N.P. Ageev, Ph.D., General Director, Novas Group of Companies - “Georezonanse” - companies, residents of the Skolkovo State Foundation;
P.G.Ageev, Deputy Director General for Science, developer of the PPT method;
A.S. Desyatkin, Ph.D., Chief Engineer of Projects; G.A. Elsukov, chief technologist.
The magazine Mining Industry No1 (119) / 2015 published an article that considered a possible “triggering mechanism” of gas-dynamic processes leading to unexpected gas and rock emissions into the working space of mines during any underground work, and also justified the need to use plasma pulsed treatment (PPT) for the advance degassing of coal seams. In order to create a wide network of microcracking in the coal seam to increase the permeability and, consequently, improve the conditions for maximum methane desorption, it was proposed to use the “Ideal nonlinear plasma-pulse source of directional controlled periodic broadband oscillations” (PPT Generator), which establishes a non-linear relationship with a multilayer nonlinear bulk elastic modulus containing a non-equilibrium dissipative dynamic system (coal - water - gas)."
A specific feature of PPT technology is the absence of man-made intervention in the stress-strain state of coal seams, which is the case, for example, with hydraulic fracturing or hydraulic dissection, and even vice versa, periodic broadband plasma-pulse action, by creating compressive and tensile stresses, improves the permeability of the coal seam without changing the physical and mechanical properties and qualitative characteristics of coal. It has been noted previously, that during PPT, due to the removal of surface tension in the pores, cracks, capillaries, the light fractions replaced the heavier ones, which led to a reduction in porosity and an increase in permeability.
In order to confirm or refute the declared capabilities of the PPT technology and to improve the treatment methodology, in March-May 2015, research and development work was carried out in the framework of R & D on passive microseismic monitoring on an area of more than 40,000 m2 in two of 4 degassing wells drilled in domes of the collapse of the developed lava. The distances between them were: 200m between А-1 & А-2; 400m between wells А-1 & А3, and А-2 & А-4 accordingly. Plasma-pulse treatment was carried out in all wells according to a specially selected technique, in order to increase permeability and, consequently, create conditions for methane desorption from both sorbed surfaces and from closed pores (vacancies) in the coal lattice. In addition, it was necessary to determine the radius of the impact zone (seismic events), which will allow in the future to choose the optimal distance for drilling wells in the domes of the collapse and conducting PPT in them.
The choice of passive microseismic monitoring for the study of process and results of PPT is related to the fact that it does not require powerful sources of probing signals, but uses permanent weak seismic fields of artificial or natural origin.
Since the seismic emission occurs in the geological environment due to the relaxation of elastic stresses by spontaneous deformations of the medium, often associated with discontinuity, the physical fundamentals of passive microseismic monitoring technology fully corresponded to the effects (compressive and tensile stresses) triggered by a plasma-pulse source of oscillations.
The process of changing elastic stresses is associated both with natural factors, mainly due to the geodynamics of the environment (tectonic movements, sedimentation, lunar-solar activity, etc.), and with the influence of various man-made exogenous and endogenous factors. The emission resulting from anthropogenic impact is the response of the environment, which is called induced seismoacoustic activity.
Thus, the purpose of the pilot works is to identify the areas of microseismic activity in the process of conducting PPT at different times in four wells drilled in the collapse domes and located at a distance of 200 and 400 m, respectively. The coal seams themselves are represented by interlaying sandy-argillaceous rocks with layers and numerous coal beds. The area on which the treatment was made is an almost perfect monocline with an angle of incidence of 9–12 ° to the southeast. The reference points in the wells vertically make up the difference of 80 m.
Simultaneously, video logging was carried out in the wells during the PPT. To do this, a digital video camera was lowered to the free water surface on the cable, which recorded in real time the process of fluid level recovery, indicating the connection of wells with the reservoir and reflected on the monitor located on the surface near the wellhead.
As a result of video logging, the growth of the hydrodynamic level in two wells of different depth after drilling and casing was recorded - from 75 to 65 m and from 67 to 45 m, respectively, with active release of gas bubbles, which indicated true reservoir pressure and inflow of carbonated water from the seams, which were treated by plasma-pulse effect.
Microseismic emission was recorded using an areal observation system - a seismic antenna based on 28 three-channel seismic stations SCOUT (developed by SKB Seismic Instrument-Making), which were installed in the projection of the horizontal wellbore section onto the surface.
During the observation period from March 23 to May 4, 2015, more than 232 GB of information was recorded. The processing of microseismic monitoring data carried out during the PPT process and after treatment was carried out by the specialists of JSC TechObraz.
A general map of the distribution of microseismic emission sources is produced, taking into account the magnitude of the reliability of the selected sources of seismic events, and their main parameters are determined: spatial coordinates; event time; frequency response; mean seismic wave propagation velocity from the source to the seismic antenna.
To reduce the influence of surface waves and man-made noise, all seismic antenna sensors were installed in boreholes up to 0.5 m deep, which were closed and covered with a layer up to 0.1 m from local soil. The total number of observation points of the seismic antenna was 84, and the intervals between them were 20–30 m. The seismic antenna aperture was 800 m from north to south and 600 m from west to east. Frequency digitization kHz (sampling period of 1 ms).
In well A-1, located in the upper part of the slope, 703 plasma pulses were shot.
The antenna recorded signals of microseismic activity at the field from an artificial radiation source, which in the spectral-temporal diagram ranged from 35 to 4 Hz.
