Well Logging: SP & GR Log
Nearly every well log includes on the left track of the log, a spontaneous potential or SP curve or a gamma-ray log. On some displays, both the SP and gamma-ray logs are rocorded together.
We saw in video module GL 402 how the SP curve results from naturally occuring electrical currents flowing between the borehole mud and the formation.
The movement of ions which causes SP currents is possible only in formations with some permeability. However, there is no direct relationship between the value of permeability or even porosity on one hand and the magnitude of the SP deflection on the other hand.
Let's examine a few basic relationships.
First, we see that the SP curve is scaled in millivolts with each division of the scale usually representing 20 millivolts. A negative reading will cause a deflection to the left and a positive reading will cause the curve to deflect to the right. In the case of a freshwater mud and saline formation waters, the potential observed opposite say a permeable sandstone bed is negative compared to the potential opposite shale. Thus, the SP curve will deflect to the left. If the salinity of the mud filtrate is greater than that of the formation water, say in the case of freshwater sands, a positive potential is developed and the SP curve will deflect to the right. If the salinities of the formation water and mud filtrate are similar such as when saltwater base drilling muds are used, there is no SP current flow and thus no SP deflection opposite a permeable bed.
The SP deflection opposite a thick clean permeable formation approaches a value called the static SP or SSP. The value of the static SP can be found by drawing a line through the SP opposite the thick permeable formation and another line through the SP opposite the intervening shale beds. The difference in millivolts between these two lines is the static SP.
Nearly every well log includes on the left track of the log, a spontaneous potential or SP curve or a gamma-ray log. On some displays, both the SP and gamma-ray logs are rocorded together.
We saw in video module GL 402 how the SP curve results from naturally occuring electrical currents flowing between the borehole mud and the formation.
The movement of ions which causes SP currents is possible only in formations with some permeability. However, there is no direct relationship between the value of permeability or even porosity on one hand and the magnitude of the SP deflection on the other hand.
Let's examine a few basic relationships.
First, we see that the SP curve is scaled in millivolts with each division of the scale usually representing 20 millivolts. A negative reading will cause a deflection to the left and a positive reading will cause the curve to deflect to the right. In the case of a freshwater mud and saline formation waters, the potential observed opposite say a permeable sandstone bed is negative compared to the potential opposite shale. Thus, the SP curve will deflect to the left. If the salinity of the mud filtrate is greater than that of the formation water, say in the case of freshwater sands, a positive potential is developed and the SP curve will deflect to the right. If the salinities of the formation water and mud filtrate are similar such as when saltwater base drilling muds are used, there is no SP current flow and thus no SP deflection opposite a permeable bed.
The SP deflection opposite a thick clean permeable formation approaches a value called the static SP or SSP. The value of the static SP can be found by drawing a line through the SP opposite the thick permeable formation and another line through the SP opposite the intervening shale beds. The difference in millivolts between these two lines is the static SP.
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