Really logs give understanding of the newest formations and you can criteria on subsurface, aimed primarily on detection and you will assessment away from perhaps active horizons.
Commitment out of saturation
Liquids saturation ‘s the fraction of pore quantity of the fresh tank material that’s full of h2o. It’s basically assumed, unless or even identified, the pore regularity not full of h2o is stuffed with hydrocarbons. Deciding drinking water and you can hydrocarbon saturation is just one of the earliest objectives of really signing.
All-water saturation determinations away from resistivity logs into the clean (nonshaly) formations having homogeneous intergranular porosity are based on Archie’s h2o saturation formula, or differences thereof.   Brand new formula is actually
To have ease, the brand new saturation exponent letter is sometimes pulled since 2. Research experiments demonstrate this particular are a reasonable worthy of getting average cases. For more exacting performs, electronic specifications towards cores commonly make most useful number for letter, a, and you can m. Whenever center measured opinions is actually not available, the prices regarding an effective and m into the Eq. 4 is going to be projected below: inside carbonates, F=1/? 2 is normally made use of; into the sands, F=0.62/? 2  (Very humble algorithm), otherwise F=0.81/? dos (a less strenuous form very nearly equivalent to new Modest formula). This type of equations are often programmed towards the spreadsheets and so are for sale in extremely diary translation 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: Rw, F, and Rt. 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 = 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.
Choice methods for choosing water saturation include analysis of cores clipped which have low-attack oils-built muds (OBMs) and you may single well chemical tracer (SWCT) tests. These separate strategies are often used to calibrate journal 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 Ventura local hookup websites 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.