Characterization and Distinction between Reservoir Faulting and Fracturing Using Borehole Electrical Imagery 

NURMI, ROY, and MAHMOUD AKBAR

More fractures and faults have been revealed using borehole electrical imagery in horizontal wells in many Middle East reservoirs than was, previously mapped. Borehole imagery, in fact, is revealing many more faults than 3-D and is being used to provide the exact position of these faults along the trajectories of horizontal wells. The identification of these individual faults is very important to reservoir modeling, but is even more critical to the immediate completion of a well when a fault is open and permeable. It appears that these faults are the source of much early water production in reservoirs. Very early water production even occurs in horizontal wells drilled for attic oil along the tops of low-permeability oil zones. Moreover, the vertical extent of these features has been confirmed by the drilling of vertical laterals and mapping the faults and related fractures with electrical imagery. These unmapped faults are important reservoir features which provide positive impact as well.

The recognition of faults and distinction from joints and fractures is rather simple if geological knowledge of fault characteristics is applied to image, interpretation. Thus, evidence for movement along the plane of the feature is critical to this distinction, however, other criteria have been found to be useful, including crack apertures, strike orientation of fracturing versus supposed faulting, changes in fracture frequency or changes in stress field associated; with the position of the suspected fault have proved helpful. In some cases, acoustic anisotropy data has provided needed stress field orientation data. 

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.