Horizontal drilling into naturally fractured reservoirs requires informationon the orientation, spacing, and connectivity of the fractures relative to thestructure and stratigraphy so that drilling depths and directions can be setfor maximum productivity. Prior to drilling, this information must come fromoutcrops, but these data may be suspect, as stress relaxation and weatheringmay have enhanced the fracture development. To what degree can outcrop data beextrapolated to rocks at depth even if the outcrop rocks are representativelithologically of the reservoir rocks, structural development (strain) is thesame, and the physical conditions at the time of deformation were the same sothat similar relative strengths and ductilities obtained at the time offracturing?
The Austin Chalk play of South-Central Texas is discussed as a case historyrelative to the extrapolation question. The rocks at outcrop and in thesubsurface were deformed under conditions of low effective confining pressureand behaved brittlely. Previously published outcrop data on orientation andspacing are reviewed and correlated with recently acquired similar data using Formation Microscanner surveys (FMS) from horizontal segments of boreholes. Itis shown that only one set of strongly oriented fractures occurs in thesubsurface (nearly vertical and striking NE) whereas at least two sets aredeveloped at the outcrop. However, the set the subsurface ialso is welldeveloped on outcrop and usually is the longest, oldest, most systematic andbest connected. In addition, the distribution of fracture spacings in thesubsurface is equal to or approaches that at the outcrop.
The characterization of the naturally fractured reservoirs involves anunderstanding of the mechanical behavior of the reservoir rock, the origin ofthe fractures, the orientation (i.e. strike and dip), spacing, and connectivityof the fractures, and the role fractures play in the fluid flow of thereservoir. Naturally fractured reservoirs such as the Austin Chalk (Texas), Bakken Shale (Montana), Purdoe-Bay clastics (Alaska), North Sea Chalks, and the Niobrara limestone (Colorado) have received a great deal of attention since theadvent of horizontal drilling.
