Spontaneous Potential (SP) Log

Spontaneous Potential (SP) LogMOHAMMAD NOORAZAM BIN ADANMUHAMMAD NURSALAM BIN ZULKIFLIMUHAMMAD AIZUDDIN BIN ZAINAL ABIDIN SHAHFARHAN HAIRI BIN KASIMMOHAMAD SHALIHIN BIN MAT SAIDIntroductionCalled self or spontaneous potential.The natural occurrence of potential different in the earth.Measure the D.C voltage difference between the surface and borehole electrode.It is compulsory for the existing of conductive fluid (drilling mud & connate water) in the borehole.Generated due to the current flowing in the mud in the borehole that results form two types of potential.Electrochemical (membrane & junction).Electro kinetic.SP Operation principleThe spontaneous-potential log is a graphic plot of the small differences in voltage, measured in millivolts, that develop at the contacts between the borehole fluid, the shale or clay, and the water in the aquifer. Two sources of potential are recognized. The first source is the streaming potential caused by electro kinetic phenomena. This electromotive force (emf) develops when an electrolyte moves through a permeable medium. The emf appears in the borehole at places where mud is being forced into permeable beds, although in water wells, streaming potentials may be generated in zones gaining or losing water.The second and most important source of SP arises in the electrochemical emf produced at the junction of dissimilar materials in the borehole.The junctions are between the following materials : Mud-mud filtrate, mud filtrate-formation water, formation water-shale, and shale-mud.SP measurement principleSP is a natural occurring potential measured in the borehole mud . This potential is created by chemically induced electric current flow.

Liquid Junction Potential (Ej) Generation MechanismOccurs at the boundary between the flushed zone and the virgin zone.Because of the high salinity of the formation water, both cations (Na+) and anions (Cl-) will migrate towards the mud filtrate.The anions (Cl-) move faster than the cations (Na+ ),and the net effect is a current flowing.Liquid Junction Potential (Ej)The Na+ ion is comparatively large and drags 4.5 molecules of water, while the Cl- ion is smaller and drags only 2.5 molecules of water.Hence, the anion Cl- will migrate more easily than the Na+ ions.More positive charges left behind in the formationwater. These positive charges restrict further Cl- migration towards the flushed zone.

Membrane Potential (Em) Generation MechanismBecause of its molecular structure, shale are more permeable to Na+ cations than the Cl- anions; a shale acts as an ionic sieve. Since Na+ ions effectively manage to penetrate the shale bed through from the saline formation water to the less saline mud column, a potential is created known as the membrane potential (Em).

ADVANTAGESIt indicates movable hydrocarbons, not just the presence of hydrocarbonIt is independent of porosity and lithology ;therefore it is more suitable to variable lithology formationsSp method is very simple and fastBetter adapted for locating top and bottom of the bed then resistivity logSupplemental information for interpretation other type of logs

LIMITATIONS/DISADVANTAGESThis method is limited to fresh muds only(not working in oil based mud)It overestimates shale volume I hydrocarbon-bearing zoneSensitive to the selection of clean reservoir and shale pointsEfficiency intrude by noise and anomalous(usually when poor insulator between probe electrode and cable)

Environmental Effect On SP OIL Base Muds & Air-Filled BoreholesDue to a complete lack of an electrical path in borehole, no SP will be generated in wells where the hole is filled with oil-based muds or air.

(2) Shaly FormationsShale will reduce the measured SP. This effect permits the shaliness to be estimated if a clean sand of the same water salinity is available for comparison.

(3) HydrocarbonsHydrocarbon saturation may reduce SP measurements, so only water-bearing sands should be used for determining Rw from the SPSo, a hydrocarbon bearing zone suppress the SP curve.

(4) Unbalanced MudsUnbalanced mud columns, with differential pressure into the formation, can cause streaming potentials that increase the SP deflection, especially in depleted reservoirs. There is no way to handle it quantitatively. This effect is called the electro kinetic SP.ConclusionSpontaneous Potential (SP) is due to combinations of two phenomena which are :Electrochemical potential is created by the contact of two solutions with different concentration. It consists of liquid junction potential (Ej) and membrane potential (Em). Electro kinetic potential is created when a solution is forced, by different pressure, to flow through a membrane. It is usually negligible.

SP logs is usually used to identify :The permeable zoneBed boundariesDepositional environment

It also usually used to compute the shale content and formation water resistivity.

REFFERENCEShttp://en.wikipedia.org/wiki/Spontaneous_potential_logginghttp://www1.uis.no/Fag/Learningspace_kurs/PetBachelor/webpage/tech%5CWell%20logging%5CUnit-2%20The%20Spontaneous%20Potential%20Log.htmhttp://petrowiki.org/Spontaneous_(SP)_loghttp://www.academia.edu/6396507/Well_Logging_and_Formation_Evaluation_PAB2084_and_PCB_2044_Spontaneous_Potential_SP_Log