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  • 1. Well Logging Course (1st Ed.)

2. 1. Reading A Log 2. Examples of Curve Behavior And Log Display 3. Electrical Properties Of Rocks And Brines 3. 1. Spontaneous Potential A. membrane potential B. Application C. Log Example of The SP 4. Spontaneous potential Spontaneous potential main usage: the identification of permeable zonesThe origins of the spontaneous potential in wellbores involve both electrochemical potentials and the cation selectivity of shales.basis for the spontaneous potential is the process of diffusion the self-diffusion of the dissolved ions in the fluids in the borehole and in the formation.Fall 13 H. AlamiNiaWell Logging Course:5 5. The mechanism of generating the liquid-junction potential Electrochemical potentials of interest to the generation of the spontaneous potential are the liquid junction potential the membrane potentialFigure schematically illustrates the situation for the generation of the liquid-junction potential. To the left is a saline solution of low NaCl concentration. To the right is one of a higher ionic concentration. Fall 13 H. AlamiNiaWell Logging Course:6 6. The liquid junction potential As is often the case, the resistivity of the drilling mud filtrate (Rmf ) is greater than the resistivity of the formation water (Rw), so:Where Vlj is The liquid junction potentialFall 13 H. AlamiNiaWell Logging Course:7 7. A schematic representation of the development of the SP in a borehole The cell marked Ed corresponds to the liquid junction potential just discussed is sketched with the polarity corresponding to a higher electrolyte concentration in the formation water than in the mud filtrate.additional source of SP is associated with the shale result of the membrane potential generated in the presence of the shale that contains clay minerals which have large negative surface chargeFall 13 H. AlamiNiaWell Logging Course:8 8. A representation of a shale On the left, consisting of rock mineral grains and small platy clay particles.On the right the distributions of ions close to the face of one of the clay minerals is shown, which illustrates the so-called electrical double-layer.Fall 13 H. AlamiNiaWell Logging Course:10 9. How does a cation differ from an anion? A cation (s)(+)is a positively (+) charged ion. It loses one or more negatively charged electrons when forming ionic compounds. (are) almost always metalsAn anion (s) (-)is a negatively (-) charged ion. It gains one or more electrons when forming ionic compounds. (are) typically nonmetalsEvery ionic compound must contain both a cation and an anion so that the compound as a whole has no charge.A common example: In the ionic compound table salt (NaCl), sodium (Na+) is the cation, and chloride (Cl-) is the anion.Fall 13 H. AlamiNiaWell Logging Course:11 10. electrical double layer We assume that shale is nearly impermeable to fluid flow, but still capable of ionic transport, although considerably altered by the presence of clay minerals.The shale acts like a cation-selective (+) membrane. This property is related to the sheet-like structure of the alumino-silicates that form the basic structure of clay minerals. At the surface of the clay minerals there is a strong negative charge related to unpaired Si and O bonds. When the clay mineral particles are exposed to an ionic solution, one containing Na+ and Cl for example, the anions (Cl-) will be repulsed by their surfaces while the cations (Na+) will be attracted to the surface charge, forming the so-called electrical double layer. Fall 13 H. AlamiNiaWell Logging Course:12 11. membrane potential Close to the clay layers, the fluid will be dominated by cations since the anions are excluded by electrostatic repulsion.In this manner, in a complex mixture of clay minerals and other small mineral particles, with pore spaces even too small to permit the hydraulic flow of water, the cations will be able to diffuse along the charged surfaces, from high concentration to low concentration while the negative Cl ions will tend to be excluded.Such a diffusion process will tend to accumulate a positive charge on the low ionic concentration side of the shale barrier, producing an attendant electric field. In the practical situation, the cations from the fluid saturating the porous sand zone diffuse through the shale to the borehole with the lower cation concentration.Fall 13 H. AlamiNiaWell Logging Course:13 12. evaluating the membrane potential In this figure a semipermeable shale barrier separates the solutions of two different salinities. A schematic representation of the mechanism responsible for the generation of the membrane potential. The diffusion process is altered by the selective passage of Na+ through the shale membrane. Fall 13 H. AlamiNiaWell Logging Course:14 13. magnitude of the membrane potential The natural diffusion process is impeded because of the negative surface charge of the shale. The Cl ions