Proppant transport is of primary importance in the design of new fracturing fluids. In the prediction of proppant settling rates, a wide variation has been found between measured proppant falling rates and those predicted using correlations based on apparent vicosity. Many fracture design computer programs calculate proppant settling rates using calculation procedures which are based on Stokes' Law or some type of power law settling velocity correlation. Much of the experimental data on which these correlations are based were derived from studies of guar, hydroxypropyl guar and hydroxyethyl cellulose. This type of correlation is inadequate to describe the settling behavior in xanthan solutions, foams, emulsions or crosslinked gels.
Data on the proppant settling velocity and rheology of xanthan polymer solutions and xanthan-hydroxypropyl guar mixtures have been gathered to use in the development of correlations for proppant transport. The xanthan-hydroxypropyl guar mixture has excellent proppant carrying capacity over a wide range of temperatures. In fact, a mixture of 15 lb/1000 gal of xanthan with 15 lb/1000 gal hydroxypropyl guar yields comparable particle settling velocities to a 40 lb/1000 gal hydroxypropyl guar solution at room temperature and superior behavior at 150° F. The exact mechanism of the enhanced particle transport by the mixture has not been determined. However, the rheological behavior of the mixture at shear rates of less than 10 s-1 is important and is a shear rate not often considered in current fluid evaluation studies.