Experimental data and analysis are presented on the static and dynamic settling of proppant in non-Newtonian hydraulic fracturing fluids. A unique High Pressure Simulator (HPS) that closely models reservoir hydraulic fractures is used to study proppant-settling behavior under both static and dynamic conditions. A system of optical fibers and Light Emitting Diodes (LED) acting as a vision system were imbedded in the platens representing fracture walls for quantification of proppant settling under various fluid injection conditions.
Hydraulic fracturing fluids tested include slurries of 35 lb/Mgal guar linear gel with 6 ppg 20/40 mesh sand, 60 and 40 lb/ Mgal hydroxypropyl guar (HPG) linear gels both with 4 ppg 20/40 mesh sand, and borate-crosslinked 35 lb/Mgal guar gel with 7.3 ppg 20/40 mesh sand. Test conditions cover a shear rate range of 60 to 120 sec-1. In addition, the effect of an enzyme breaker on proppant settling rate under static conditions was studied. Tests were conducted with a fracture gap width of 0.375 in. under ambient conditions.
Analysis of the dynamic condition indicates that proppant settling within the linear and crosslinked gels investigated is non-linearly dependent on shear rate. Under static conditions, linear gel slurries settled 40 to 50 times faster than the crosslinked gel containing breaker. It is also shown that an increase in shear rate induces proppant settling in borate-crosslinked 35 lb/Mgal guar gel while it reduces the settling rate in 35 lb/Mgal guar linear gel.
When a particle settles in a given fluid and its fall is not affected by the boundaries of the container or by the presence of other surrounding particles, the process is referred to as "free settling" or "single particle settling". However, when the particles are near each other (in the form of a slurry), the motion of one particle is greatly influenced by the presence of other particles. This process is called "hindered settling".
Particle settling occurs when the gravity vector is greater than the forces contributing to particle suspension. However, the forces that contribute to particle suspension decrease as the fluid moves through surface equipment and downhole through tubing at high shear rates. The reduction in suspending ability occurs as the result of the thermal and shear history effects experienced by the fluid. Within the fractures, the gravity vector becomes dominant and particle settling begins to occur. The build up of settled particles along the bottom of the fracture causes a restriction to flow. As a result of settling, the area open to flow above the bed reduces and therefore, fluid velocity increases across the top of the sand dune. This makes the fluid, again, capable of re-suspending and moving the proppant while proppant deposition stops. This stage is called bed height equilibrium. It is important to note that at equilibrium conditions, fluid is capable of re-suspending and transporting proppant at the same rate that proppant settles. The settling rate, therefore, equals the transport rate and hence equilibrium occurs.
Particle settling is an important issue in hydraulic fracturing because the expected improvement in productivity of a well after hydraulic fracturing depends mainly on the fracture length and fracture conductivity, both of which depend on proppant settling.
The complexity of measuring proppant settling in multi-particle environment has been simplified by measuring single particle settling velocity. Measurement of single particle settling velocities in Newtonian fluids is simple and straightforward. This is due to the properties of these fluids, which state that viscosity is independent of shear rate.