June 17, 2014 - Nom Geo
Spontaneous Potential ( SP ) log and Formation Water Resistivity
Spontaneous potential measures the naturally occuring potentials in a wellbore for each depth. In petrophysics it is useful for formation water resistivity, clay content and bed thickness estimations. In geology it is useful for facies identification, textural character and bed correlations between logged wells.
To explain further, there are two types of potential being measured.
1)electrochemical potential – measurement of cations vs anions – always relevant
2) electrofiltration, also known as electrokinetic or streaming potential.- relevant where water resistivity is high and where permeability is very low.
What the SP curve is showing is the changes in the electrochemical properties of the formation waters due to lithology.
The SP curve takes advantage of the electrical difference across a membrane of water with different ionic compositions, namely the difference in salinity. It operates much as a battery, where the difference results in a current flow.
In most cases it is the shape of the generated log curve, and the size of the positive or negative deflection that is useful for lithological determination.
Using the example of a shallow permian well from our previous post on the Rwa method (Groundwater salinity post) the SP curve is:
Because bed thicknesses required in the SP method for water resistivity calculations need to be above 3 metres (where the SP correction factor is at a hefty 6x to 10x) the zone the calculation will be made for one zone only, the 280 to 290 m interval. To recap, the water resistivity calculated for this zone show a pronounced drop in value, indicating a high salinity water zone, or a zone where there is conductive clay.
The method will be used as a calculation;
The SSP value will be derived from the SP value with a correction factor of 1.1x from the following chart:
From the graph of the SP curve in our chosen interval, at 284.8 m the max SP reading is equal to -123.76 Mv.
Hence the SSP = -135.3 mv
From wireline logs at 284.8 m the formation temperature is:38.31 celsius or 100.96 fahrenheit and mud resistivity is equal to:
Rm (from log header) x (surface temp (F) +7/Depth Temp (F) +7) and this is equal to 0.152 ohm m.
We will assume that the value of the mud filtrate Rmf is equal to the MRRS log value which at that depth is 5.516 ohm m.
and Rwe = Rmf/Rmfe = 5.52/66.39 = 0.083 ohm m
Empirically water resistivity, Rw is calculated as follows;
Using ii) above: Rw = (77x 0.083+5)/(146-337x 0.083) = 0.0965 ohm m,
If we compare this with the values of the Rwa method of 0.09 ohm m this result is very close and hence both methods return a consistent result.
There is a problem with this method, and again we return to the initial assumptions. Despite the closeness of the results, the method has not precluded the influence of bound ions in clays influencing the result. In other words, is the water resistivity really represented by the 0.09 ohm m value?
Additionally I have chosen the SP left deflection at 272.3 m at -129.79 ohm m which using the same methods above results in a value for Rwe = 0.104. Since this is less tha 0.12 the same formula as the example above can be applied to give; Rw = 0.117 ohm m which is not far from the Rwa method result of 0.13 ohm m. from 274.3 m to 279 m.
In the above examples the values for Rmf was used in the calculated Rwe value. whereas in the graphical methods the Rm / Rmf value can be used.
For this post I would like to acknowledge the following works which helped me to follow the methods.
USHIE, FA, 2001: FORMATION WATER RESISTIVITY (Rw) DETERMINATION: THE SP METHOD. Journal of Applied Science and Enviromental Management 2001; Vol 5 (1), 25 – 28
My next post will look at using the Sp curve to identify facies.
Please leave a comment if you wish.