Fracture analysis using Stoneley waves in a coalbed methane reservoir

Abstract

The production of coalbed methane gas depends upon various factors such as permeability, gas content, gas saturation and seam thickness. The low permeability in coal is a constraint in the gas extraction process. However, the presence of natural fractures and cleats in coal increases the permeability; therefore, its identification is necessary to establish the prospectivity. Hence, a methodology is used to process the Stoneley waveform in the Bokaro coalfield located in India to obtain slowness magnitude from the semblance analysis. The separation of directly transmitted and reflected Stoneley waves determines the reflection coefficient and attenuation, and its analysis emphasizes the presence of fractures. In coal seams, the average reflection coefficient ranges from 0.42 to 2.44 and attenuation ranges from 0.2 dB to 12.68 dB. The Stoneley slowness versus frequency plot shows a constant slowness with increasing frequency in the non-fractured seams, whereas a dispersive trend is observed in the fractured seams. Fractures are also distinguished from the cross-plot of compressional/shear versus Stoneley velocity. The observation of higher reflection coefficient, attenuation and slowness-frequency dispersion in coal seams indicates extensive fractures/cleats that corroborate with the resistivity image log correlation. Thus, the study helps to distinguish between fractured and non-fractured coal seams from the integrated approach of reflection coefficient, attenuation and dispersion analysis of Stoneley waves supported by the identification of fractures from resistivity image log for optimization in coalbed methane production in the development phase.

Anisotropy and fracture analysis for coalbed methane reservoir development in Bokaro coalfield, India

“…Semblance processing determines the slowness in the time domain that favours the low‐frequency limit of the wave containing the highest signal‐to‐noise ratio. For a dispersive wave, the strongest signal can occur at frequencies higher than the low‐frequency limit, and hence the resulting slowness is too high (Banerjee & Chatterjee, 2021b). The dispersion result can give erroneous if adequate dispersion corrections are not applied.…”

“…Index map of India showing the location of the Bokaro field, which is the study area. The geological map of the Bokaro coalfield drawn after Banerjee and Chatterjee (2021a,b) represents the geological formations and well locations B1 and B2.…”

“…The response of emitted transmitter's signal is short and decays rapidly with time, whereas the recorded receiver's signal consists of the signal train with increasing magnitude with time. These signal trains are the composite waveform that is received in successive arrival of (i) compressional wave, (ii) shear wave, (iii) Rayleigh wave, (iv) mud wave, and (v) Stoneley waves (Banerjee & Chatterjee, 2021b). The P, S and St components have different characteristics.…”

“…But, in an anisotropic medium, a phenomenon called birefringence occurs where S-wave splits into fast and slow components with a different group and phase velocities (Sayers, 2009). The last arrival of the St-wave propagates through a solid-liquid interface along the borehole wall and is sensitive to fractures and permeable formation resulting in velocity reduction and amplitude decays (Stoneley, 1924;Tang & Cheng, 1993;Banerjee & Chatterjee, 2021b). Moreover, the properties of P, S and St waves are affected by anisotropy in the formation.…”

Permeability of large‐scale fractures with ununiform proppant distributions in coalbed methane development

“…While the matrix, whose porosity is usually less than 10% and permeability is less than 1 mD, hosts most of the gas in medium and high-rank CBM, the fractures provide the pathways for the gas to flow. [1][2][3][4][5] One of the most significant and successful technological approaches for utilizing unconventional hydrocarbon resources is hydraulic fracturing, particularly when combined with horizontal wells. This technique creates fractures in the formation with the high pressure fluid, like slick water, supercritical CO 2 , liquid N 2 , and foam, [6][7][8] to address issues with low porosity, poor permeability, and lower natural productivity in reservoirs.…”