Effect of Quick-Freezing vs Saturation of Oil Well Cores

It is perhaps not widely realized that extraction and saturation processescarried out on oil well core samples alter the properties of these samples tovarying degrees. On the other hand it is felt by some that quick-freezing ofcore samples increases their permeability and porosity significantly.Accordingly, laboratory tests were carried out on 49 pairs of horizontallyadjacent samples in order to differentiate between the effect of quick-freezingper se on permeability and porosity of the samples, as distinguished from theeffect of the identical saturation treatment on permeability and porosity ofthe companion samples. Also, additional field data were obtained on comparisonof frozen vs unfrozen companion samples.

Laboratory Investigation of Freezing vs Saturation Effects

Procedure

The samples used in these tests were two-cm cubes cut in horizontallyadjacent pairs from cores from eight Gulf Coast and Mid-Continent wells, whichcores had not previously been frozen. These samples were extracted with carbontetrachloride, dried, and air permeabilities run in the conventional manner.They were then evacuated and saturated with brine of 25,000 ppm sodium chloridecontent, and porosities determined by gain in weight. The samples werepartially desaturated by evaporation down to an average brine saturation of 68per cent. One sample from each pair was quick-frozen by covering with dry iceafter wrapping in a single layer of paper, and allowed to remain frozen forabout two hours; the companion sample from each pair was not frozen. Afterthawing the frozen sample, all samples were immersed in tap water overnight inorder to leach out most of the brine. Air permeabilities were re-run, and thesamples were again saturated with brine to determine a second porosityvalue.

For purposes of averaging of data, the samples were grouped according tofour permeability ranges, from 0 to 10, 10 to 100, 100 to 1,000, and 1,000 to3,840 md. Average permeability and porosity changes for the frozen vs theunfrozen adjacent samples are shown in Table 1.

Discussion

As may be seen from Table 1, the averages of the per cent permeabilityincreases for the quick-frozen samples ranged from 3.8 to 12.9 per cent amongthe four permeability groups.

T.N. 151