Amott index

Figure 9. (a) Effect of adsorbed surfactant on the Amott index... 

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 A qualitative term referring to the water- or oil-preferring nature of surfaces, such as mineral surfaces. Wettability may be determined by direct measurement of contact angles or inferred from measurements of fluid imbibition or relative permeabilities. Several conventions for describing wettability values exist. See also Amott Test, Contact Angle, Wettability Index, Wetting. 

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 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 index (WI) and oil index (OI) reflecting degrees of wettability between neutral and strongly wetting. 

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... 

The main shortcoming, reported by Anderson [12] for the Amott methods, is their insensitivity near neutral wettability. In addition, the RDI relates relative volnmes of imbibition, but one also needs to look at these individual volumes to obtain a better understanding of flnid displacement as represented by this index. More recently, Ma [15] reported that the Amott test also does not discriminate adequately at strongly water-wet conditions and proposed an imbibition rate method for wettability (see below). Other shortcomings include variations in laboratory procednres. The temperatnre and length of time employed for the spontaneons imbibition and the pressure used during the forced displacement cycle are often modified to match specific field parameters or for ease of measurement in the laboratory. Consequently, the qnantitative values that are obtained can differ from one laboratory to another for reasons that are not related just to the wettability of the rock sample. 

The relationship of SII to the Amott WI can be seen in Figure 6. This index is the same as the Amott WI at both neutral and highly water-wet conditions, 0 and 1, respectively. The denominator in equation 17 is normally estimated using a correlation obtained from conventional WI tests and can be formulated in terms of porosity, cp, and initial water saturation, Swi, both raised to powers. 

Summary. The Amott, USBM, Spontaneous Imbibition Index, imbibition rate, and capillary pressure are all displacement methods applicable to porous media and the possible evaluation of wettability alteration by surfactants. However, these methods must be complemented by more fundamental studies using contact angles or adhesion studies (Wilhelmy), etc. to meld the understanding of surface interactions with the macroscopic displacement of fluids. To comprehend how a surfactant alters the contact angle on a flat surface provides only part of the information to predict how the surfactant will interact in porous media. To measure only the fluid displacement in porous media provides little information on surface interactions. NMR and/or cryomicroscopy could help span this gap. Cryomicroscopy can directly look at pore surfaces, but for the moment, it is difficult and time consuming to use. Both techniques provide more of a qualitative measure of wettability than quantitative, but they are tools that can complement and help bridge between more fundamental measurements and quantitative displacement methods. 

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