Borehole Geophysics for Unconventionals

Attention has turned away from conventional gas reservoirs, characterised by a static density stratification and a hydrocarbon-water contact, to formations that act as both source and reservoir (shale gas and coal bed methane) or where gas is a solid component (gas hydrates). These unconventional resources may contain free or dissolved gas, augmented by adsorbed gas (shale and coal bed methane) or caged gas (gas hydrates). Compared with conventional reservoirs these unconventional resources often have large, ill-defined geographical areas (shale gas), constitute thin discontinuous seams that may vary locally (coal bed methane), or show scale variable concentrations (gas hydrates). These make for unusual physical reservoirs and their evaluation requires a reassessment of physical properties and a new approach to the interpretation of borehole geophysical responses. The focus of this paper is the role of borehole geophysics in respect of three different types of unconventional gas: shale gas, coal bed methane (CBM), and gas hydrates.