Experiment Study on the Mechanisms of Sand Suspension and Settling of Proppant in Fracturing Fluids

The sand carrying performance of a fracturing fluid directly affects the transport and placement of proppant in formation fractures and the effective flow conductance of the fractures after fracturing. A model "XS-I" instrument for simulation experiment on sand suspension and proppant settling has been developed to study the suspension of three ceramsite proppants (particle sizes of 70/140 mesh, 40/70 mesh and 30/50 mesh) in a SRFP-1 fracturing fluid. Using XS-I, the amount of settled proppants, settling rate and the changing pattern of the amount and settling rate with time in suspension fluid were studied and the main factor affecting the sand carrying capacity of a fracturing fluid was obtained. Laboratory experimental results show that the settling of proppants in suspension fluid can be divided into three stages:fast settling, slow settling and stabilized equilibrium. The main factor affecting the suspension capacity of a fracturing fluid is the viscosity of the fracturing fluid, with particle sizes and sand/fluid ratio being the minor affecting factors. Fracturing fluids of low viscosity can only suspend the 70/140 mesh proppant (time for the proppant to completely settle is 10-20 min), but the suspension of the 40/70 mesh and 30/50 mesh proppants in low-viscosity fracturing fluids is poor (time for the proppants to completely settle is only 1.0-5.5 min), and generally speaking the low-viscosity fracturing fluids have poor suspension capacity. Fracturing fluids of medium viscosity have good suspension to the 70/140 mesh proppant (only 9.9%-11.1% of the proppant was settling). When sand concentration is less than 15%, fracturing fluids of medium viscosity have good suspension to the 40/70 mesh and 30/50 mesh proppants (time for the proppants to completely settle is 80 min-240 min). Fracturing fluids of high viscosity have good suspension to large particle size proppants (30/50 mesh) with high concentrations (25%-30%); only 12%-13.1% of the proppant settles down. This means that a high-viscosity fracturing fluid can be used as a sand carrying fluid at the major sand mixing stage. The results of this study provide a new evaluation method for testing the sand carrying capacity of a fracturing fluid and for evaluating and optimizing proppants, and can be used in optimizing fracturing job parameters and selecting proppants.