Well logs provide understanding of the latest formations and you can requirements regarding the subsurface, lined up generally at the identification and you may research regarding perhaps productive horizons.

## Commitment away from saturation

H2o saturation is the tiny fraction of pore quantity of the fresh new tank rock that’s full of drinking water. It is generally believed, except if if not identified, that the pore frequency maybe not filled up with h2o is filled with hydrocarbons. Deciding drinking water and hydrocarbon saturation is one of the basic objectives out of better signing.

## Clean formations

All-water saturation determinations away from resistivity logs into the clean (nonshaly) formations having homogeneous intergranular porosity depend on Archie’s liquids saturation formula, or variations thereof. [1] [2] The fresh new equation was

To possess simplicity, the newest saturation exponent letter is often taken as dos. Research tests show this try a good well worth to have average cases. For lots more exacting functions, electric dimensions to your cores will build finest quantity getting letter, good, and you may meters. Whenever core measured values are unavailable, the values away from a and you may m inside the Eq. cuatro will be projected below: within the carbonates, F=1/? dos is normally put; during the sands, F=0.62/? 2 [3] (Simple algorithm), or F=0.81/? dos (a less complicated form almost comparable to the new Humble formula). Such equations can be programmed on the spreadsheets and are generally obtainable in most record interpretation 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.

Choice approaches for deciding drinking water saturation is research off cores slash with low-intrusion oils-founded muds (OBMs) and you may unmarried really chemical substances tracer (SWCT) testing. Such independent methods can be used Tacoma free hookup dating sites to calibrate diary analyses.

## Resistivity compared to. 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.