Abstract

Image logs are very effective for characterizing fractured reservoir rocks occurring in subsurface. In the initial stages, characterization was limited only to fracture detection and orientation. Imagery provides a reasonably clear picture of the fractures that cross the wellbore. This clarity is due to resistivity contrast between the fracture(s) and host rock which is capable of characterizing the fractures as— open, closed or partially open. In later years, this contrast has been stretched to quantify aperture sizes based on the concept that higher fracture conductivity is due to large size fracture aperture.

A relatively lower conductivity therefore shall lead to a computation of lower aperture size. People in the industry have now accepted imagery derived fracture aperture sizes more authentic than core derived fracture analysis. This is because representative core sample recovery in fractured reservoir is difficult and handling of the core sample during analysis is delicate. There is always a fair chance to add fresh porosity component to samples during the preparatory and/or measurement stage.

Fracture aperture is an important input to workout fracture porosity, fracture permeability and evaluating fracture intensity. In the present study, this application has been stretched to the coal beds to characterize cleats in addition to fractures. This has been done using equations that are applicable to clastics and carbonates. Cleats and fractures within the coal beds have successfully been identified and their aperture values are quantified with same degree of confidence— enabling analyst formally predict the flow potential of the coal seams under CBM exploration. In the final conclusion, it is noted that fracture aperture sizes are always on the higher side compared to apertures in cleats.