Neutron density crossplot porosity charts were constructed for clean, liquid saturated formations and boreholes filled with water or water based mud. This chart should not be used for air or gas filled boreholes. Additional charts are available for the sidewall
neutron tools.
The separation between the quartz, limestone, and dolomite lines indicate good resolution for these lithologies. Points for the common evaporites, salt and anhydrite, are also identified.
Key Points
1. Errors in choosing the matrix pair does not result in a large error in the porosity value. only applies when shale and gypsum are not present 2. Neutron porosity is always shown in limestone units. 3. Most commonly used
for quick lithology determination.
4. Points that plot between the lithology lines can be assumed to have a matrix approximately proportional to the distance between the two lithology lines.
6. Gas in the pores can cause the points to plot above the sandstone line.
If lithology is known, the correction for gas is parallel to the gas correction line back to the matrix line.
Neutron and density logs are often used together, the chart method is common, but other equations are also used. Usually they involve some type of averaging to account for the effect of clays and of gas on these logs. Typically RHOB will be converted to density porosity and neutron porosity will be in the apporopiate matix.
Simple average
neutron +
density / 2Sum of the Squares Method
((
neutron 2 +
density 2) / 2) 1/2 Gulf Coast Method (more emphisis on density)
neutron +
densityThe neutron - density porosity is often termed total porosity, perhaps because integrating the neutron it is obviously higher than would be expected in shaly sands. One method for correcting this is to subtact the portion of the porosity that related to clay or in this case shale. This new term is often dubbed effective although it may have no relation the the effective pore space the correction is usually in the right direction.
effective =
total (1-Vsh)Rw from SP
The steps in estimating formation water resistivity from the SP are:
2. Read the maximum deflection from the base line (maximum is used because most sources of error cause the SP to read low).
3. Calculate temperature at depth of interest. Use linear interpolation between surface temperature and recorded BHT if no better temperature data is available.
4. Decide is a streaming potential (Ek) correction should be made,.
Subtract any streaming potential from a negative SP, and add it to a "reversed" SP. 5. Calculate Rmf at Formation Temperature
(Use Arps Formula or Schlumberger Chart Gen-9).
6. Find Rmfe at formation temperature from Rmf, using Schlumberger chart Sp-2
7. Find Rmfe/Rwe , using Schlumberger chart Sp-1, or solving:
o Ec = (61 + .133 T F) log (Rmfe/Rwe)
8. Find Rw (at Formation Temp.) from using Rwe , chart Sp-2.
Only experience in a specific area will tell you how accurate the answer is likely to be. Generally the calculated Rw will be usefully accurate if the following apply:
1. Formations are thick enough for full SP development, and are electrically non-shaly. 2. Rmf is less than 1 ohm-meter (preferably less than .5 ohm-meter) so that streaming
potentials are not high.
3. Formation waters are principally NaCl, and salinities are not less than 10,000 ppm.
Beyond these limitations, the SP can normally be used quantitatively only by applying empirical methods that have been found to work when checked against drillstem test or production test recoveries of
uncontaminated formation water.
Sources of Error
There are many potential sources of error when making Rw estimates from the SP. Users should be
aware of them, and of how large the effects can be, to use the curve intelligently.
Fortunately, the errors are seldom all additive, and frequently they largely cancel each other. Below is a list of the principal assumptions used in the SP theory that may not be true, and that may not be adequately corrected for:
1. Mud filtrate, assumed to be a NaCl solution, seldom is. Errors are greatest for fresh muds.
waters are more saline than about 10,000 -20,000 ppm. Fresher waters have a wide range of composition, and deviate most from NaCl composition for the freshest waters, usually of meteoric origin. Very saline water can have significant concentrations of divalent ions, particularly calcium and magnesium.
3. Activity and resistivity are assumed to have a linear relationship. This introduces large errors from very salty waters, particularly above 100,000 ppm.
4. Streaming potentials can be a major part of the SP if muds are more resistive than 1 ohm-meter, and/or if the hydrostatic pressure due to the mud is much higher than formation pressure.
5. The total static potential (SSP) may be higher than the observed SP used in calculations. The error is important in highly resistive and/or thin beds.
6. Mud filtrate invasion can lower the recorded SP, because of very deep invasion so the electrochemical cell is far from the borehole, and the SP currents are largely in the formation. Very shallow invasion at logging time (because invaded fluids have dissipated) can produce a shale potential across the mud cake that can largely cancel the normal SP. 7. Clay minerals that are electrically charged (almost any clay except some kaolins) will
reduce the SP sharply for quite low concentrations.
8. SP theory assumes that adjacent shale beds are perfect shale membranes, impervious to anions. If this is not true, the measured SP will be lowered.
With so many potential sources of error, plus the fact that the SP is often recorded carelessly, it is surprising that the curve is as useful as it is. In many prospecting and producing areas, quite good Rw values can be obtained by using
the simplified theory given here. Simple empirical corrections can often be derived from local experience, that permit even more reliable answers.
R
wa(apparent water resistivity)
The apparent water resistivity is a very useful and widely used calculation. It can used as an input to the water saturation calculation, or as a quicklook technique for identifying potential hydrocarbon zones. Starting with the Archie water
saturation equation
If water saturation is assumed to be 100%, and solving for Rwa the equation reduces to:
Most users set a =1 so:
To use as quick look technique Rwa = Rw only in 100% wet formations; in
hydrocarbon bearing formations, Rwa computed from the above equation will be
greater than Rw.
Key Points & Assumptions
1. The Rwa technique assumes that Rdeep = Rt invasion must be shallow enough that the
deep resistivity is true resistivity 2. Rw (or salinity) is relatively constant 3. Lithology and shale effects are negligible
4. Zone selected for calculation are assumed to be 100% water saturated
5. Rwa has advantages over other sources of Rw , because is calculated from the same tools in the same environment the final saturation will not be subject to errors of a, and m.