which otherwise would diffuse more readily are prevented from traversing the shale membrane,whereas the less mobile Na ions can pass through it readily.The result is that the effective mobility of the chlorine in this case is reduced to nearly zero.magnitude of the membrane potential VmFall 13 H. AlamiNiaWell Logging Course:15 14. SP Measurement In the case of lower NaCl concentration in the mud, the voltages add, resulting in a more negative voltage in front of the sand than in front of the shale zone.The membrane potential provides about 4/5 of the SP amplitude, since the absolute value of mobilities enters in its potential, rather than the difference as in the liquid-junction potential.The SP is measured, between an electrode in the borehole and a distant reference. Fall 13 H. AlamiNiaWell Logging Course:17 15. natural potential vs. static spontaneous potential The shale baseline represents the natural potential between the two electrodes, without electrochemical effects, andis ideally a straight line from top to bottom.The static spontaneous potential (SSP), is the ideal SP generated by electrochemical effects when passing from the shale to a thick porous clean (shale-free) sand if no current flowed.Fall 13 H. AlamiNiaWell Logging Course:18 16. potential drop In practice the electrode can only measure the potential change in the borehole. Although the mud is usually less resistive than the formation, the area for current flow is much smaller in the borehole than in the formation, so that the borehole resistance is usually much higher than the formation resistance.Most of the potential drop therefore takes place in the borehole with the result that the measured SP amplitude in the center of the bed is close to the SSP. Fall 13 H. AlamiNiaWell Logging Course:19 17. The determination of Rw In the best of cases, the measurement of the SP allows the identification of permeable zones and the determination of formation water resistivity. Since the mud filtrate resistivity can be measured, the formation water resistivity can be calculated using factors that are well known for NaCl solutions.A deflection indicates that a zone is porous and permeable and has water with a different ionic concentration than the mud. Fall 13 H. AlamiNiaWell Logging Course:20 18. effective water resistivities vs. actual resistivities In practice the electrochemical potential is often written in terms of effective water resistivities (Rmf e) and (Rwe) rather than actual resistivities. These are equal to Rmf and Rw except for concentrated or dilute solutions. In concentrated solutions,below about 0.1 ohm- m at 75 F, the conductivity is no longer proportional to the number density of charge carriers and their mobilities. At high concentrations the proximity of the ions to one another is increased; their mutual attractions begin to compete with the solvation to reduce their mobilities.In dilute solutions of most oilfield waters, other ions than Na+ Cl become increasingly important.Numerous charts exist for the determination of Rw from the SP, knowing Rmf and temperature.Fall 13 H. AlamiNiaWell Logging Course:21 19. Other applications of SP log The SP is also used to indicate the amount of clay in a reservoir. The presence of clay coating the grains and throats of the formation will impede the mobility of the Cl anions because of the negative surface charge, and thus spoil the development of the liquid-junction potential.The ideal SP generated opposite a shaley sand when no current flows is known as the pseudo static potential (PSP).In addition to these quantitative interpretations, elaborate connections have been established between the shape of the SP over depth and geologically significant events.Some examples of using the SP curve to determine patterns of sedimentation are given in Pirson [10]. Fall 13 H. AlamiNiaWell Logging Course:22 20. SP vs. other logging techniques The measurement of the SP is probably the antithesis of the high-tech image of many of the other logging techniques. The sensor is simply an electrode (often mounted on an insulated cable, known as the bridle, some tens of feet above any other measurement sondes) which is referenced to ground at the surface. The measurement is essentially a dc voltage measurement in which it is assumed that unwanted sources of dc voltage are constant or only slowly varying with time and depth. Fall 13 H. AlamiNiaWell Logging Course:25 21. shale and clean sand beds along with the idealized response of SP logging deflections to the left correspond to increasingly negative values. In the first sand zone, there is no SP deflection since this case represents equal salinity in the formation water and in the mud filtrate.The next two zones show a development of the SP which is largest for the largest contrast in mud filtrate and formation water resistivity.In the last zone, the deflection is seen to be to the right of the shale baseline and corresponds to the case of a mud fi