Abstract: Fault-Related Fractures in Middle East Reservoirs Revealed by Borehole Imagery in Horizontal Wells

Roy Nurmi

Formerly difficult to detect, strike-slip faults and related fracturing were found to be significant features in many Middle East reservoirs using borehole imagery. While 3-D seismic improved fault mapping, 3-D borehole imagery in horizontal wells is revealing still more faulting and fault-related fractures. Good success has also been achieved in guiding horizontal well trajectories to intersect the maximum of fractures, however, in some reservoirs it is desirable to avoid the fault-related fractures as these are a major cause of water production.

Faults and related fractures may be closed by cataclastic deformation and/or mineralization, or reactivated and opened by later stresses and/or dissolution. The match of greatest fracture frequency, largest apertures modelled and the strike of the faults suggests that structural and stress modelling is the key to mapping their reservoir orientation once constraining data from 3-D seismic and borehole imagery is available. Whereas, the preliminary data on systematic spacings between strike-slip faults in some reservoirs and also aperture variations suggests that fractal characteristics may be the key to modelling the lateral distribution of fracture attributes which make these features either barriers, baffles or high permeability flow paths.

A number of different types of borehole imagery have been used to unravel the characteristics of reservoir faulting and relating fracturing, including resistivity imagery acquired while drilling. Higher resolution microresistivity imagery and deeper, more quantitative lateral log resistivity imagery logged and sonic and ultrasonic imagery provide more detail but are acquired after drilling.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994