Capillary Pressure andWater Saturation

Capillary pressures and corresponding water saturations (Sw) vary between facies, and with porosity/permeability and gas column height. Threshold entry pressures and corresponding heights above free water level are well correlated with permeability. (See figure below). This is consistent with the relationship between pore throat size and permeability. The figure shows that for rocks with in situ Klinkenberg gas permeability below approximately 0.003 md, threshold entry heights are greater than the gas column heights available in the Council Grove and therefore the samples have Sw=100%. Synthetic capillary pressure curves were constructed from capillary curves from 91 cores representing the range in facies and permeability shown in the figure below.Capillary pressures in each facies can be represented to be a function of porosity. The figures to the left for the NM sand/ siltstones and the Pkst/Pkst-Grnst facies illustrate that with decreasing porosity and permeability, threshold entry heights and heights necessary to decrease Sw increase.

Differences in Sw between porosities increase with decreasing height above free water level. High porosity NM Sandstones exhibit lower entry pressures than similar porosity carbonates but have higher “irreducible” water saturations and threshold heights increase greater with decreasing porosity. Note that NM Silt/ Sandstone with < 6% do not appear on the figure because of high entry heights. Differences in Sw between facies increase with decreasing porosity and decreasing height above free water (figures below). For example, at 7% porosity (which represents >50% of all Mdst/Wkst) at 200 ft above free water, Mudstones are 100% water saturated while in Grainstones Sw ~40%. Because differences decrease with increasing height, saturations for all facies approximately approach a similar “irreducible” saturation at gas column heights above ~300 ft except for samples at low porosity where saturation differences are still evident.