Core … Does it Really “Ground Truth” Reservoir Properties?

Core … Does it Really “Ground Truth” Reservoir Properties?

Juan Cottier Juan Cottier
Director & Lead Geologist at MMbbls Limited

Firstly, to get a terminology comment off my chest: for me using the term “ground-truth” is like using “passionate” or “synergy” or “hunkering down” … we all think we know what it means but it is lazy speech and does not carry a substantial or clear enough definition to be useful. So when we say “ground truth” we aren’t really explaining what we really mean ... and that ... might be at the core (forgive me) of the problem.

That said: the main crux of my concern is that the drilling, recovering to surface, handling, boxing, transporting, plugging, cleaning & drying and finally the very act of measuring will all change the core properties from that of the reservoir. Sure, there are all sorts of core-lab corrections, overburden correction being foremost, but the damage (literally) has already been done. It seems most unfair that e-logs are treated with all manner of suspicion and considered “uncertain” whilst core is considered “ground truth”. We always correct a Phie CPI log to the RCA porosity, not vice versa.

Are we being told that you can take a 25 year old bloke from Glasgow and put him in a bar in Prague for the weekend knowing that he will behave differently but if we take a 110 million year old piece of Albian reservoir from 2km depth in the Ivory Coast and send it to a lab in Aberdeen it will behave the same?

Don’t get me wrong, I am a big fan of core (I’ve previously spoken of my admiration for sedimentologists and their contribution to reservoir understanding and Field Development) my concern is that core gets an easy ride. Maybe it’s because it takes so much effort to get onto a drilling programme, and that it costs so much to recover, and that RCA takes months (and SCAL always takes 2 years) … and so at the end of that process we can’t really be heard to say … “those permeabilities are roughly indicative”, and let's face it ... reservoir permeability is the holy grail of our work.

And then we need to talk about scale. Big things and small things. Core plugs, simulation cells and DSTs.

Using my old University; the Geology Dept Williamson Building of Manchester University for scale comparisons.

Image#1: A decent XY representation of a relatively small simulation cell. Courtesy of Google Maps.

Image#2: Taken at base or RED Z-arrow - The height of the Williamson Building (>10m) is at the DST scale ... whilst the ashtray (bottom left) is at the whole-core scale. Courtesy of my wife.

I like probe-permeametry, I remember it arriving in Aberdeen in the mid90s and it was knock-out then and pretty cool now. Nobody seriously suggests that probe-permeametry values are accurate for a reservoir but they do give a wonderful picture of the range, variation and heterogeneity or a reservoir.

I also like DSTs, it's very hard to argue against DSTs (though the H in the kH is often up for debate). So whilst probeperm is at the mm scale and core-plugs are at the cm-scale, DSTs are performed over metres and simulation cells are 100s m2 … and yet contradicting core plug RCA values is frowned upon, indeed even forbidden by some including many reserves auditors, because core "ground-truths". Probeperm and RCA, CPIs, field outcrop analogues and DSTs all help us fill in the picture between the mm scale and the 10s to 100s of metres scale.

Most things may be considered useful ... but nothing is “ground truth”.

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