Over the last four decades, solutions for cased-hole measurements have reached significantly improved levels. While the intrinsic physical measurement process has remained stable, the generation of tools have greatly advanced.
Unfortunately, despite advancement, there is still one critical piece of information missing and that’s a porosity measurement. Specifically, an accurate gauge of the presence of formation gas or a measurement to combine with neutron porosity. In the absence of this resource, primary formation assessment in cased wellbores risk ambiguity.
In hopes of closing this gap, the first intrinsic measurement for cased-hole formation evaluation was introduced in 2015. The measurement (FNXS) is related closely to atom as opposed to bulk density. Gas volume is assessed through inelastic cross sections with little elemental variance. But due to the fact elements like oil, rock and water can have similar numbers of atoms per unit volume, the FNXS measurement responds to liquids and rock pretty much the same way.
The new measurement does have unique reaction to gas, gas volume and total porosity. This makes it an excellent solution for use in combination with other cased-hole measurements. The FNXS measurement is also a good tool for obtaining total porosity using info acquired in a cased wellbore. This would be accomplished through a combination of liquid-filled porosity based on neutron porosity with FNXS gas volume.
Not being able to measure gas formations has long been a challenge when conducting primary evaluation in cased-holes. Since the 1960s, evaluation (open hole) has used gamma-gamma density measurements for identification of gas-bearing formations with a conventional neutron-density gas crossover effect.
The FNXS measurement will be instrumental, replacing the required density measurement and offering a far more robust solution than a cased-hole density measurement. It will supply that one missing piece of information, changing the entire industry.