A New Look at Gravel-Pack Carrier Fluid

Abstract

Summary For gravel-pack carrier fluids, less viscosity is needed downhole than for suspending and pumping high gravel loadings on the surface. To achieve this dual viscosity, breakers are added to degrade the viscosifying polymers. Unfortunately, viscosity breakback criteria are not well-defined. In addition to the type and concentration of breaker and temperature, the apparent viscosity breakback time of a polymer solution also depends on the shear rate at which the viscosity is measured. As a result, industry breakback data generally disagree because of the variety of instruments and shear rates used to measure viscosity. This paper discusses new viscosity breakback criteria for polymer-viscosified gravel-packing carrier fluids that are based on gravel settling. Because settled, noncompacted gravel remains fluid, the time required for gravel compaction to begin is a useful criterion for maximum slurry placement time. Similarly, the time for the entire settled gravel column to become compacted is useful as a minimum shut-in time to ensure a stable pack. These criteria correlate well with fluid performance and apply to slurries with different initial viscosities (including crosslinked gels), brine densities, and gravel sizes. These times also can be correlated with low-shear-rate (0.03 second−1) Brookfield carrier-fluid viscosities when it is more convenient to take viscosity rather than gravel-settling data. Data useful for formulating gravel-pack carrier fluids are also presented. Although this paper does not define optimum downhole carrier-fluid properties, the new criteria reflect downhole conditions better than most previous criteria. They should give better correlation with model studies and field experience and, in turn, should lead to a better understanding of the requirements for optimum downhole performance.