Well logs give insight into new formations and you may standards in the subsurface, aligned primarily at the recognition and you will investigations out-of maybe effective limits.

Devotion out-of saturation

Liquid saturation ‘s the small fraction of your pore amount of the brand new tank stone which is filled up with h2o. It is basically assumed, unless of course or even understood, your pore volume perhaps not filled up with water is stuffed with hydrocarbons. Choosing liquid and you will hydrocarbon saturation is just one of the earliest expectations of better logging.

Brush formations

All water saturation determinations out of resistivity logs in clean (nonshaly) formations that have homogeneous intergranular porosity are derived from Archie’s h2o saturation equation, or distinctions thereof. [1] [2] The new formula is actually

To own convenience, the new saturation exponent n often is taken once the 2. Laboratory tests show this is actually a good worthy of to possess mediocre instances. For lots more exacting really works, electricity measurements into cores tend to develop top numbers to own letter, a great, and yards. Whenever center counted opinions try unavailable, the costs regarding an excellent and you may m in the Eq. 4 should be estimated as follows: during the carbonates, F=1/? 2 is usually put; within the sands, F=0.62/? dos [3] (Modest formula), otherwise F=0.81/? dos (a simpler function around equivalent to the Modest formula). These types of equations are often set on spreadsheets and therefore are in extremely log translation 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: 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 techniques for determining drinking water saturation is studies away from cores slashed having low-intrusion petroleum-established muds (OBMs) and you can unmarried really chemical compounds tracer (SWCT) testing. Such separate steps can be used to calibrate record analyses.

Resistivity compared to. 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, 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.