Reservoir Characterization to Inexpensively Evaluate the Exploitation Potential of a Small Morrow Incised Valley-fill Field

Kansas Geological Survey
Open-file Report 2002-9

Core Petrophysical Properties

Capillary Pressure and Water Saturation

Fluid saturations in the Minneola unit can be determined from both electrical wireline log interpretation and from capillary pressure data. The capillary pressure curves show a relationship of increasing threshold entry pressure with decreasing permeability that is consistent with decreasing pore throat size with decreasing permeability (Figure). “Irreducible” wetting phase saturations (Swi) increase with decreasing permeability similar to other Morrow sandstones. The more permeable cores exhibit transition zones of only several feet while the least permeable cores exhibit transition zones of 20-30 feet. Based on the saturation versus height profiles the more permeable reservoir intervals were at or near “irreducible” saturation on discovery. For the less permeable rock, water saturations were considerably greater and reflected the high amount of bound water due to shalyness. For all cores, the wetting phase saturation at approximately 75 feet above free water level is within 5-10% of the “irreducible” saturation that can be achieved by these reservoirs in Kansas, given available structural closures in the state. Comparison between curves indicates that entry pressure, “irreducible” wetting phase saturation, and the capillary curve curvature (reflecting increasing pore throat size heterogeneity) increases with decreasing permeability.

Generalized Capillary Pressure Curves

To provide capillary pressure curves for the reservoir simulation it was necessary to develop generalized curves that represented the specific permeabilities that might be assigned to a gridcell. Equations to construct generalized capillary pressure curves were constructed based on the relationships evident from the entry pressures in the air-mercury capillary pressure curves and the capillary pressure curve shapes, and from the saturations evident in the air-brine capillary pressure analysis. The relationships between increasing entry pressure, “irreducible” wetting phase saturation, and the capillary curve curvature (reflecting increasing pore throat size heterogeneity) with decreasing permeability were utilized to develop equations that would predict the capillary pressure curve using permeability as the independent variable.

Entry pressure, or the first pressure at which wetting phase desaturation begins and similar to R35, exhibits a strong correlation with permeability and can be predicted using:

Pcowentry = 1.311 log10ki + 3.364

Where Pcowentry is the oil-water entry pressure and ki is the in situ permeability.

To model the changing capillary curve shape with decreasing permeability an empirical function was developed that predicted curve shape and “irreducible: saturation from ki:

Pcow = Pcowentry + exp(Pca + Pcb Sw 2)


Sw = water saturation (%)
Pca = 1.363 log10ki + 9.64
Pcb = -0.29 log10ki 3 + 0.85 log10ki 2 - 0.64 log10ki - 0.85

Using the above equations capillary pressure curves were constructed for six general classes of permeability and used in the reservoir simulation model.

Single-Point Air-Brine Capillary Pressure

“Irreducible” brine saturation (Swi; Sw at 75 ft above free water) is strongly correlated with permeability (ki) for “clean” and “shaly” lithologies and Swi decreases with increasing ki. Shaly sandstones exhibit a significant greater increase in Swi with decreasing permeability than clean sandstones. This is consistent with increasing surface area and an increase in micorpores with increasing clay and shale content. Clean sandstones exhibit little increase in Swi with decreasing ki, consistent with only modest increase in surface area and microporosity with quartz or carbonate cementation. "Irreducible" saturation can be predicted using the following equations:

Shaly -Very Shaly: Swi = -28.89 log10kinsitu + 78.6
Clean-Slighlty Shaly: Swi = -14.55 log10kinsitu + 46.8

The standard error of prediction for these equations is 5+1%. It is important to note that the higher “irreducible” saturations of the lower porosity and permeability sandstone samples diminishes their relative contribution to the total storage and flow capacity of the reservoir compared to the higher porosity and permeability samples. The predicted irreducible water saturations are consistent with water saturations measured in the reservoir from wireline logs.

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Last updated March 2002