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Geochemical logs
Geochemical logging is still struggling to find applications. Schlumberger’s latest incarnation is called the environmental capture sonde (ECS).[1] Applications lie primarily in rock and clay typing for reservoir description. The goal is to add additional elemental concentrations to the formation model.
What can be learned
Natural gamma ray spectroscopy measurements provide data for:
- Potassium
- Uranium
- Thorium
The capture spectroscopy tool detects:
- Silicon
- calcium
- Iron
- Sulfur
- Gadolinium
- Titanium
- Chlorine
- Hydrogen
In analyzing the data, elemental concentrations are derived and processed to obtain a complete mineralogical description.
There are several sources of ambiguity. Most importantly, uncertainties in the measurements of the elemental concentrations are not explicitly accounted for. Because elemental concentrations derive from least-squares deconvolution of unresolved gamma ray spectra, they are not determined independently. Furthermore, sensitivities to different elements vary by orders of magnitude and often are very small, requiring large integration times. Finally, element-to-mineral mapping is not sufficiently unique. For best results, a limited suite of minerals must be selected before analyzing the data. The correct choice of a mineral suite depends on knowledge of local mineralogy from other experiences, such as core in an offset well. This arises in part from incomplete and inaccurate elemental analysis and partly from the nearly infinite variety of minerals and the small number of elements. As an example, quartz and opal contain the same elements but are quite different in their impact on a reservoir.
Mineralogy can in turn be related to such properties as:
- Permeability
- Porosity
- Cation/ion-exchange capacity
Unfortunately, the minerals-to-petrophysical-properties inversion is not unique either, partly because the tool has no information about the physical configuration of the minerals (for example, grain size or fractures). Even when geochemical logging can give accurate elemental abundances, conversion of those numbers to mineralogy and petrophysical parameters such as permeability still requires a locally calibrated interpretation model. With limited goals and careful local calibration, geochemical logs do provide useful information. The logs remain tied to a local database and ad hoc knowledge to relate mineralogy to petrophysical properties.
References
- ↑ Herron, S.L. and Herron, M.M. 1996. Quantitative Lithology—An Application for Open and Cased Hole Spectroscopy. Paper E presented at the 1996 SPWLA Annual Logging Symposium.
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