Anisotropy analysis for optimized gas production in Coalbed Methane reservoir, Bokaro eld, India

24 The efficient production of Coalbed Methane (CBM) gas is facing challenges due to the larger 25 dewatering period from fracture connectivity to the aquifer zone. Also, commingled production 26 from well makes it more difficult to identify the coal seam-wise problem. Therefore, prior 27 knowledge of sub-surface fractures in coal seams is necessary to execute an accurate 28 simulation model for planning hydraulic fracturing treatment. This paper highlights the studies 29 in Bokaro CBM reservoir to mitigate challenges in few wells by characterizing anisotropy, 30 determining fast shear wave polarization angle, maximum horizontal stress direction, fracture 31 orientation, and analysis of low resistivity signature. Both the fast shear wave polarization 32 angle and fracture orientation in resistivity image are observed in the same direction (N26°- 33 35°E) in coal. The fast and slow shear slowness versus frequency plot concludes stress-induced 34 anisotropy resulting from fractures that are supported by resistivity image and drilling core. 35 Processing of the resistivity image log shows the maximum horizontal stress is along NE-SW 36 direction, as identified from drilling-induced fractures. The observation of low resistivity 37 signature with resistivity ranging from 0.4 to 0.8 ohm-m in few wells confirms the presence of 38 conducting minerals such as siderite and pyrite from the x-ray diffraction studies of sidewall 39 core. The present work guides in making production, drilling, and hydraulic fracturing design 40 strategies to better understand the fluid propagation for optimized CBM production and will 41 also help in future geomechanical studies.

The presence of opening mode fractures; face and butt cleats in coal is connected to stress-70 induced anisotropy. Stress-induced anisotropy is derived from the S-wave velocities using 71 advanced sonic logging data indicates azimuthal anisotropy or horizontal transverse isotropy 72 (HTI) due to the variation of physical properties in the azimuthal direction around the wellbore 73 (Stevens and Day 1986;Cheng and Cheng 1996). The anisotropy mechanisms can be by azimuthal anisotropy, St-wave responds to VTI anisotropy is established but P-wave is least 82 affected by anisotropy (Hornby et al. 1999;Sun et al. 2003). The dipole flexural S-mode is 83 affected and it gives erroneous results due to borehole failure or breakouts in an elliptical 84 borehole (Grandi et al. 2003). During data processing, the higher relative dip angle between the 85 borehole axis and formation beds is necessary to be considered (Tang and Patterson 2005;De 86 and Schmitt 2005).

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Analysis of stress-dependent coal anisotropy is conducted for studying and determining the gas anisotropy from fractures in coal is established using seismic data was shown by many authors 93 (De et al. 1998;Gray et al. 2002;Vetri et al. 2003). Processing of cross multipole array 94 acoustics log data is required to know the dispersion characteristics of S-waves with frequency 95 for its identification of stress-induced and intrinsic anisotropy. Stress direction is required for 96 geomechanical application, oriented perforation, and effective fracturing in objective 97 formations (Fletcher et al. 1996;Todorovic-Marinic et al. 2004). 98 In a previous study on west Bokaro Coalfield (CF), the resistivity image log has identified the

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The routine operational activities include the perforation of coal seam followed by hydraulic 137 fracturing (HF) treatment. Subsequently, the perforated and HF coal seams are pumped 138 continuously using an artificial pumping machine for dewatering and gas production.

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Dewatering extracts the water volume from the coal seam to desorb the existing gas in an 140 adsorbed state by achieving the critical desorption pressure. Once the critical desorption pressure is attained, the gas break occurs (Sun et al. 2017). Initially, only water is produced 142 without gas, but once gas breaks are attained, water production declines, and gas production 143 increases. A longer dewatering period is the challenge for cost optimization. The commingled 144 production from 05 seams with the period (days) shows larger dewatering (m 3 /day) and smaller 145 discontinuous gas break (m 3 /day) in W-2 and W-3, whereas in well W-4 no gas break is 146 observed (Figures 3a, 3b, 3c). The water and gas production in these 03 wells are different as 147 wells are drilled in different locations at different times but the recording interval of production 148 data is constant as shown in figure 3. Well W-2 is recorded from February 2019 to October 149 2019, but no major gas break is observed, hence the well was closed. Similarly, well W-3 is 150 recorded from 2017 to 2019 but the well was closed from January 2018 to January 2019 due to 151 well complication, after that it was again recorded with the observation of few discontinuous 152 gas breaks. The production data in well W-4 from September 2018 to June 2019 shows no gas 153 break and hence the well was closed. The average duration of production in these 03 CBM 154 wells was continued for 6-8 months as discontinuous/no gas breaks were observed.

