Didn't read the article but from what I understand drilling fluid for environmentally sensitive areas tends to be water with bentonite clay and a small amount of thickener like barium sulfate or calcium carbonate (i.e. seashells) in it.

The independent analysis [1] concluded that "there are currently no adverse environmental, scenic, recreational, or economic impacts resulting from the drill break or presence of Remaining Materials 50 to 70 feet below the sea floor, nor is there a reasonably conceived scenario (e.g. earthquake, tsunami, long-term coastal erosion) that would expose the Remaining Materials to the surrounding environment and result in future impacts."

The drilling fluid is in a borehole ~60 feet underground, and has hardened into a solid (as it's designed to do when not under the mechanical stress of drilling). It's mostly water and clay, with a few additives, all of which are below their ecotoxicity threshold concentrations, even at full concentration in the borehole. And if the drilling fluid ever entered the ocean (which it won't), it would be diluted such that "concentrations of the drilling mud additives and their chemical constituents would be orders of magnitude below ecotoxicity threshold concentrations (and therefore non-hazardous to natural resources) and undetectable".

[1] https://oregoncoastalliance.org/documents_20/edge-cable/06_F...