Spontaneous Potential (SP)
This is a record of the direct current voltage differences between the naturally occurring potential maintained by a movable electrode in the borehole and a fixed potential located at the surface. The potential is as a result in the difference between the salinity of the drilling and formation fluid.
The SP log can be used to identify:
- Permeable formations
- Boundaries of permanent formations
- Porosity determinations
- Correlations
- The resistivity of formation water
The readings opposite shales are consistent and are referred to as a “shale baseline”. Opposite permeable formations the SP will move to the left or to the right. The formation boundaries are usually indicated by an inflection point. The log will lose resolution in highly resistive beds, or those with little or no permeability. It is NOT to be run in drilling fluid that is salt or oil based, or air.
Limitations
- Spurious spikes caused by external electrical signals.
- Abnormality caused by highly resistive formations.
- Noisy signal cause by improper grounding.
- Gamma Ray (GR)
Rw from SP
- Determine formation temperature
- Find Rmf and Tf
- Convert Rmf at Tf to a Rmfe value
- Compute the Rmfe/Rwe ration from the SP
- Compute Rwe
- Convert Rwe at formation temperature to a Rw value
