A Technique for Detecting Incomplete Saturation of Cores

Cores and core plugs are cleaned, dried, and saturated with brine or oil as part of many measuring procedures. The validity of the measurements depends directly on saturation being complete without any trace of residual gas in the core. Errors introduced by residual gas vary from about a square-root dependence for formation factor, through a linear effect for porosity, to what may be a severe, larger-than-linear effect for acoustic and permeability measurements. permeability measurements. A typical saturation procedure comprises evacuation of the core to a pressure of less than 15 Pa, admission of degassed fluid to the evacuated vessel containing the cores, and imposition of at least 5 MPa hydraulic pressure on the fluid while the cores remain immersed. There is 0.18 gm/m3 air left in a vessel evacuated to 15 Pa, The solubility of air in water is 20.2 gm/m3 at atmospheric conditions; therefore, if residual air at 15 Pa in the pore space of a core dissolves in a brine that saturates the core, then that air is less than 0.9% of the solubility of air in water. Similar arguments are true for other gases; therefore, this procedure ensures no residual free-gas saturation in the core after hydraulic pressure is released. The possibility exists that the procedure actually will not be possibility exists that the procedure actually will not be conducted as planned; a valve or pump seal may leak, a gas bubble may be trapped in the brine lines, and so forth. The cores still will be fully saturated, provided the leak is smaller than the capacity of the undersaturated brine to dissolve air. The following technique can detect free-gas saturation in excess of 0.01% porosity, or 0. 12 gm/m3 of air. Thus, all significant leaks will be detected, except for a narrow range that exceeds the dissolving capacity of the brine, but by less than 0.12 gm/m3.Fig. 1 is an enlarged drawing of a glass vial that can be "saturated" along with the cores. As fluid enters the saturation, vessel, a sample of residual gas in the vapor space of the vessel is trapped in the vial when the liquid level rises above the top of the cutouts in the lower rim. At the end of the cycle, any free gas left in the vial appears as a bubble at the tip of the capillary. The visual limit for bubble detection is about 0.4 nm3, which corresponds to 0.01% porosity for a 4-ml vial. The presence of a very small gas bubble in the vial conservatively indicates incomplete saturation because the vial is a closed trap once the liquid seals the base, whereas the core itself is flushed further by displacement and imbibition as the brine level rises around the core.Any vial with a configuration similar to that shown in the figure may be used for this procedure. Such vials are readily made by a glass blower from standard borosilicate glass tubing. The only special requirement is to maintain as square an end as practical on both the inside and outside of the capillary tip to allow clear observation of any gas bubble present.