Spontaneous Potential Logs
Spontaneous potential (SP) results due to two factors which are 1. salinity difference between formation connate water and mud filtrate (liquid junction potential) 2.Presence of ion selective shale membrane(membrane potential).
- Liquid Junction potential:-This type of potential develop across the boundary between mud filtrate and formation water due to salinity difference between mud filtrate and formation water in permeable formation.When an salt solution (sodium chloride) with higher concentration water ( formation water) comes in contact with lower concentration salt water(mud filtrate),ion transfer will initiate.As chlorine ion migrate quickly to less concentrated solution as compared to relatively slower sodium ion.This effect results into more positive sodium ion in formation water and more negative chlorine ion in mud filtrate resulting into potential difference across the junction.
Liquid Junction Potential
- Membrane potential :-This type of potential results due to introduction of shale between fluid of two different concentration of NaCl.As shale by nature is negatively charged,negative chlorine atoms are repelled by the shale membrane whereas positive sodium ions are allowed to pass through the membrane from more concentrated solution to less concentrated solution.Most of the times mud filtrate is less concentrated solution and formation water is more concentrated solution.This results into development of negative chlorine ion in formation water at shale formation boundary and positive ion on other side,this leads to development of membrane potential.
As SP is measured by changes in voltage measured between downhole electrode relative to ground surface.SP does not have any specified origin value,An shale baseline is established by interpreter in nearby 100% shale zone and measurements are referenced to that shale base line.Unit of SP is milli volts (mv). There is no separate tool for SP measurement,it is usually an passive measurement run with old resistivity tools or sidewall cores.
Spontaneous potential development in formation
As one reason for SP is salinity differences between formation connate water and mud filtrate and salinity is inversely proportional to resistivity , amount of SP deflection can also be attributed due to contrast between Resistivity of mud filtrate to resistivity of formation water (Rmf/Rw) .SP development and amplitude can be explained as below:-
- Bed thickness:- In thick permeable bed with large Rmf/Rw contrast ,Sp curve will show large and consistent deflection around Static Spontaneous potential (SSP)
- Rmf/Rw >1 :-SP curve against permeable formations will be negative ,Amplitude will depend upon Rmf to Rw contrast.Negative SP is referred as normal SP.
- Rmf/Rw=1:- There will be no SP deflection.
- Rmf/Rw <1 :- SP curve against permeable formations will be Positive ,Amplitude will depend upon Rmf to Rw contrast.Positive SP is referred as reverse SP.
Primary application of SP logs are:-
- Identification of permeable formations
- Well to well correlation of permeable formation
- Formation water resistivity calculation
- Volume of shale estimation
- Depositional environment from shape of SP curve