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Unformatted text preview: aced next to the detector and the tool is used to make a measurement. The calibration is designed such that the calibrated readings of the tool accurately report difference in the amount of radiation from each of the radiation sources. 60 Geology & Geophysics in Oil Exploration Mahmoud Sroor Geology & Geophysics in Oil Exploration Spontaneous potential: (SP)
The spontaneous potential log (SP) measures the natural or spontaneous potential difference (sometimes called self-potential) that exists between the borehole and the surface in the absence of any artificially applied current. It is a very simple log that requires only an electrode in the borehole and a reference electrode at the surface. These spontaneous potentials arise from the different access that different formations provide for charge carriers in the borehole and formation fluids, which lead to a spontaneous current flow, and hence to a spontaneous potential difference. The spontaneous potential log is given the generic acronym SP. Origin of SP current: • Electrochemical components • Electro kinetic components Tool Operation: The tool is extremely simple, consisting of a single electrode that is connected to a good surface earth point via a galvanometer for the measurement of DC potential. A small 1.5 V battery is also included commonly to ensure that the overall signal is measured on the correct scale. Uses of SP: • The detection of permeable beds • The determination of Rw • The indication of the shaliness of a formation • Correlation Notes: • The SP tool has a poor resolution. So it can be used for correlation. • The drilling mud salinity will affect the strength of the electromotive forces (EMF) which give the SP deflections. If the salinity of the mud is similar to the formation water then the SP curve may give little or no response opposite a permeable formation; if the mud is more saline, then the curve has a positive voltage with respect to the baseline opposite permeable formations; if it is less, the voltage deflection is negative. In rare cases the baseline of the SP can shift suddenly if the salinity of the mud changes part way down hole. • Mud invasion into the permeable formation can cause t...
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This note was uploaded on 01/05/2011 for the course GEOL 1 taught by Professor Jenniferalford during the Spring '10 term at Indian Institute of Technology, Roorkee.
- Spring '10