PRODUCTION-SCALE FLOW EFFECTS OF COMPACTION BANDS IN SANDSTONE

Thin, tabular, low-porosity and permeability compaction bands (CBs) form pervasive arrays in the Aztec Sandstone that extend over square kilometers of exposure in the Valley of Fire, southeastern Nevada. Abundant evidence of CB influence on patterns of paleo fluid-flow alteration, coupled with modeling of effective permeability, indicates order-of-magnitude impacts at the outcrop scale. But do these effects persist to larger scales? To address this issue, we created a detailed CB map covering some 40 acres of nearly continuous exposure in the Valley of Fire, making it possible to model fluid flow at scales relevant to reservoir and aquifer production.

A variety of flow simulation scenarios based on this raw map, to which neither interpretative nor geostatistical adjustments were made, reveal significant potential impacts. For example, the driving pressure between injector-producer well pairs separated up to 200 m increases about three-fold relative to the homogeneous, isotropic CB-free case. The bands as mapped also induce considerable driving-pressure anisotropy, with pressure across the dominant trend exceeding pressure along it by as much as 35% for wells spaced 100 m apart. Directional effects are even more pronounced in two-phase transport simulations. For a typical five-spot scenario with central injector, favorable alignment to the dominant band trend can increase the efficiency of oil recovery by more than a third over less favorable production configurations. Finally, simulations of contaminant migration through the CB array reveal a strong influence on plume shape and concentration, which rivals that of the regional pressure gradient and could seriously impair containment and recovery efforts if not taken into account.

Fortunately, in the absence of detailed subsurface data, the gross hydraulic effects of CB arrays can be approximated as a homogeneous permeability anisotropy with kmax oriented parallel to the dominant band trend. Thus, armed with only spatially limited data on the orientation, volume fraction and relative permeability of CBs present, production and recovery efforts could be managed to mitigate and possibly capitalize on their impact.