Well Log Analysis & Consulting
We offer well Log analysis, consulting, teaching and coaching to those of the oil and gas industry who are in need of the most rigorous log calculations of water saturation, porosity and permeability available. The rigorous interpretations provide the best Net-to-Gross Pay and reserves available.
Our Philosophy of Interpretation
1) Eliminate guesswork. One can calculate the cementation factor, “m”, and the saturation exponent, “n” using full mineralogy. The Archie equation is Sw^1/n=(1/porosity^m)*(Rw/Rt). Water saturation is more accurate when the appropriate “n” and “m” are used. Full mineralogy provides a method to calculate “m” and “n” to ensure consistency, infinitely better than the Pickett plot. We always calculate full mineralogy.
2) Why would one use Archie, Simandoux or Indonesian equations for Sw? Answer: to check for an approximate saturation that may be 100% in error. Otherwise, Sw equations should not be used because CEC – corrected Dual Water and Waxman-Smits-Thomas et al, is available to be used.
3) What can be done if there are no elemental (Capture or Inelastic Spectroscopy) measurements to supply elements for a full mineral interpretation? If neural net or Geological Analysis by Maximum Likelihood Systems (GAMLS) are available, the elements can be predicted from offset wells where the elements were run in same field. Hence, there is opportunity and encouragement to run at least one set of elemental measurements per field. The carbonate content in the Duvernay fields, for example, require measurements on every well. Alternatively, 50 or so core mineral measurements coupled with offset well element measurements. We predict elements, in all wells, using the best prediction clustering software on the market: GAMLS. In addition, we can predict any missing curves from CMR/NMR or Sonic Scanner.
4) In bad-hole, where density is bad, masquerading as high porosity, due to the rugosity effect on density pad contact, we null the bad-hole density and predict again, using the good-hole density. How do we know bad density? Compare sonic porosity from Raymer-Hunt-Gardiner transform, limestone matrix, to density porosity on limestone matrix. Density porosity should rarely (almost never) read above sonic porosity.
5) Valid formation water resistivity is requirement of a valid interpretation. Use the log data to derive a consistent result. First, use the spontaneous potential (SP) and an estimate of local Rw, to calculate an approximate variable Rw Then, interpret with full mineralogy to calculate the wet resistivity, Ro (i.e. F*Rw=Ro). When full mineralogy is known, then cation exchange capacity (CEC) is also known as it is an attribute of each clay family. The CEC effect is corrected on Ro, so that Ro and true resistivity, Rt, can be compared in all low resistivities, which are often shales. Previous SP methods required a clean water sand. This method does not. Hence, the data determines the correct Rw to use.
6) Many analysts who use a Vsh method believe they have the right answer if they can get close to RCA core porosity and core Sw. However, to be ‘right’ they need to also be close to core mineralogy, core elements, core permeability. I say ‘close to’ because one must allow for differences in weight and volume fractions as a result of the volumes and depth differences being measured. One solution might be to consider averages. One must not drown when crossing a stream that averages two feet deep, with a 10 ft drop in the middle. Plotting and depth correcting core data points to logs is a difficult but necessary first step to a valid interpretation. A correlation of Vsh to a core clay weight fraction is the first step down the wrong road. One needs a correlation to all the minerals. Not just lithology like Vsh. Conclusion? Use elements to derive mineralogy. At least check cuttings analyzed with QemScan. XRD can work only if done right and if illite and muscovite can be separated via elemental analysis.
7) Bottom line: do the best possible Mineral-based interpretation rather than Vsh-based interpretation, utilizing every input one can find, including offset wells, core mineralogy and RCA, formation.