Horizontal wells have been applied in many types of reservoir conditions with a variety of applications. The most successful of these has been exploiting the very large horizontal permeability anisotropics associated with naturally fractured reservoirs. Log analysis has always been difficult in naturally fractured reservoirs; very little has been published regarding horizontal well logs. This paper summarizes Union Pacific Resources (UPRC) experience logging dozens of horizontal wells in several different naturally fractured reservoirs, summarizes the improved information resulting from this experience, contrasts various well logs applicable in such areas, and describes a method for categorizing fracture apertures used to quantify the total horizontal well fracture index CRH).
UPRC has logged dozens of horizontal wells in the Bakken, Niobrara, Austin Chalk, Buda, Georgetown, and Second White Specks formations with oriented microresistivity devices (FMS), borehole acoustic televiewers (BATV), four and six arm dipmeters, and sonic logs. Formation evaluation objectives have included determining the depth of most intensely fractured intervals, identifying and quantifying number and aperture of fractures encountered, locating faults and bed boundaries, determining fracture azimuths, and for use in subsequent production operations. The use of vertical and 45 degree ‘pilot’ holes is compared, showing significantly superior results using a 45 degree well to detect the fractured intervals. The applicability of the various tools are described with examples of each being presented. The UNIX-workstation based methodology for approximate quantification of fracture azimuths into aperture-based categories is described, along with the procedure to determine the RFI. A horizontal well log presentation is discussed that illustrates difficulties in mud log interpretation.