Petrophysical evaluation of fracture sytems in coal bed methane (CBM) bearing coal seams in relation to geological setting,3 exploration blocks, Botswana

Abstract

This study is focused on the Coal Bed Methane resources of Botswana with specific reference to the Central Kalahari basin where prospect license blocks forming the focus of this study are located. The aim of this study is to evaluate the fracture network in the coal seams and the fracture systems in the surrounding coal bearing sedimentary sequences and their contribution to dynamic flow. Coal bed methane sources are dual-porosity media documented on the natural fracture network, seen as micropores (matrix/natural fractures) and macropores (cleat). The coals of this region belong to the Ecca Group’s Morupule Fm (Permian) (70 m), focus of this study and have been preserved in the extensive Karoo basin within the Southern Africa region. Fractures can easily be identified in Acoustic Televiewer logs (ATV) and their orientation and structural character interpreted by rose plots, tadpoles and stick dip plots. In-situ stress fields have been determined from breakout structural evaluation and maintains a general E-W dip direction and N-S strike, thus most fractures are orientated optimally with inferred in-situ stress and enhancing flow potential in pore systems. A qualitative (MID plots & M-N cross-plots) and quantitative description of the fracture system is fundamental to the petrophysical evaluation, and involves the estimation of fracture parameters (fracture porosity, resistivity fracture index and both horizontal and vertical fracture indices).