Fractured Limestones Versus Fractured Sandstones

John C. Lorenz
Sandia National Laboratories, Albuquerque, NM

Limestones and sandstones commonly contain similar fractures that strongly influence system permeability. However, their differences in chemical and mechanical properties dictate important differences in the probability of fracturing and the potential effects of the fractures on reservoirs. For example, although laboratory tests show that limestones and sandstones have similar strengths (i.e., they fail compressively at similar pressures), limestones are more compliant, requiring larger amounts of strain before reaching the failure limit. Thus, under the same conditions, limestones should have fewer fractures than sandstones. This is the case in examples from the Holbrook Basin of Arizona, where fractured beds of Coconino Sandstone are interbedded with more sparsely fractured beds of Kaibab Limestone, even though the beds are of similar thickness. Likewise, because limestones are readily soluble, hairline fractures are commonly healed immediately after fracturing, and therefore the bulk mechanical properties of limestones are commonly not significantly altered by the presence of small fractures. If the host limestone is contaminated by clay or organic matter, however, the calcite-filled fractures are more susceptible than the host rock to failure and fracture reactivation, as well as to dissolution. This is illustrated by examples from the Lisburne Limestone and the Austin Chalk. Fractures in limestone may also provide initial pathways for significant secondary dissolution of the host rock itself, leading to cavernous porosity that is rare in sandstones. Finally, the permeability along limestone fractures may be more susceptible to damage than sandstone fractures during the pressure cycles associated with production from a reservoir.