Measurement of Proppant Transport of Frac Fluids

Abstract

Abstract The object of hydraulic fracturing is to produce a propped fracture extending from the wellbore. Extensive time and effort are expended in measuring rheological properties of crosslinked fluids (without proppant) under simulated downhole temperature and shear-history conditions.Normally, it is assumed that if a fluid meets certain minimum viscosity conditions, it will transport proppant successfully. Measurements of actual proppant transport under dynamic simulated downhole conditions have been attempted and described, but such measurements have been very cumbersome and require significant equipment and expense to execute. Such measurements are well beyond the scope of routine QA/QC analyses. A proppant viscometer recently constructed can measure fracturing fluids containing propping agents across a wide range of concentrations. The device was designed to work as a conventional Fann Model 50-type viscometer. This unique viscometer has been used to measure typical fracturing fluids containing realistic concentrations of proppants, at temperatures and times up to several hours, representative of actual fracturing treatments. The measurements show regions of elastic transport typical of viscoelastic fracturing fluids where proppant is transported efficiently, usually followed by regions of purely viscous transport where proppant slowly settles. The advantage of the new proppant viscometer is that all components of a fracturing fluid, including proppant, can be tested. Shear-history effects of proppants on frac fluids are usually unknown or ignored, but such effects were observed in this work. Breakers were usually added to the fluids, showing reasonable times when the crosslinked fluid no longer transported proppant efficiently and proppant began to settle. This paper shows how different types of fracturing fluids can support proppant based on their chemical type, i.e. metal and borate crosslinked fluids, linear gel fluids, and surfactant gel fluids. Proppant concentrations are also considered. The physical characteristics of the proppant viscometer are also addressed. Introduction Most hydraulic-fracturing treatments use a gelled fluid to create a subterranean fracture and partially fill the fracture with propping agents. When the fracture closes and the fluid is recovered, a conductive channel into the reservoir remains. Proppant transport is a function of (1) wellbore and fracture geometry; (2) volumetric rate; (3) proppant size, concentration and specific density; and (4) carrier-fluid rheology. Instruments such as Fann Model 50 viscometers are available for measuring viscosity at high temperatures and pressures, but elasticity is much more elusive to measure. Additionally, most viscometers, such as the Model 50, are designed only to handle the "clean fluid systems," e.g. without proppant. By default, we generally assume that higher viscosities will do a better job of transporting proppant, as well as generating the desired fracture geometry. There have been several attempts1–9 to characterize transport properties of fluids using modified bench-scale viscometers or through large slot or pipeline apparatus, but those efforts are very expensive and are not performed usingroutine quality-assurance measures. Several rheological properties directly impact a frac fluid's performance:apparent viscosity,yield stress,dynamic viscosity,rheomalaxis (irreversible thixotropy),viscoelasticity (for example G', G"), andthe related issue of turbulent-drag reduction. In laboratory research, sample volume is often very limited, thus necessitating rheological testing and evaluation of small quantities. Also, most bench-top rheometers use batch mode, that is, small samples are placed in a testing chamber as opposed to flow-through testing, as is the case for pipe viscometers.This presents the challenge of simultaneously:Imparting viscometric shear history that simulates the wellbore travel path.Not exceeding the proper mechanical energy input; the bench-top batch process should impart about the same amount of integrated work as the wellbore path.Maintaining satisfactory thermal balance, e.g. being sure not to create localized "hot spots" in the bench-top process because of its batch mode of operation