Fractures may be characterized by a variety of borehole logging methods including electrical pad type imaging, azimuthal resistivity imaging, ultra sonic scanning acoustic anisotropy, and acoustic waveform properties. Some methods, such as electrical contact are quite definitive, allowing for details to be visually interpreted al high resolution these methods also tend to have rather shallow depths of investigation and are sometimes limited by borehole geometry. Acoustic methods, which can yield much deeper fracture information, are more deductive in nature but can often yield complementary information to other image logs and provide both near- and far-well bore analysis of stress field indicating where failure is likely. While acoustic techniques such as crossed-dipole anisotropy analysis yield a wealth of information they must be carefully analysed to distinguish formation responses from the effects of hole conditions.
A review of the current industry methods of fracture characterisation is presented, with various available and developing technologies. Examples of interpretation, quality indicators, joint interpretation of multiple techniques, and tips on novel visual displays for simplified interpretation arc included.
Three main technologies are available for borehole imaging: electrical arrays ultrasonic scanners and acoustics methods. Of these methods, electrical imaging is most definitive because the analyst can visually identify fractures in the image of the borehole wall. Fracture detection by acoustic methods, on the other hand, is more deductive in nature: the changes to propagating acoustic waves caused by fractures must be differentiated from similar effects that are caused by petrophysical or stratigraphic features. The resolution of the available fracture characterisation technologies from high resolution to low is
Wireline Electrical Imagers
UltraSonic Scanners
LWD Electrical Imagers
Acoustic Methods
Depth of investigation or the available fracture imaging technologies falls in a similar order to the resolution scale with higher resolution tools reading shallowt'r than tower resolution ones.
Detectable borehole fractures can be either natural in origin or induced by the drilling process. Natural fractures reflect the tectonic stress fields that created them and will align themselves with the greater two of the three principle axes of stress. In areas without significant regional tectonic activity, the greatest stress is usually the weight of the overburden and is oriented to the vertical axis. Within these areas, most natural fractures are generally oriented to within a few degrees of vertical.
In areas where strong regional tectonic forces exist, stress fields can assume almost any orientation resulting in fracture orientations ranging from horizontal to vertical and any angle in between.
Fractures created by the drilling process are generally created parallel to the axis of the borehole, provided the principle stress directions are also parallel to the borehole. However, induced fractures can also cross the borechole obliquely, as when the principle stress directions are not co-linear with the borehole axis.
Fractures can be classified as open, closed or healed. Open fractures are fluid-filled and may have significant porosity. Closed factures have their opposing faces touching and usually have insignificant amounts of porosity.
