Abstract

In a previous WTGS Bulletin article and Fall Symposia Paper, we reported on a ‘scale down’ experiment from carbonate cores taken from the SACROC Unit in Scurry County, Texas. These samples were analyzed as twelve inch samples then cut into six inch whole core samples and re-analyzed. Then, those pieces were further cut into two inch whole core samples and analyzed again. Finally, a few additional samples were plugged (1-½’ diameter) and analyzed one last time. These experiments indicated that plugs and small (less than 6’ in length) whole core samples did not accurately predict the vertical permeability of full length (12”) cores. Smaller samples who’s ‘parent sample’ was less than 2mD typically displayed 3 Orders of Magnitude (OoM) variation, over-estimating by 2 OoM or under-estimating by 1 OoM. Higher quality parent samples (>10mD) were more accurately predicted by smaller subsets. Small samples typically over-estimated their parents by 1 OoM or under-estimated them by ½ OoM, with the range decreasing as parent sample quality increased. Horizontal permeability predictions from small subsets (2’ tall whole-core samples and 1-½’ diameter plugs) faired better when compared to 6’ length parent samples (the maximum size available). These datasets ranged within 1 OoM of parent samples (+ and -).

‘Conventional Wisdom’ holds that carbonates are, by nature, more variable than siliciclastics. Therefore, one should not be surprised that small sample sizes would impact expected permeability distributions. That same wisdom holds that siliciclastic deposits should not see a significant sample size impact on larger scale permeability predictions due to the more ‘linear’ nature of their (frequently gravity-driven) depositional processes. However, historic datasets from the Pennsylvanian Morrow Formation sands of the Postle Field in Texas County, Oklahoma indicate that 1-½’ diameter plugs over-estimate horizontal perms from 6’ whole core samples by 1 OoM and under-predict by 2 OoM.

In late 2010, a new core was taken through 4 individual sand bodies in the Postle Field. We will discuss the preliminary results of a similar ‘scale-down analysis’ run on two of those sands with new core.