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

This paper presents a study of hydration, crosslinking, and rheological properties of 15-to 25-lbm/1,000-gal optimized boratecrosslinked guar fluids. Hydration and crosslinking properties of fluids were measured in a new turbulent flow-loop viscometer to simulate the high shear mixing of fracture fluids in surface equipment and the pumping of the fluids down tubulars. Fluids were then heated and measured in laminar flow to simulate fluid flow in a fracture. The effects of conditioning crosslinked fluids in high shear rate flow on laminar flow properties were observed.Hydration rates were measured for guar at different levels of shear history. Optimum shear levels and times for hydration were observed for the time-dependent base gel fluids. The properties of the crosslinked fluid were critically dependent upon the base gel fluid state of hydration before crosslinking. Properly crosslinked fluids did not shear-thin back to base gel levels but maintained significant viscosity even at high shear levels.Power-law index values were measured for low-polymer borate fluids with a three-viscometer method that uses constant shear rates. Lower values of nЈ were found for the new fluids compared with conventional borate fluids with similar polymer loadings. Proppant transport properties were qualitatively observed in a transparent slot model showing good transport for both an optimized 25-lbm/1,000gal borate fluid and a conventional 35-lbm/1,000-gal borate fluid.

A recent research project was conducted to determine whether a stable, fine-textured, 95% gas content foam could be made. The viscosities of 95-quality N2 foams were measured in a recirculating flow loop viscometer. Only foam prepared from 2% of an anionic surfactant with plain water had uniform, fine-bubble structure (texture) at 95 quality. All other combinations of additives or other foamers (nonionic, amphoteric or anionic, and/or 0.24% guar) produced unstable foams at 95 quality and stable foams at lower quality. Foams that were unstable at 95 quality typically contained large slugs of N2 gas within the foam structure. Unstable, high-quality foams did not invert phases to form a mist. Instead, such foams were mixtures of very small and very large bubbles. The net viscosity and stability of those fluid systems were lower than that of a uniform, fine-textured foam. Yield points were measured for fine-textured foams at 70 to 95 quality. These new yield points were higher than yield points in earlier data. Introduction Foam fracturing is a standard technique used in North America to stimulate low-permeability reservoirs, including shales and coal seams. Foams are attractive because the water content of a foam fluid is very small, reducing the damage potential to sensitive formations. A foam fluid consists of a high percentage of a gas-internal phase and a lesser percentage of a liquid-external phase that includes a stabilizing surfactant called a foaming agent. Foams are viscous fluids used for creating fracture geometry and transporting proppant into the fractures. Foam at nitrogen qualities of 70 to 90 have been applied effectively in the formation types mentioned above. Alternatively, even pure N2 without proppant will stimulate gas production from these low-permeability formations in some areas. Liquid CO2 with a small concentration of proppant has also been reported to stimulate gas production. One way to minimize the amount of water injected is to use a foam quality higher than 90. Although the water content of a 90-quality foam is only 10%, 95- or higher quality foam contains even less water (5%). The danger of increasing the quality is that at some point the liquid will stretch to cover so much surface area of bubbles that the foam may collapse or even invert phases to become a mist. Assuming sufficient foam stability, 95- quality foam has enough viscosity to place proppant. This paper shows the requirements for building high-quality (95% gas content) foam fracturing fluids. The effects of external phase composition, foaming agent type and concentration, shear rate history, and gas type (N2 and CO2) on viscosity development were examined in the study. Experimental Base liquids consisted of either tap water or 0.24% guar polymer in tap water. Foaming agent was mixed into the base fluid at a concentration of 2% (by volume) unless otherwise noted. Liquid phase composition and foamer types are listed in Table 1. The foamer surfactants that were tested are typical of commercial materials used in foam fracturing service work; two anionics, one nonionic, and one amphoteric were tested. Anionic surfactant number one was used for all experiments except where noted. The base liquid was pumped into the recirculating flow loop viscometer and pressurized to 1,000 psi. Nitrogen gas was bled into the pipeline loop while the loop was being recirculated at 1,000 sec-1. To generate 95-quality N2 foam, N2 was pumped into the loop, displacing 613 mL of liquid from the 645-mL volume of the loop through a backpressure regulator. Liquid displacement was measured by trapping the effluent in a beaker on a digital balance. When gas was added, viscosity was monitored and foam was observed through the visual port for the appearance of large gas bubbles. P. 265

Fracturing gels are formulated to have a high viscosity sufficient to generate fracture geometry and transport proppant materials. Fracturing fluids are subjected to extensive QA/QC testing on high-temperature rheometers to ensure they have adequate viscosity to perform the intended task and break within a time frame suitable for the operation performed. Although most rheological tests incorporate only the gelled fluid without proppant materials, it is assumed that the fluid will transport proppants to their desired location.A field location reported that treatments containing certain proppants were screening out more frequently. Tests were performed on a slurry viscometer (SV). It was discovered that not all proppant materials of the same nominal size and density were transported equally when added to a fracturing gel. The typical properties of the borate gels were measured, but no significant differences were found that would account for differences in transport. All the proppants had nominal 20/40 mesh size. A laser analysis of the particles indicated small differences in the distribution of sizes. However, the size analysis did not fit the trend that "bigger settles faster."Scanning electron micrographs showed differences in surface roughness between the ceramic materials and sand. The proppants were coated with a tacky substance to neutralize any chemical absorption effects. The coated proppants then settled in a manner consistent with their physical characteristics (size and density), but the tacky material caused agglomeration and faster settling than before. The coated proppants were treated chemically to temporarily disable the tackiness, and it was observed that all the materials settled in a manner consistent with their physical characteristics.It was found that certain 20/40-mesh proppants were suspended twice as long as other proppants in the same gel environment. The proppant effect was measured with the SV. The trend from the SV instrument correlated well with the screenout rate observed in the field.Analysis indicated a surface catalytic effect of ceramic proppants causing faster decomposition of oxidizing breakers and accelerated gel breaking. This effect was absent in fluids with uncoated sand. Some resin-coated proppants were found to also accelerate breaking because of leaching of chemical compounds from the coatings. Therefore, it is not correct to assume that all proppants added to fracturing gels will be transported equally. Viscosity measurements of gels without proppant might not give accurate information about the behavior of gels with proppant if the proppant interacts with the gels. IntroductionThe majority of oil and gas wells in the Unites States, and a large number of international wells, are stimulated by hydraulic fracturing. A great deal of study and effort goes into the development and production of fluids that are able to withstand the temperatures and duration of fracturing treatments for subterranean formations. The fracturing fluid must generate fracture geometry and carry p...

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