Capillary Pressure Properties of Mesaverde Group Low-Permeability Sandstones in Six Basins, Western U.S

Alan P. Byrnes¹, Robert M. Cluff², John C. Webb², Daniel S. Osburn¹, Andrew Knoderer¹, Owen Metheny¹, Troy Hommertzheim¹, and Joshua P. Byrnes^1
1Kansas Geological Survey, Lawrence, KS
²The Discovery Group, Inc, Denver, CO

Drainage and imbibition air-mercury capillary-pressure properties were measured for over 100 Mesaverde Group low-permeability sandstones from six basins in the Western U.S. For all samples pore-throat diameters associated with the threshold-entry pressure (Pe) decrease with decreasing permeability. Stressed (4,000 psi NCS) and unstressed curve pairs for high-permeability cores (k>1 mD) are nearly identical; however, with decreasing permeability the unstressed and stressed threshold-entry pressures diverge. For all sample pairs this difference is greatest at Pe and the curves converge with decreasing wetting phase saturation (Sw) down to 30-50%, where the stressed curve crosses the unstressed curve and thereafter exhibits 0-5% lower Sw with increasing capillary pressure.

The data imply that confining stress exerts principal influence on the largest pore throats and that pore throats accessed at non-wetting phase saturations below approximately 50% are not significantly affected by confining stress. This is consistent with these smaller pores comprising pore space within pore bodies or in regions of the rocks where stress is not concentrated.

Hysteresis analysis involving three drainage-imbibition cycles for each sample were performed on 32 samples and residual mercury saturation was measured for over 200 samples where initial mercury non-wetting phase saturation (Snwi) corresponds to conditions near “irreducible” wetting-phase saturation (Swirr). The relationship between Snwi and residual non-wetting (Snwr) saturations following imbibition is well characterized by a Land-type relationship: 1/Snwr*-1/Snwi* = C, where Snwr* = Snwr/(1-Swirr), Snwi* = Snwi/(1-Swirr), and C = 0.55 at Swirr = 0. Results indicate that residual non-wetting phase saturations (e.g., gas) are high following imbibition.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado