Mr. Scott Montgomery's article on oil well core preservation (OGJ, Nov. 15, 1999, p. 84) reminded me of my first oil field job: core analyst for Core Laboratories Inc. in Midland, Tex.
Snyder field was booming and the entire Permian Basin was active. The Spraberry trend was just getting started. We brought in hundreds of feet of core every day for analysis including the San Andres, Wolfcamp, Fusselman, 3-Bar, Woodford (a notorious pipe-sticking shale), Ellenburger, Clear Fork, and impressively Canyon Reef from Scurry County. Shales like the Woodford, by the way, were usually cored accidentally, and rarely, if ever, analyzed.
We performed two types of analysis: 1) conventional, as it was called, which destroyed a good part of the core, and 2) whole core, which entailed vacuum distillation and then permeability measurement by the Klinkenberg method. The whole core method was developed to improve porosity measurement in vugular formations and rapidly gained popularity as Snyder field grew in size.
Most customers never saw the cores again after they came into the lab. All they were interested in was the analysis. But some did, and we duly delivered the core back to them if they requested it. Ninety percent of the cores were crushed for yard fill or hauled off to the city dump.
I had a friend who was a professor of geology at Texas A&M, and I often sent him curious specimens that were fated for the rock pile. Every day in the course of our work we saw several species of brachiopod, crinoid stems, and vugs, some of them oil-filled. The Fusselmand had remarkably beautiful geodes, some oil-filled, some not.
Even today I can remember a 6-in. section of Canyon Reef that was nothing but crinoids, more or less perfectly preserved. It was barely cemented and we could extract individual crinoids with stems intact. This was, of course, a rarity.
Later in life I became a reservior engineer and found my early experience in Midland invaluable. Seeing all those reservoir rock specimens laid out in the lab gave me an insight into porosity, permeability, and fluid saturation that, while not unique, was not common either. We observed that cores laid out in the yard weathered rapidly, especially shales and shaly limestones. Cores subjected to vacuum distillation weathered even more rapidly.
Sometimes after a rare summer rain, cores that had been in the yard for some months became rows of sludge. Of course, the denser rocks of low permeability held up pretty well. It was the good reservoir rock that disintegrated quickly.
I agree with Mr. Montgomery that we ought to preserve as much core as economically possible. But even under ideal conditions, cores deteriorate over time. As important as the cores are the analyses that reveal the rock properties and give some indication of original fluid content. Mile after mile of analyses exist, mainly in musty oil company archives. It would be great if the core repositories could also contain the analyses that once determined the fortunes of long-vanished oil companies.
Linton Morrell
Biarritz, France
