Amott-Harvey indices

Amott-Harvey Indices, in which spontaneous imbibition of, individually, water and oil compared with the maxima possible under forced (pressure) imbibition. These are used to calculate water (WI) and oil (OI) indices reflecting degrees of wettability between neutral and strongly wetting. 

Interpretation of NMR well logs is usually made with the assumption that the formation is water-wet such that water occupies the smaller pores and oil relaxes as the bulk fluid. Examination of crude oil, brine, rock systems show that a mixed-wet condition is more common than a water-wet condition, but the NMR interpretation may not be adversely affected [47]. Surfactants used in oil-based drilling fluids have a significant effect on wettability and the NMR response can be correlated with the Amott-Harvey wettability index [46]. These surfactants can have an effect on the estimation of the irreducible water saturation unless compensated by adjusting the T2 cut-off [48]. 

Amott Test A measure of wettability based on a comparison of the amounts of water or oil imbibed into a porous medium spontaneously and by forced displacement. Amott test results are expressed as a displacement-by-oil (60) ratio and a displacement-by-water ratio (Sw). In the Amott-Harvey test, a core is prepared at irreducible water saturation and then an Amott test is run. The Amott-Harvey relative displacement (wettability) index is then calculated as 6W — 60, with values ranging from — 1.0 for complete oil-wetting to 1.0 for complete water-wetting. See also reference 8, Wettability, Wettability Index. 

Wettability Index A measure of wettability based on the U.S. Bureau of Mines wettability test in which the wettability index (W) is determined as the logarithm of the ratio of areas under the capillary pressure curves for both increasing and decreasing saturation of the wetting phase. Complete oil-wetting occurs for W = —oo (in practice about —1.5), and complete water-wetting occurs for W = oo (in practice about 1.0). Another wettability index is derived from the Amott-Harvey test. See also reference 8, Amott Test, Wettability. 

Amott-Harvey. The wettability test devised by Amott [13] and its modification, the Amott-Harvey Relative Displacement Index (RDI) [14] are the most common quantitative measures of wettability employed for porous media by the oil industry. It relies on measurements of the saturation changes produced by spontaneous imbibition for both water. 

Figure 4. The relationship of saturation states used for the Amott wettability indices or Amott-Harvey Relative Displacement Index to capillary pressure. ASws is the saturation change from a to b. ASw is the saturation change from b to c. ASos is the saturation change from c to d. ASof is the saturation change from d to a.

The Amott-Harvey methodology combines these indices into a single expression, the Relative Displacement Index, RDI, by... 

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