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3-D Modeling of the Santa Terezinha Coalfield, Brazil: Implications for Estimation of the Coalbed Methane Potential

Heinz H. Burger1, Wolfgang Kalkreuth2, Michael Holz2, and René Prissang1
1 Freie Univ. Berlin, Berlin, Germany
2 Instituto de Geociencias, UFRGS, Porto Alegre, Brazil

The geometrical modeling of a coal basin in Brazil was made possible due to conventional analysis of borehole data and a detailed study of the sequence stratigraphy. The most important controls for the coalbed methane (CBM) potential in the Santa Terezinha coal field in Rio Grande do Sul state, Brazil are a) basin geology, i.e. reservoir size, depth and thickness of coal seams; b) coal characteristics, i.e. coal rank, coal type and coal grade; c) in-situ methane content, measured by means of gas desorption tests and isotherm adsorption tests.

The geometrical 3D-model of the Terezinha coal field is focused on a set of coal seams hosted in a succession of sandstones, mudstones, and shales of Early Permian age. The combination of logs from a rather small number of boreholes and information from paleogeography and sequence stratigraphy provide a powerful tool for the construction of a consistent 3D-model of the coal basin.

Assessment of coal rank is based on vitrinite reflectance measurements of 120 coal samples obtained from 27 borehole locations. Methane adsorption tests carried out in a number of coal seams indicate a general capacity of 2 - 6 m3/ton of coal. This result corresponds to those methane values calculated on the basis of empirical considerations including coal rank, ash yields and depth.

The combination of geometrical 3D-modeling and coal property modeling is the basis for estimating the CBM potential of the Terezinha coal field and for selecting an optimal location for a first coalbed methane test well.