Well logs bring understanding of the fresh new formations and you will standards from the subsurface, lined up mainly on identification and review away from possibly energetic horizons.

Commitment regarding saturation

Liquids saturation ‘s the tiny fraction of your pore quantity of brand new tank rock that is filled with drinking water. It is generally believed, until otherwise understood, your pore regularity maybe not filled with drinking water is filled with hydrocarbons. Choosing h2o and you may hydrocarbon saturation is amongst the first objectives off really signing.

Brush formations

All water saturation determinations from resistivity logs when you look at the clean (nonshaly) formations which have homogeneous intergranular porosity are derived from Archie’s water saturation picture, otherwise variations thereof. [1] [2] The new equation is

To have simplicity, new saturation exponent letter is normally drawn given that dos. Research studies demonstrated that is actually a reasonable worth getting mediocre circumstances. To get more exacting functions, electrical proportions with the cores will build greatest number getting letter, a, and you will meters. Whenever center counted opinions was not available, the prices out-of good and you can meters when you look at the Eq. 4 is estimated the following: in the carbonates, F=1/? 2 can be put; in sands, F=0.62/? 2 [3] (Simple formula), or F=0.81/? 2 (a less complicated mode nearly comparable to new Modest formula). These types of equations are typically set to your spreadsheets and they are available in most log interpretation software.

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.

Option tricks for determining liquids saturation become study regarding cores clipped having lower-attack petroleum-created muds (OBMs) and unmarried better chemicals tracer (SWCT) tests. Such separate measures are often used to calibrate record analyses.

Resistivity versus. 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, gay hookup Cairns 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 ohm•m, as shown.