The Field Emission Scanning Electron Microscope allows a different approach to understanding the structure and concepts behind the design of drilling fluids.
Information obtained from the preliminary studies indicates changes may be required in our thinking on what are acceptable fluid parameters, such as fluid loss and how to control this effectively.
Many papers published in the past have shown the basic pore and pore throat structure of sandstone formations. Our observations indicated that these openings could range from one to 100 microns depending on the formation, grain size distribution, cementing materials, clays, etc.
Observations made of the standard A.P.I. water loss filter paper showed that the paper had no structure that could be compared to a sandstone. Several authors in the past have addressed the subject of water loss control (references 3, 4, 5, 6). Looking at the difference between filter paper and a sandstone raised the question of what role Bentonite plays in creating a filter cake. The sample of Bentonite used in this study was taken from a bag of commercial drilling gel. The sample was prepared in several different manners and looked at with a Scanning Electron Microscope to try to answer some of the questions.
The Field Emission Scanning Electron Microscope (F.E.S.E.M.) operates at a filament voltage of 4 to 5 K.V. versus the much higher voltages usually used for Electron Microscope work. Because of the low beam voltage, specimens do not normally require a conductive layer to bleed off the excess energy charge
The lack of coating on the specimens allows us to document clays in a sample, expose the sample to fluids and know there is no barrier preventing fluid contact with the specimen. The sample may be redried then checked for alterations to the clay minerology.
The F.E.S.E.M. has a useable resolution of 100 angstroms so, at higher magnifications, the picture quality drops off. An operating condition of the picture quality drops off. An operating condition of the machine is that the specimen and gun must be kept under very high vacuum (10 to - 10 torr), so specimens must be desiccated adequately. Also, the low beam voltage is not high enough to generate X-rays so, the machine cannot be equipped with a microprobe X-ray attachment. The size of particles can be measured using the micron lines on the bottom of each picture. The line on the left indicates 1 micron and picture. The line on the left indicates 1 micron and the number of lines on the right indicates the power of 10.
i.e. 0 lines on right; line on left = 1 micron 1 line on right; line on left = 10 microns 2 lines on right; line on left = 100 microns 3 lines on right; line on left = 1000 microns
As work progressed, it was found that air or vacuum drying collapsed the gel fabric, so freeze drying techniques were used to try to retain the delicate clay structure. Freeze drying techniques have been used extensively in the field of Medical Research to prevent the collapse of delicate tissue. It was also prevent the collapse of delicate tissue. It was also noted, that even using very sophisticated gold coating techniques, clays were being damaged.
The first sequence of pictures (Series A) were taken of the dry bentonite mounted on an aluminum specimen stub with a thin coat of 5 minute Epoxy. An attempt was made to see if any crystal structure was apparent at various magnifications up to a maximum of 45000X. No crystal structure was found.
The second sequence of pictures (Series B) shows the results of air drying a hydrated sample of the same gel. Note the collapsed amorphous-like structure at higher magnifications. The final picture of the sequence shows a small piece of Bentonite which was not completely hydrated and it is possible to see the collapsed gel material grading through uncollapsed to the unhydrated, unchanged portion.
The third sequence of pictures (Series C), shows the hydrated gel that was freeze dried and photographed up to a magnification of 30,000 times. photographed up to a magnification of 30,000 times.