It is known that the fundamental period of the resonant frequency of free oscillations of the Earth is in the range of 58–60 s and the fundamental frequency ranges from 0.2 to 10 Hz and higher, depending on the geological characteristics of the rock. Consequently, the identified periodic broadband signals of the same magnitude, separated by equal periods of time from the PPT source, just fell into the range of fundamental frequencies, initiating shear, transverse oscillations.
As a result of the interpretation, maps of the registered microseismic events and the energy density of the microseismic emission projected on the day surface were built. In the first case, in the study of TID in well A-1, the total number of events corresponded to the number of pulses. The energy of methane emission from coal and the creation of microcracks was 3004 kJ. The maximum shear stress energy was 1950 kJ, the compression energy was 42.88 kJ, the separation energy was 35.83 kJ.
The map was built in a projection on a horizontal plane (X - West-East; Y - North-South). Coloration indicates the density of the accumulated energy of microseismic events. The coordinate axes in meters, the values on the axes show the distance relative to the wellhead No1 (white point).
The highest energy density of microseismic events was observed under a seismic antenna, in the area of the bottom hole of the A1 well. The area of microseismic activity has an ellipsoid shape with a predominant direction of development in the south-west azimuth of 225є. The approximate linear dimensions of the microseismic activity area are 1500 meters in the direction from north to south and 1250 meters in the direction from West to East.
Interesting results were obtained when conducting microseismic monitoring of PPT at well A-3, which differed in the direction of an increase in depth from well A-1 by more than 100 m taking into account reference points. In this well, 1180 periodic wideband pulses were shot.
Maps of the registered microseismic events and the energy density of the microseismic emission projected on the day surface were built.
The methane emission energy from coal was 2215 kJ, the maximum shear stress energy was 1446 kJ, the compression energy was 19.1 kJ, and the separation energy was 18.16 kJ.
The highest energy density of microseismic events was observed under a seismic antenna, in the area of the bottom hole of the wells. The area of microseismic activity stretched from north to south. The approximate linear dimensions of the microseismic activity area are 1350 meters in the direction from north to south and 1000 meters in the direction from West to East.
All indicators: energy costs and zone of influence - in the second case were lower than in the first. Obviously, this is due to the fact that only 326 pulses were interpreted instead of 1180.
The percentage of total maximum separation stress energies of 78.70 kJ for well No1 and 37.27 kJ - for well A-3 from the maximum shear stress energies of 1950.60 kJ and 1446.49 kJ of microseismic events, respectively, showed the ratio of the energies spent on opening/closing cracks to the total energy. As can be seen, the energy spent on opening/closing cracks accounted for only 4% of the total seismic energy.
It is difficult enough to explain such a phenomenon. However, we have repeatedly noted that with the use of PPT in oil wells, along with the receipt of positive effects, an additional increase in the dynamic level was observed 3 months after treatment, which may indicate the duration of the process of a given self-modulation. In addition, it cannot be excluded that the plasma-pulse treatment in well A-3 was carried out 10 days after a similar treatment in well A-4, which was located in the same area. Considering the duration of synchronous oscillations and the process of self-modulation, the previously specified oscillations were simply extinguished by subsequent ones, which were initiated and investigated in well A-3.
However, looking ahead, it should be said that the wells A-3 and A-4, which fell into the zone of influence of a larger number of initiated plasma impulses, began to actively release methane. Suffice it to say that the annular pressure in the wells increased to 10 atm, when 110 and 140 m remained to the top of the reservoir, respectively, and the extracted water was saturated with gas, which occupied at least 10–12% in production volume.
According to the analysis, over the observation period, the process of volume reduction prevailed over the process of volume increase, and the process of closing cracks, respectively, prevailed over the process of their opening. In other words, the light phase (gas) passed into a free state from pores, cracks, and capillaries, which led to a decrease in volume, with simultaneous opening of closed pores and cracks and the development of a network of microcracks in a solid interstitial solution (coal). This is eloquently demonstrated by the indicators of the energy of methane emission from coal, in the first case 3004 kJ and 2215 kJ in the second.
In general, it can be stated with sufficient certainty that the observation of the kinematic and dynamic characteristics of the microseismic activity area in the area of the observation system installation indicates an increase in reservoir permeability over a considerable distance over the area, which is more than 1 km2.
As a result, all 4 wells were connected by a single system of seepage channels, and in the formed funnel, active desorption of methane occurred, much of which passed into the free state.
Conclusions:
Seismic and geophysical studies have confirmed the claimed capabilities of plasma-pulse technology when used in hydrocarbon productive formations, in particular, in coal deposits.
Under plasma-pulse action, a common funnel was formed, in which 4 wells turned out to be interconnected, and when the mode of operation on one well changed, the others immediately reacted in the form of dynamic level fluctuations or in the form of annular pressure growth.
It can be preliminarily assumed that the development of an anomalous micro-fracture network after applying PPT in coal seams over a large area will in the future allow for adjusting the grid and during the construction of degassing wells in the collapse domes, allow degassing of coal seams in advance or lowering the methane content in the coal to a safe level, and utilizing the methane for mine's purposes.
There is no doubt in the long term that there is a positive economic component, namely, minimizing the costs of degassing measures and drainage of layers, speeding up the miner machinery, recycling recoverable methane (purity 95–98%) to generate electricity and heat and, consequently, as a result - reducing the cost of coal produced.
In conclusion, it should be said that the design of wells, their construction, as well as the mode of their operation is a separate subject for consideration.