Investigation of the Affects of Bentonite in Cement-Bentonite Grouts used for Monitor Well Completion
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Bentonite is one of the most common additives to cement to form grouts for completion of monitor wells. Recent studies have indicated that these grouts may not be the most appropriate method for completing monitor wells, because of fractures that form during curing of the grouts. Within grouts examined with optical microscopy, a reaction halo is present around the fracture. Further microprobe examinations indicate a zonation of elements around the fractures. Studies on cement-bentonite grouts that are used for slurry walls have indicated that montmorillonite dissolution and the formation of secondary minerals can lead to fracturing of the cement-bentonite grouts. This study will examine cement-bentonite grouts, which were used in the optical microscopy and microprobe study, to determine if secondary minerals can be identified. The study used powdered x-ray diffraction to examine the minerals present within four differently prepared cement-bentonite grouts and one neat cement grout. Three minerals were identified with x-ray diffraction in all mixtures of cement-bentonite grouts and the neat cement-grout. Those minerals were calcite, vaterite, and portlandite. The three minerals had variable intensities within each grout. There was one unidentified peak within the grouts that contained Aquagel®. Based on the results of the x-ray diffraction combined with the results of previous work, the unidentified peak was determined to be calcium silicate hydrate, which is a common secondary mineral formed from the dissolution of montmorillonite. It was concluded that the variation in intensity of the identified minerals was likely the result of excessive or inadequate hydration depending on the mixing method. It was also concluded that the fracturing within these grouts is the result of the dissolution of montmorillonite and formation of the secondary minerals. Based upon these results, it is clear that the common practice of completing monitor wells with cement-bentonite grouts in most geologic conditions is inappropriate. It is recommended that cement-bentonite grouts should only be used to complete monitor wells that are in fractured hard rocks, because the fracturing of the grout will be similar to that of the local rock.
Haney, Stephen (2015). Investigation of the Affects of Bentonite in Cement-Bentonite Grouts used for Monitor Well Completion. Master's thesis, Texas A & M University. Available electronically from