Better logs render insight into the fresh formations and you can criteria regarding the subsurface, lined up mostly at the identification and you can analysis out-of possibly active perspectives.
Determination regarding saturation
H2o saturation is the fraction of the pore volume of new reservoir material that’s filled up with drinking water. It is generally presumed, unless if you don’t identified, your pore volume maybe not full of water is stuffed with hydrocarbons. Choosing drinking water and hydrocarbon saturation is among the basic objectives from really signing.
Brush formations
All-water saturation determinations of resistivity logs in the clean (nonshaly) formations with homogeneous intergranular porosity depend on Archie’s h2o saturation equation, or differences thereof. [1] [2] The fresh new equation is actually
Having ease, the saturation exponent n is oftentimes pulled while the dos. Laboratory tests have shown that is actually a reasonable worthy of to have average cases. To get more exacting really works, electricity specifications on cores tend to establish greatest wide variety for letter, a good, and you may yards. Whenever core counted opinions try unavailable, the prices out of an effective and you may meters in Eq. 4 can be projected as follows: during the carbonates, F=1/? dos is normally made use of; within the sands, F=0.62/? dos [3] (Modest formula), or F=0.81/? 2 (an easier function about comparable to the fresh Modest formula). Such equations can be programmed into the spreadsheets and are for sale in very log 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: Rw, F, and Rt. The deep resistivity measurement (induction or laterolog) must be corrected, therefore, for borehole, bed thickness www.datingranking.net/local-hookup/chula-vista/, and invasion (see the page Formation resistivity determination for more details). It is almost never safe to make the assumption “deep = Rt.” 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 Rw 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 techniques for deciding drinking water saturation were research regarding cores clipped with lowest-invasion oils-centered muds (OBMs) and you may single really agents tracer (SWCT) tests. These types of separate methods are often used to calibrate log analyses.
Resistivity against. porosity crossplots
Eq. 7 shows that for Rw constant, ?Sw 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, Sw = 1 and Rt = 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 Sw will also fall on a straight line, because in Eq. 7 the coefficient is constant for constant values of Rw and Sw.
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 Sw = 1 is drawn from the pivot point (? = 0, Rt = ?) through the most northwesterly plotted points. The slope of this line defines the value of Rw 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 Rw = R0/F, Rw = 0.065 ohm•m, as shown.
