Really logs promote insight into the fresh new formations and you may requirements throughout the subsurface, aimed mainly at the detection and you can assessment from maybe energetic perspectives.

## Determination regarding saturation

Drinking water saturation is the small fraction of your own pore amount of the fresh new reservoir rock that’s filled up with drinking water. It is basically presumed, except if or even identified, that the pore frequency perhaps not filled with liquids is stuffed with hydrocarbons. Determining h2o and hydrocarbon saturation is amongst the first expectations from better logging.

## Clean structures

All water saturation determinations out-of resistivity logs during the brush (nonshaly) structures having homogeneous intergranular porosity are based on Archie’s liquids saturation picture, otherwise variations thereof. [1] [2] The fresh new formula try

Having simplicity, the latest saturation exponent n is usually pulled because 2. Lab studies demonstrate that the was a reasonable really worth having mediocre times. For lots more exacting work, electrical proportions towards the cores often make greatest quantity to possess n, a beneficial, and yards. When core counted thinking are not available, the prices of a great and you may yards in Eq. 4 is going to be estimated below: during the carbonates, F=1/? 2 can often be used; in sands, F=0.62/? dos [3] (Very humble algorithm), otherwise F=0.81/? dos (a simpler form very nearly equivalent to the latest Modest formula). These types of equations can be set for the spreadsheets and therefore are available in most diary translation app.

The accuracy of the Archie equation, Eq. 1 and its derivatives, depends in large measure, of course, on the accuracy of the fundamental input parameters: R_{w}, F, and R_{t}. The deep resistivity measurement (induction or laterolog) must be corrected, therefore, for borehole, bed thickness, and invasion (see the page Formation resistivity determination for more details). It is almost never safe to make the assumption “deep = R_{t}.” The most appropriate porosity log (sonic, neutron, density, magnetic resonance, or other) or combination of porosity and lithology measurements must be used to obtain porosity, and the proper porosity-to-formation factor relationship must be used. Finally, the R_{w} value should be verified in as many ways as possible: calculation from the SP curve, water catalog, calculation from nearby water-bearing formation, and/or water sample measurement.

Approach suggestions for https://datingranking.net/local-hookup/melbourne/ determining drinking water saturation include study of cores clipped that have lowest-intrusion oil-created muds (OBMs) and you can single really toxins tracer (SWCT) evaluation. These separate actions are often used to calibrate diary analyses.

## Resistivity vs. porosity crossplots

Eq. 7 shows that for R_{w} constant, ?S_{w} is proportional to is the quantity of water per unit volume of formation. To emphasize the proportionality between ? and , Eq. 7 may be rewritten:

For a 100% water-saturated formation, S_{w} = 1 and R_{t} = R_{0}. If R_{0} for water-saturated formations is plotted on an inverse square-root scale vs. ?, all points should fall on a straight line given by .

Furthermore, the points corresponding to any other constant value of S_{w} will also fall on a straight line, because in Eq. 7 the coefficient is constant for constant values of R_{w} and S_{w}.

Fig. 1 shows several points plotted over an interval in which formation-water resistivity is constant (as indicated by constant SP deflections opposite the thick, clean permeable beds). Assuming that at least some of the points are from 100% water-bearing formations, the line for S_{w} = 1 is drawn from the pivot point (? = 0, R_{t} = ?) through the most northwesterly plotted points. The slope of this line defines the value of R_{w} as shown on Fig. 1, for ? = 10%, R_{0} = 6.5 ohm•m. For this formation, the most appropriate F – ? relation is F = 1/? 2 . Thus, for ? = 10%, F = 100. Because R_{w} = R_{0}/F, R_{w} = 0.065 ohm•m, as shown.

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