Spontaneous Potential - Interpretation

Interpretation

SP can be affected by several factors that complicates the interpretation. Beside petrochemical component, SP is also affected by electrokinetic potential and bimetallism. Besides, SP is also affected by the following factors:

• Bed thickness (h); Since SP is a measurement of electrical potential produced by current in the mud, its amplitude approaches the SSP value only when the resistance to current due to formation and adjacent beds is negligible compared with that of the mud. This condition is met only in thick bed. In thin beds, the SP is proportionally reduced.
• True resistivity (R_t) of permeable bed; As R_t/R_m increases, the SP deflection decreases, and the bed boundaries are less sharply defined. Presence of hydrocarbons also attenuates SP.
• Resistivity of invaded zone (R_xo) and mud resistivity (R_m); SP increases with increase of R_xo/R_m
• Diameter of invasion (d_i); SP decreases as invasion deepens
• Ratio of mud filtrate to formation water salinities: R_mf/R_w
• Neighbouring shale resistivity (R_s); SP increases with increase of R_s/R_m
• Hole diameter (d_h); With increasing hole size, the value of SP is reduced