$388 offer expires on November 29 midnight! (California time)
First year full license + learn set.
Only $388 (Regular price: $688)


REFERENCE

Calculate Shale Volume from NeutronDensity porosity differenceEstimating the Shale Volume (VSHALE) from Gamma Ray (GR), remains the most popular standard technique to compute an initial curve of rock shaliness. However, the approach is far from perfect due to several criticisms:
If modern logs are available, it is good idea to compare several Shale Volume estimates. The Neutron Porosity to Density Porosity difference is perhaps after the GR, the next preferred method to calculate the Volume of Shale. Provided that the reservoir under study does not contain natural gas, shaly sands will show higher neutron porosity values than density porosity values. Conversely, if neutron porosity is smaller than density porosity (called gas crossover), it is a very strong indication of gas presence. If there is no gas present in the formation, the neutron porosity to density porosity difference tends to have a linear response with rock's clay content. This is because the neutron porosity tool is very sensible to the high hydrogen index of clays, while the bulk density tool does not. Even better, the difference does not depend upon Gamma Ray, so it provides a GR independent estimator:
Clay minerals like kaolinite, chlorite, illite, and montmorillonite, have a high hydrogen index. However, the minerals quartz & feldspar, calcite, and dolomite, present in clean sandstones, limestones, and dolostone matrices, don't. The table below highlights clay minerals properties. Notice how their chemical formulas contain hydrogen atoms, while quartz, feldspar, calcite, and dolomite, don't. The figure below shows typical rock mineral petrophysical properties. Compare H atoms content in the clay formulas.
Since the neutron porosity tool signal depends strongly upon sandstone, limestone, or dolostone matrix lithology (the bulk density tool doesn't), it must be converted or corrected to the actual reservoir rock matrix column before applying the difference equation between neutron porosity and density porosity. The GeolOil Functions module provides the necessary conversions between matrix lithologies. In most of the cases, the logging company delivers a neutron porosity curve calibrated to a 100% pure limestone matrix lab sample (regardless of the actual reservoir lithology). Then, if the actual reservoir consists of sandstones, its signal must be corrected or converted to a sandstone lithology. If the actual reservoir column is complex and have a variable matrix lithology or succession, GeolOil will gladly take a target matrix density lithology curve (instead of a constant) for the conversion. The figure below shows at its bottom the Functions Panel to convert NPHI neutron porosity from limestone to some sandstone matrix
The figure below shows the GeolOil VSH Functions Panel to compute Shale Volume
The log curves plotted below show the result of a new generated .LAS file:
The neutron to density difference method usually does not need corrections for matrix radioactivity or VSH overestimation. It is indeed an excellent VSH estimator that should be computed any time the neutron porosity and bulk density curves are available. However, if the caliper shows a unstable borehole, density porosity washouts might show and the results will be severely affected or unusable. The figure below compares VSH computed as Linear GR index, Larionov and NeutronDensity difference.




Content, and web design © 20122020 GeolOil LLC. You are welcome to refer them. Please give us fair credits under Creative Commons License CCbyND 