Objective:
The ultimate objective of assessing the effect of PPT on the permeability of the coal seam is further divided into smaller tasks and goals:
- Registration of micro-seismic emission detected in the process of passive microseismic monitoring.
- Building of a 3D geomechanical model.
- Creating a discrete fracture model (Discrete Fracture Network).
- Calculation of the value of secondary permeability resulting from the plasma-pulse effect on the coal seam.
- Creating a hydrodynamic model of coal seams.
Significance:
- The quantities of recoverable methane from a coal seam greatly rely on the quality of the coal, the thickness of the seam, lithostatic pressure and the size of the desorption funnel formed around the well.
- In turn, the size of the funnel depends on the filtration properties of coal and the geological structure of the coal seam.
- Furthermore, geological structure of coal and the matrix are sensitive to permeability changes which are occurred due to the formation of macro, meso and microcracks during PPT.
- Hence, the consequential significance of investigating the permeability changes in the coal structure is directly linked to the enhancement or diminution of the quantities of recoverable methane.
Registration of micro-seismic emission detected in the process of passive microseismic monitoring:
- The results obtained allowed to contour the area of PPT
action around the wells - The observed kinematic and dynamic characteristics
are further useful to build a 3D Geomechanical model
Fig 1: A 3D model of the distribution of microseismic events in the PPT process in wells A1 and A3 (hypocenters are shown by colored circles). The color indicates the magnitude of the seismic energy released
Building of a 3D geomechanical model:
- Geomechanical models are crucial in order to estimate the stress-strain state of the coal-rock strata.
- This enables us to integrate a variety of structural and geological information, to construct a reliable discrete model of fracturing (Discrete Fracture Network)
- The prerequisite data for building this model are a 3D digital geological model and the results of micro-seismic monitoring from the previous task
Fig 2: General view of a 3D geological model around wells A1-A4
Creating a discrete fracture model (Discrete Fracture Network):
- Figure 3 shows a Discrete Model of fracturing of coal seams 45-48, built using 6 trends that determine the appearance of newly formed cracks or aperture (degree of opening) of primary and secondary fracturing
Calculation of the value of secondary permeability resulting from the plasma-pulse effect on the coal seam:
Fig 4: The scheme of technogenic permeability resulting from the plasma-pulse effect on coal seams 45-48.
Fig 5: Secondary permeability maps built using geological data for the A5-A20 well area **
- An important observation that was revealed through the geophysical analysis is the fact that the parameters affecting the formation of man-made fractures, do not have a radial distribution, instead they form two systems of directions – diagonal (North-eastern and North-western orientation) and orthogonal (Sublatitudnal and submeridional orientations).
- This reflect on the actual mechanism of the formation of man-made cracks occur in the initially weakened zones that exist in the coal matrix.
Creating a hydrodynamic model of coal seams:
Hydrodynamic model is constructed of the methane filtration that occurred as a result of desorption of coal seam.
Fig 6: Depression funnel that occurred 400 days after the start of water pumping
- Calculations of the degassing process in the wells required the usage of a grid consisting of technogenic permeability values, which resulted from the plasma pulse effect on coal seams 45-48.
- Based on the theoretical understanding of the methane adsorption process due to coal seam, calculations were performed featuring the volumes of gas that were released due to formation of desorption funnel from water pumping.
- It should be noted that the calculation of rising reservoir pressure due to the pumping of water after time t, is crucial and a necessary process.
- Finally, by observing the difference between these parameters will evidently characterize the magnitude of the desorption funnel that has risen around the well.
Fig 7: A 3D model of the character of the drainage zones of the A5-A20 well in the lava area 48-9
- The structure of drainage zones makes it possible to estimate the volumes of gas that is in a sorbed state on the coal seam
- By calculating the volumes of gas within each drainage zone, it is possible to reverse-engineer and estimate the amount of methane that must be removed to achieve a methane content of 13 m3/t
Results of Hydrodynamic model:
- Gas volumes contained in coal seams 45 and 48 within the desorption funnel of wells were estimated
- The volumes of gas that need to be recovered from the reservoir to achieve the threshold gas content of 13m3/t were estimated
- The time period for which the gas content falls to a safe level is calculated
- The permeability model played a key role in the understanding of parameters of the wells and predict the following:
- Increased water content of some wells
- Low gas flow rates of others
Key findings:
A wide range of findings were collectively discovered from the various tasks that were performed and mentioned above. The results obtained opened the doors to a very new perspective in coal seam degassing process using PPT within the Erunakovskaya-VIII mine section. Also, an additional discovery were the modern methods that can be used to assess the filtration-capacitive properties of the coal strata using the 3D computer simulation.
From the Geomechanical Model:
- Successful estimation of the stress state of the coal strata, to create a discrete fracture model and calculate the secondary and man-made porosity and permeability using special geophysical methods of micro-seismic sounding.
- Identification of drainage areas of the methane-coal wells A1, A3 and A4 were first identified.
- Geophysical monitoring revealed that the area of influence of PPT extend beyond 900 meters from the well.
- A network of filtering channels are created along the large cracks that were formed during implementation of PPT.
- Presence of these channels is further confirmed by observing the changes in the flow rates of the producing wells.
From the Hydrodynamic Model:
- Successful estimation of methane reserves within the area of producing wells
- Methane production rates were plotted for A1, A3 and A4 wells.
- The results of secondary and man-made permeability which were previously determined were confirmed through the Hydrodynamic model
- The model played a key role in understanding the mechanisms of water cut in A2 well, low flow rates in A3 well and the significant rise in the methane flow rate in the A4 well after decommissioning of A1 well.
Conclusions:
- Furthermore, geomechanical model enabled us to establish the anthropogenic impact on the coal seam that occurred in initial environment conditions that were later disturbed by tectonic cracks, which occurs in a specific geodynamic setting.
- Natural fracturing plays a crucial role in the formation of filtration channels in the coal.
- Comparison of technogenic and secondary permeability allowed us to create a filtration model for the projected area of A5 – A20 wells in the contour of the lava 48-9.
- According to results of hydrodynamic modelling, it was revealed that there is a danger of encountering high flow rates of water flow in well A18 (similar to well A1). Extreme wells A5 and A20 have the largest drainage area.