Lecture itemsLecture itemsSelf Potential Log* Theory of measurement. -Shale-base line& Sand line -SSP, PSP and SP log readings * Factors affecting on log readings. * Applications.- Resistivity Logs * Definition. * Types * Units& Presentation. * Theories of measurement. * Factors affecting on log readings. * Applications.

1 -Electrokinetic Potential (can be neglected)

2 -Electrochemical Potential 1 )Membrane Potential2 )Liquid Junction Potential

Rmf/Rw (Salinity effect) Fresh mud: negative SP, Saline mud: positive SP.

Shale or clay content Shale reduces SP. Permeability Presence of hydrocarbon Bed thickness: SP decreases when bed thickness

decreases. Invasion: Reduces SP. Mud filtrate: The magnitude and direction of SP

deflection from the shale baseline depends on relative resistivities of the mud filtrate and the formation water.

Resistive formations

Factors affecting SP log measurements

Clean Laminated Structural Dispersed

Rw calculation from SP

Mathematical Calculation of Rw from SSP (modified after Bateman & Konen, 1977) Rmf at 75oF = Rmf temp

* x (temp + 6.77)/81.77 Correction of Rmf to 75o K = 60 + (0.133 x Tf) Rmfe / Rwe = 10 – SSP / K** Rmfe = (146 x Rmf – 5) / (337 x Rmf + 77) Rmfe formula if Rmf at 75oF < 0.1 Rmfe = 0.85 x Rmf Rmfe formula if Rmf at 75o > 0.1 Rwe = Rmfe / (Rmfe / Rwe) Rw at 75oF = (77 x Rwe + 5) / (146 – 377 x Rwe) Rw at 75o formula if Rwe < 0.12 Rw at 75oF = - [0.58 – 10 (0.69 x Rwe –0.24)] Rw at 75oF formula if Rwe > 0.12 Rw at formation temperature = Rw at 75o x 81.77 / (Tf + 6.77) *Rmftemp = Rmf at a temperature other than 75oF **The e subscript (i.e. Rmfe) stands for equivalent resistivity.

Differentiation between shaly, clean and shale zones.

Differentiation between Permeable and non-permeable zones.

Calculation of Rw.Determination of the volume of shale.For correlation purposesFor sedimentological analysis and facies

studies.

Applications

Notice how the shale baseline shows a distinctive drift with depth. This characteristics is commonly caused by an increases in relative oxidation of the rocks that are close to the land surface. The highest sandstone in the well has a muted deflection on the SP log as compared with the lower sandstones. This contrast is an immediate indication that water in the upper sandstone may be significantly fresher than waters of the lower sandstone. In other wells it is not uncommon to see sandstone units where the SP deflection goes to the right of the shale baseline. In these instances, the drilling mud filtrate is salter than the formation water.

A good example of this phenomenon is shown in the figure attached. In the upper sandstone, "U", the SP log shows a deflection to the right, indicating formation water to be fresher than the drilling mud, while in the lower sandstone, "L", the deflection is to the left, showing the formation water to be more saline.

Flow chart from oil-industry log analysis to estimate formation water resistivity, Rw, in deep formations from the SP log. RMF is mud filtrate resistivity measured at temperatureTmf and recorded on the log header; Tf is the temperature of the formation, generally estimated by interpolating between the bottom-hole temperature (BHT) at total depth (TD) and mean annual temperature at the surface; SSP is the static self-potential measured on the log between the "clean line" and "shale line" in millivolts (mv) and with associated sign (positive or negative).