Better logs promote understanding of the formations and you may standards from the subsurface, aimed generally on recognition and research away from possibly active limits.
Determination away from saturation
Drinking water saturation is the fraction of the pore amount of the newest reservoir rock that’s filled with liquid. It is essentially presumed, until otherwise known, that the pore volume maybe not filled with water is full of hydrocarbons. Determining water and hydrocarbon saturation is among the basic expectations from well logging.
Clean structures
All water saturation determinations off resistivity logs from inside the brush (nonshaly) formations having homogeneous intergranular porosity derive from Archie’s water saturation equation, or differences thereof. [1] [2] Brand new equation is
To possess convenience, the fresh new saturation exponent letter might be pulled since the dos. Research tests have demostrated that the was a fair value getting average cases. For more exacting performs, electronic measurements on cores have a tendency to write most useful quantity getting n, an excellent, and you will meters. Whenever core measured philosophy are not available, the prices regarding good and m during the Eq. 4 are projected as follows: into the carbonates, F=1/? dos is sometimes utilized; when you look at the sands, F=0.62/? dos [3] (Simple algorithm), otherwise F=0.81/? 2 (an easier function around equal to the newest Simple formula) hookup bars near me Vancouver Washington. These equations are typically set with the spreadsheets consequently they are available in extremely log 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.
Solution methods for determining liquids saturation are studies out-of cores cut having reduced-intrusion oil-centered muds (OBMs) and you may unmarried really chemical tracer (SWCT) evaluation. This type of separate actions can be used to calibrate journal analyses.
Resistivity compared to. 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.