A Comparative Study of Mechanical Properties of Gondwana Ccoal Seams from Damodar Valley Coalfields and Their Implications in CBM Exploitation

Kaustav Nag, S. Durga Prasad, Bhanu P. Singh, and Rajesh Kumar Srivastava
E & D Directorate, ONGC, Dehradun, India

CBM reservoirs differ fundamentally from conventional petroleum reservoirs. Most of the gas in place in coals (greater than 90%) is adsorbed in the coal matrix. The adsorbed capacity of a coal seam varies non-linearly as a function of pressure. The production of gas from coal seams involves a three stage process in which gas i) flows from natural fractures ii) desorbs from the cleat surfaces iii) diffuses through the coal matrix to the cleats.
The present paper deals with a comparative study of the mechanical properties of coals from Jharia, Bokaro and North Karanpura coalfields using electrolog data. The Poisson’s ratio and Young’s Modulus of coals have been calculated using Vp/Vs ratio and standard set of equations. The mechanical properties of coal are important indicators of the stress level of coals which has a direct bearing on hydraulic fracturing. For commercial production of gas from coals, hydraulic fracturing is often a pre-requisite. The values of PR and Young’s modulus as derived from the present study are found to be higher than the laboratory determined data which are determined through high frequency acoustic pulses on core samples.
The Poisson’s ratio which is the ratio of the lateral strain to longitudinal strain for some of the coal seams in Jharia and Bokaro are quite high indicating possible boundaries for hydraulic fracture. These data can also be used with reasonable reliability to predict the variation of the minimum horizontal stress with depth. Moreover the Young’s modulus in coals and the surrounding horizons determine the width of the fracture. A large contrast between Young’s modulus of coals and in the surrounding zones would help in fracture containment.

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