailed sensitivity analyses, the simulation results reveal the hot fluid injection rate as the most important operational parameter to be optimized for a successful thermal flood. The numerical runs show that that for single-phase core-flood simulation, the effect of temperature on the rock absolute permeability and porosity can be neglected without introducing any significant errors in the estimated recovery and temperature profile.

Diffusion coefficients of supercritical CO2 in oil-saturated cores under low permeability reservoir conditions

CO2 diffusion in oil-saturated porous media with low permeability is of great importance for the project design, risk assessment, and performance forecast of carbon capture and storage (CCS) or enhanced oil recovery (EOR). This paper developed a method to determine CO2 diffusion in oil-saturated cores under low permeability reservoir conditions. Core, crude oil and experimental parameters were taken from the representative low permeability reservoir. In the solution of the mathematical model, oil saturation was introduced in to diffusion equation, an oil-phase swelling caused by gas dissolution was considered, but a water-phase swelling was not, which is in agreement with the actual diffusion situation. The error caused by the state equation of carbon dioxide was eliminated, improving the calculation accuracy of the diffusion coefficient. The effects of pressure (6.490–29.940MPa), temperature (70–150°C), oil saturation (0–63.58%) and permeability (8.62–985.06mD) on the diffusion coefficient of supercritical CO2 in low-permeability reservoirs were studied. The order of the diffusion coefficient is from 10−10 to 10−9m2/s. The results show that with an increase in pressure and temperature, the CO2 diffusion coefficient in the porous media saturated with oil firstly increases significantly and then the rate of increase gradually slowed down. The CO2 diffusion coefficient increases greatly with the oil saturation in porous media. The CO2 diffusion coefficient first increases greatly with permeability, and when the permeability of the core is greater than approximately 100mD, it remains almost stable. The experimental results can provide theoretical support for CO2 transport in porous media.

3-D visualization of diffusive and convective solvent transport processes in oil-saturated porous media using laser technology

A technique to visualize miscible displacement in porous media is introduced in this paper. After saturating the model made of different sized glass beads with oil, solvent was introduced to mix and displace it. The refractive indices of saturated and injected fluids were made the same by mixing them with lower and higher indices of refraction. Refractive index matching made the model transparent. Fluorescent dyes that were only visible with excitation of laser were dissolved in the solvent. A laser sheet scanned the model while synchronous pictures were taken by two high speed cameras from two sides of the model. Two groups of models representing diffusive (no injection) and convective (injection of solvent) interaction, respectively, were considered to test the visualization system: (1) solvent diffusion under purely static conditions, and (2) injection/production through a pair of horizontal wells. Visual data were analyzed and the limitations of the visualization technique introduced were presented.

Three dimensional visualization of solvent chamber growth during the VAPEX processes: An experimental approach using laser

VAPEX is a non-thermal process proposed as an alternative to SAGD due to its applicability in thinner reservoirs, advantage of energy efficiency, produced oil quality, and environmental saving. Its high cost caused by the injected solvent, however, requires further efforts to optimize the process. This, in turn, entails a clear understanding of the physics of the process. To acquire this understanding, a 3-D visualization technique was applied to a VAPEX model packed with glass beads. The oil saturated porous media and the solvent injected into the model processed refractive indices matched with the glass beads, which made the model transparent. Laser illuminated the yellow fluorescent dye dissolved in the solvent to distinguish it from the oil. Two high-speed cameras recorded the process from two sides of the model while a laser sheet moved along the model. A dome-shaped solvent chamber with viscous fingering on its top was observed. The injection pressure influenced the size of the chamber especially in the horizontal direction, which was not possible to identify through standard 2-D tests. Also, the solvent spread along the ceiling of the model and then displaced the oil down. The boundary of the solvent and oil was tilted and even curved at different injection rates. Finally, variable injection rate cases were compared with constant rate trials and an optimum operation plan was proposed through the analysis of the production rate, produced oil quality, and 3-D images of the chamber growth.

Synthesis and Characterization of Yttrium Iron Garnet (YIG) Nanoparticles Activated by Electromagnetic Wave in Enhanced Oil Recovery

Due to the geographical location and technological limitation, various novel enhanced oil recovery (EOR) methods has been proposed to recover the remaining oil from a depleted oil reservoir. Research on application of nanoparticles either on its own or coupled with other stimulating agents has been growing enormously and some of them have shown a promising future. In high temperature and high pressure reservoirs, thermal degradation will cause failure to the conventional chemicals. In this work, temperature-stable YIG magnetic nanoparticles with an electromagnetic wave has been proposed as a new candidate for reservoir stimulating agent. The purpose of nanoparticle injection is to increase the sweep efficiency in the reservoir by increasing the viscosity of displacing fluid. In this research, Yttrium iron garnet (YIG) nanoparticles have been injected into a waterflooded oil saturated porous medium to recover the remaining oil in the presence of an electromagnetic wave. At the sintering temperature 1200°C, a mixture of hematite and YIG was obtained, suggesting a higher temperature for single phase YIG. From VSM analysis, the average magnetic saturation, coercivity and remanence are 18.17 emu/g, 21.73 Oe and 2.38 emu/g, respectively. 1.0 wt% of YIG nanofluid was prepared and subsequently injected into the pre-saturated porous medium in the presence of square electromagnetic wave of 13.6 MHz. As much as 43.64% of the remaining oil in place (ROIP) was recovered following the injection of 2 pore volume of YIG nanofluid.

Seismoacoustic emission of an oil-producing bed

Results are presented from a study of seismoacoustic emission appearing in an oil-saturated porous geological medium under the acoustic force action in a borehole. It is shown that dynamic nonlinear processes in the producing bed are activated under the internal elastic action on the stratum, changing the energy state of the medium, and this change can be seen as a change in the acoustic emission pattern. The correlation between the high-frequency part of the acoustic emission spectrum and the low-frequency one is found, indicating the development of this process in space at different scale levels.