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Similarly the reflected signal V(t) is recorded in the acquisition coordinate, that is expressed as: coordinate system, U11 and U22 is the reflected signal along S1 and S2 direction.

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The above equation from time domain to frequency domain is expressed as: wave, δ1 and δ2 are the two-way travel time, and f1 and f2 are the filter function for the fast and 176 slow S-wave set for the attenuation, geometric spreading, and reflection coefficient.

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By applying rotation in data matrix in both source and receiver coordinate, the output of rotated 178 data matrix is expressed as:

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Considering f1(ω)=f2(ω), the equation (5) Where in WRT and VRT, R represents receiver direction and T represents transmitter direction, diagonal elements of the rotated matrix is expressed as: 192 Where N represents samples number in the search window and k and p are the integers.

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The flowchart of integrated work to determine and characterize anisotropy, to understand sub-196 surface fracture system, stress direction, and to study core and production data using available 197 data is presented in figure 5. The reason for larger dewatering and less/no gas break are 198 analyzed from anisotropy analysis. LRS is observed below KT seam is concluded from the 199 XRD study. the dip magnitude. The magnitude is calculated from the following formula:

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Where h is the height of crest and trough in meters, d is the borehole diameter in meters. Error analysis is the measure of accuracy that identifies and detects uncertainty in the result.

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The error in degree is obtained from the difference in the observed and predicted magnitude of  Figure 13, 14, 15, 16, and Table 1 354

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The outcome of the study assist in optimized gas production in CBM reservoir from the 356 following conclusions:

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(1) Anisotropy is analyzed from travel time delay of fast and slow shear wave. The fast shear 358 wave polarization direction, fracture orientation, and SH direction matched well and are 359 observed along N26°-35°E (NE-SW).

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(2) Anisotropy associated with a coal seam is stress-induced due to the splitting phenomena in 361 slowness versus frequency plot. The stress-induced anisotropy is associated with the fractures 362 and cleats.

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(3) Intrinsic anisotropy is caused due to parallel fractures in shale in Bokaro coalfield.

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(4) The reason for the larger dewatering period in wells is due to the fracture extension through 365 HF treatment in the coal seam connecting the aquifer zone.   Logs-Application to Seismic Well Tie, paper BH/RP 4.7. Expanded Abstracts, Thomsen, L. (1988). Reflection seismology in azimuthally anisotropic media. Geophysics, 53, 503 3, 304-313.

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Identifying vertical productive fractures in the Narraway gas field using the envelope of       Dewatering (m3/day) and gas (m3/day) production pro le versus period (days) in wells (a) W-2 (b) W-3 (c) W-4.

Figure 4
Diagram representing S-wave splitting in multi-source and multi-receiver acquisition.

Figure 5
Flowchart displaying the steps followed in the integrated work.    The error with depth point in (a) Seam-X (b) Seam-IX (c) Seam-VIII (d) Seam-VI+VII and (e) Seam-V.

Figure 10
Anisotropy analysis representing seam-VIII in well W-2 illustrates maximum and minimum energy (%), DTS and TTS anisotropy (%), fast and 180° apart S-azimuth, resistivity image with dip.

Figure 11
Fast and slow S-wave slowness (us/ft) from depth interval 115.0-450.0 m in well W-2 and 03 points (a, b, c) marked in the log for anisotropy characterization.

Figure 14
Correlation of fractures at 356.0 m and 357.5 m between drilling core and resistivity image log from depth 356.0 m to 362.5 m.

Figure 16
Cross-plot of intensity (cps) versus 2 (degree) from the XRD studies of SWC at 418.0 m in well A-1 showing the presence of siderite and pyrite.