Better logs give understanding of brand new formations and you may conditions regarding subsurface, lined up generally at recognition and you can analysis from perhaps effective limits.

Determination from saturation

Water saturation ‘s the fraction of your pore volume of the fresh new reservoir rock that is full of h2o. It’s essentially presumed, unless of course otherwise known, that pore frequency perhaps not filled with liquids is filled with hydrocarbons. Deciding liquid and you may hydrocarbon saturation is amongst the basic objectives off really logging.

Brush formations

All water saturation determinations away from resistivity logs from inside the clean (nonshaly) structures having homogeneous intergranular porosity derive from Archie’s drinking water saturation equation, or variations thereof. [1] [2] The fresh new picture is actually

Getting simplicity, new saturation exponent n is normally pulled because the dos. Lab experiments demonstrate this particular was a reasonable well worth getting average times. For lots more exacting work, electronic proportions into cores usually create greatest quantity to own letter, a beneficial, and you may meters. Whenever core mentioned beliefs was unavailable, the costs out of a great and m for the Eq. cuatro is projected below: when you look at the carbonates, F=1/? dos is normally used; for the sands, F=0.62/? 2 [3] (Very humble algorithm), or F=0.81/? dos (a less complicated mode around equal to the fresh new Humble formula). These equations are easily set with the spreadsheets and tend to be available in very 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.

Solution tricks for deciding h2o saturation tend to be investigation away from cores reduce with lowest-attack petroleum-based muds (OBMs) and you will unmarried really toxins tracer (SWCT) assessment. These independent strategies are often used to calibrate journal analyses.

Resistivity against. 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 = R₀. If R₀ 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%, R₀ = 6.5 ohm•m. For this formation, the most appropriate F – ? relation is F = 1/? 2 . Thus, for ? = 10%, F = 100. Because R_w = R₀/F, R_w = 0.065 Nanaimo hookup apps ohm•m, as shown.