The industry is constantly looking for ways to increase completions efficiency, increase well production, and reduce cost. Unconventional completions, on average, use around 4.5 million gallons of water carrying up to 10 million pounds of frac sand. Volumes vary greatly, from under 1 million gallons of water carrying a couple million pounds of frac sand to greater than 10 million gallons of water carrying up to 20 million pounds of frac sand. Changes in the loading of additives can greatly impact the completion costs. Understanding the advantages, or disadvantages, of any changes made to the frac design can help with efficiency and costs, if implemented correctly. The research in this paper highlights how shear stress, proppant transport, and the deposition profile change based on various loadings of commonly used Friction Reducer (FR) mixed in fresh water and brine water.
A 1,500-foot, 0.62-inch internal diameter flow loop is utilized to develop a shear history based on a shear rate equivalent of 100 barrels per minute (BPM) down 11,000 feet of 4.5-inch internal diameter (ID) casing/pipe. FR and High Viscosity Friction Reducers (HVFR) are evaluated at 0.75, 1.5, 3.0 and 5.0 gallons per thousand (gpt) loadings in fresh water and brine as well as guar with a loading of 5.0 gpt in brine (effective polymer loading 20 lbs./1000 gals). Pre-shear and post-shear viscosity readings are recorded for each gel concentration.
The shear conditioned fluid is then utilized to construct the test slurry for the subsequent transport/deposition test. The test slurry is injected into a large slot flow apparatus (10′H × 20′L), with a transverse fracture configuration (top-middle-bottom injection points along the vertical centerline) with an average 0.375-inch width tortuous slot flow path. A typical test entails approximately 750 lbs. of proppant, comprised of 80% by weight of 100 mesh proppant 1.5 pounds of proppant added per gallon (PPA) and 20% by weight of 40/70 mesh proppant at 1.5 PPA. Slurry test pump rate target is 13 gallons per minute (gpm) to simulate the lower shear rate in a far-field fracture environment. Fluid viscosity measurements are recorded post blender and post slot flow apparatus. A deposition profile is generated for each FR loading and job design.
The data are compiled to demonstrate the effects of shear history on the carrier fluid viscosity, as well as to document transport and deposition behavior based on the various loadings of the different friction reducers and pump schedules.