Better logs provide understanding of the new formations and criteria about subsurface, aligned mostly on recognition and you can testing from maybe effective limits.
Determination away from saturation
Liquid saturation ‘s the tiny fraction of pore number of brand new tank stone that is filled with liquid. It is generally thought, unless of course otherwise recognized, your pore regularity perhaps not filled up with drinking water is full of hydrocarbons. Determining h2o and you will hydrocarbon saturation is among the very first expectations away from better signing.
Brush structures
All-water saturation determinations of resistivity logs in brush (nonshaly) formations which have homogeneous intergranular porosity are based on Archie’s liquids saturation formula, otherwise distinctions thereof. [1] [2] The fresh new formula was
To own convenience, the latest saturation exponent n can be taken because dos. Laboratory tests have indicated that try a reasonable worthy of to have average circumstances. To get more exacting really works, electronic proportions towards the cores usually build greatest quantity getting letter, an effective, and you will yards. Whenever center mentioned philosophy was not available, the costs of a and you may meters inside the Eq. cuatro can be estimated the following: within the carbonates, F=1/? dos is usually used; when you look at the sands, F=0.62/? 2 [3] (Modest formula), otherwise F=0.81/? dos (a simpler mode about equal to the brand new Simple algorithm). This type of equations can be programmed into spreadsheets and so are obtainable in extremely diary interpretation application.
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). datingranking.net/local-hookup/liverpool-2/ 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.
Approach methods for choosing water saturation are analysis off cores slash with reduced-attack oils-founded muds (OBMs) and you can solitary really agents tracer (SWCT) evaluation. These independent steps are often used to calibrate log analyses.
Resistivity versus. 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.
