Fracturing gels are formulated to have a high viscosity sufficient to generate fracture geometry and transport proppant materials. Fracturing fluids are subjected to extensive QA/QC testing on high-temperature rheometers to ensure they have adequate viscosity to perform the intended task and break within a time frame suitable for the operation performed. Although most rheological tests incorporate only the gelled fluid without proppant materials, it is assumed that the fluid will transport proppants to their desired location. A field location reported that treatments containing certain proppants were screening out more frequently. Tests were performed on a slurry viscometer (SV). It was discovered that not all proppant materials of the same nominal size and density were transported equally when added to a fracturing gel. The typical properties of the borate gels were measured, but no significant differences were found that would account for differences in transport. All the proppants had nominal 20/40 mesh size. A laser analysis of the particles indicated small differences in the distribution of sizes. However, the size analysis did not fit the trend that "bigger settles faster." Scanning electron micrographs showed differences in surface roughness between the ceramic materials and sand. The proppants were coated with a tacky substance to neutralize any chemical absorption effects. The coated proppants then settled in a manner consistent with their physical characteristics (size and density), but the tacky material caused agglomeration and faster settling than before. The coated proppants were treated chemically to temporarily disable the tackiness, and it was observed that all the materials settled in a manner consistent with their physical characteristics. It was found that certain 20/40-mesh proppants were suspended twice as long as other proppants in the same gel environment. The proppant effect was measured with the SV. The trend from the SV instrument correlated well with the screenout rate observed in the field. Analysis indicated a surface catalytic effect of ceramic proppants causing faster decomposition of oxidizing breakers and accelerated gel breaking. This effect was absent in fluids with uncoated sand. Some resin-coated proppants were found to also accelerate breaking because of leaching of chemical compounds from the coatings. Therefore, it is not correct to assume that all proppants added to fracturing gels will be transported equally. Viscosity measurements of gels without proppant might not give accurate information about the behavior of gels with proppant if the proppant interacts with the gels.
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