Borehole Geophysics - Spontaneous Potential (SP)

The objective of this Activity is to provide a basic framework and criteria for the use of SP logs. An exercise will be used to as a teaching tool for this Activity.
The Spontaneous Potential (SP) log is a measurement of the electrical potential (voltage) that occurs in a boring when fluids of different salinities are in contact. The electrical potential is produced by the interaction of formation water, conductive drilling fluids, and certain ion-selective sediments (clay). The potential is generated when permeable formations are invaded by drilling fluids, which allow different fluids to be in contact. Impervious formations, like clay layers, have no invasion. Thus, no SP currents are generated and the SP curve is flat and featureless. Permeability contrast allows the development of a membrane potential that yields an electrical potential and the SP curve deflection. The magnitude of the SP curve deflection is determined by the ratio of the resistivity of the mud filtrate (Rmf) and the resistivity of the formation water (Rw). Formation water is defined as naturally occurring groundwater within a specific formation. The SP log is useful in: (1) identifying potentially transmissive beds; (2) defining bed boundaries and bed thicknesses; (3) correlation of sands and clays between different boreholes; and (4) determining the formation water resistivity, water conductivity, and total dissolved solids.