How Well Do We Really Understand Subsurface Fractures?

Open fractures are notoriously difficult to locate and characterise in the subsurface but are very easy to anticipate in a general way with only a moderate understanding of their structural geology setting and rock types and very little understanding of their mechanical evolution. But a general interpretation is not adequate. How can we become more educated in our prediction of fracture location, character and effect on reservoir performance? Part of this problem involves a number of individual beliefs that may be incomplete or outdated; part is because even apparently supported beliefs, when taken together, are inconsistent. What do we actually know about fracture formation and development and how does it deviate from what we think we know? If we are clearer in our understanding of what we really do know then our thinking about fractures and their character will be clearer, and hopefully more useful. We outline some common beliefs about fractures, primarily from outcrop observations and theory, and assess them individually. We then look at them together assessing how they support or negate each other and place them in a geomechanical context. Finally we consider how experimental results can help consolidate and progress our understanding.