A brief discussion of Eric van Oort’s paper
(AADE-16-FTCE-77) on testing shales against drilling fluids for compatibility
By now those of us in the drilling/drill-in fluids (DF/RDF) community are used to having our eyes opened by Dr. van Oort on this and other topics related to drilling and completion operations. In addition to being a prolific researcher and author, he is a veteran of almost thirty years in our industry, beginning with Shell and continuing in his current role as the Lancaster Professor of Petroleum Engineering at the University of Texas at Austin. His reputation is such that when I heard from a colleague that he would be presenting this paper at the AADE Fluids Technical Conference and Exhibition, I made immediate plans to attend in person. I wasn’t disappointed.
The test of most good papers is that when you read them, a light begins to dawn and you finish them thinking, this was so obvious…why didn’t I think of that? AADE-16-FTCE-77 is just that sort of paper.
Let’s dive a little deeper.
Another test of a good paper also lies in its relationship to a concept already well understood in the industry, but not yet having been firmly applied to the particular topic being covered. Another test AADE-16-FTCE-77 passes with ease. In this case, the industry has understood the importance of preserving core for study at formation pressures and temperatures (or as near as possible in some HPHT cases), for decades. Entire industries have built up around performing core testing under simulated downhole conditions. How is it then, that when testing drilling/drill-in fluids for shale compatibility, the predominant tests that are performed initially for screening candidate DF/RDF formulations, are often done on formation material that has had no attempt at preservation, and are performed at ambient conditions?
It is a conundrum.
Speaking as someone who has commissioned testing like Professor van Oort describes, and made material inclusions for shale inhibition in an RDF formulation based on that testing…I feel a little foolish. This is probably another good test of a technical paper’s quality.
I encourage you to read the entire paper and provide a link at the end of this article for down loading it. For those whose time and interest in the topic are more limited, I will cherry-pick the essence of the paper for you in the next couple of paragraphs.
One postulation that van Oort puts forward in the paper that resonates with the informed reader is that, one of the effects of poor sample preservation is desiccation of the sample. The impact of this can be that when the sample is re-hydrated, air that has seeped in to fill the voids (remember nature abhors a vacuum), is pressurized and may literally blow the sample plug apart. If this happens during, say, a liner swell test a strong swelling response to the fluid will be recorded. This has the potential to create a test artifact that may contribute to highly misleading results. A potential example of this shown below. Was the test fluid the problem here?
Middle-East shale appears to swell, delaminate and fall apart in a linear swell test when exposed to a KCL/glycol fluid. No attempt had been made to preserve this material in its natural state, and the test was done under ambient conditions. An oil-based fluid was selected largely on this testing.
Van Oort’s second big takeaway is that when the effects of confining pressure and cementation are removed, i.e. in sample that have dried out or been ground up, the swelling effects can be exacerbated. This opens the exit door for a lot of standardized shale testing that is routinely done under ambient conditions across the industry. Tests like: CEC testing, Bulk Hardness, and Linear Swell Testing.
Finally, van Oort is of the opinion that the above atmospheric, or ambient testing is so misleading that these tests should be discontinued. He argues (persuasively to my mind) that while these tests are inexpensive, if they produce confusing or misleading results, why do them?
van Oort closes by giving us some ways to legitimately go forward in shale testing, including some that may be done at ambient conditions. These include the Accretion test, Dispersion test, and Odeometer test done under applied load, and a series of tests run under pressure, or applied load to gauge bore hole stability. These include the Triaxial Failure test, the Pressure Transmission Test, and the Modified Thick-wall Cylinder test.
In my view, this is top-notch work by one of the industry’s leading researchers, and the point of this article was to showcase it to a wider audience than those, like me, fortunate enough to be in attendance at its presentation. Hopefully, many readers of this article will want to read the entire paper. This can be done by pasting the following link into your search line.
If you do read it, I’d love to hear your thoughts. Please come back to this article on Drillers.com and make a comment.
Dave Messler is an oilfield veteran with 38-years of experience in the industry. He is currently a Senior Trainer at ACME Fluids Training, a small organization that focuses on teaching the development of reservoir drill-in fluids (RDF’s) to the industry.