Near Infrared Spectroscopy Finding Use in Energy Industry
Near Infrared Spectroscopy (NIR) is an easy mineral identification technique that allows for onsite, real time mineral identification. Energy operators may be most familiar with this technique as a remote sensing tool, where aerial spectral surveys are conducted to identify seeps, hydrocarbon altered soils or identify regional mineralogy. However, NIR can also be used as a portable contact field tool able to quickly identify minerals in the field or during drilling.
NIR has been successfully used for decades to characterize mineralogy in and around ore deposits as NIR is particularly successful at differentiating fine-grained mineral species that are impossible to identify in the field. The use of NIR enables exploration geologists to differentiate between mineralogy alteration types (argillic, propylitic, phyllic, etc) that lead them to deposits faster.
As mineralogy and clays in particular become more important in the energy sector, unconventional technologies such as NIR field devices will become more important to produce this valuable information for energy operators.
NIR is field portable, with results available in seconds. The technique can differentiate between fine grained clay and chlorite species.
Minimal sample prep time is needed. The one caveat here is that the sample does need to be dry. However, since only the sample’s surface is measured, often the heat from light in the NIR probe will dry the sample surface enough in a few seconds to get a measurement.
With the advent of newer, handheld technologies, instrument operation is easy and can be done by technician-level employees. This technique measures light reflectance, so the energy source is a halogen lamp. There are no radiation safety or transportation issues. It provides consistency in geologic interpretation.
NIR is a safe and easy technique that can identify many important minerals. Full range NIR devices cover the visible and near infrared range from 350 to 2500 nanometers. NIR, quite simply, measures light reflectance: shine a halogen light on a rock surface and a reflectance spectrum that bounces back from the rock surface is collected by a spectrometer.
Minerals reflect differently
Different minerals in a sample reflect or absorb portions of the light energy to produce a characteristic and diagnostic spectrum which is based on the molecular composition of the mineral. The reflectance spectrum of the unknown sample is then matched to a library of mineral spectra and, in some cases, up to seven minerals can be identified in a rock sample. This spectral matching process is entirely automated and is completed in seconds.
NIR is primarily a surface technique as the light barely penetrates the rock surface and so the technique only measures surface mineralogy. NIR is a quick mineral identification tool for determination of mineralogy in drill core, drill cuttings and from hand samples. There are no safety issues and the new handheld NIR mineral identification tools make it extremely easy to use in the field for mineral identification. Mineral results can then be used to make field decisions or can be exported over to third party software for 3D modelling or other purposes.
While NIR cannot detect all minerals, it can identify fine grained alteration minerals important to oil and gas operations. NIR is used by exploration geologists and mineralogists to quickly field identify important alteration minerals like clays and chlorites as well as minerals from other mineral groups like carbonates, sulphates and iron oxide minerals.
The ability to differentiate different species of clay is something NIR does better than XRD in most cases. XRD struggles to detect clay minerals that are present in lower quantities or those with poor crystalline structures (Hosseininejad, 2014); something that NIR can do in the field.
The type of clay can tell the geologist a lot about formation temperature. For instance, the illite/smectite transition is often an excellent indicator of thermal maturity (Pollastro, 1993) as illites and smectites are clay species minerals that can be readily identified using NIR. In addition, NIR can monitor chemical shifts in clays and micas as chemistry reacts to changing environmental factors – a popular exploration tool for mining geologists. For instance, a shift from lower pH phengitic mica to higher pH paragonitic mica can be seen spectrally by monitoring slight wavelength shifts and is important in determining proximity to a deposit (Hermann et al, 2001). A similar spectral wavelength feature in low temperature and high temperature chlorites can also be monitored by NIR (such as Amera, 2007 and White et al., 2010).
The ability to quickly identify clays in the field has many implications in oil and gas exploration. As noted by Jiang (2012), “the presence of clay minerals strongly influences the physical and chemical properties of conventional sandstone, carbonate and unconventional shale.”
Clay minerals can be used to interpret thermal history, correlate strata, as indicators of hydrocarbon generation and help determine porosity. Quickly identifying clay mineralogy in the field and monitoring spectral features such as those described in the previous paragraph can greatly assist exploration efforts. NIR can even be used for clay quantification that Jiang notes can help avoid exploration failures as well as minimizing costs.
Carbonates are an important group of exploration minerals that are easily individually identified by a strong carbonate absorption feature in the near infrared region. Identification of this absorption feature allows for easy differentiation of calcite, dolomite and other carbonates. By monitoring the location of this feature, the geologist can get an immediate idea of porosity/dolomitization from the location of this feature on the scale as it will shift to reflect the Ca/Mg ratio.
Example energy applications
The energy industry is realizing the importance of clay minerals in both conventional and unconventional exploration. Being able to rapidly identify those minerals and monitor changes in mineralogy improves and hastens the exploration process. Some applications where NIR can assist energy operators include:
Whether you just want to get a quick assessment of mineralogy before you start a process, quickly pre-characterize samples so you send meaningful samples to the lab, onsite core analysis for real time decision making or monitor mineralogy from a drilling program, NIR expedites exploration efforts and minimizes costs for oil and gas operators.