Measuring understanding is notoriously difficult. Indeed, in formulating learning outcomes the word 'understanding' is usually avoided, but in the sciences, developing understanding is one of the main aims of instruction. Scientific knowledge is factual, having been tested against empirical observation and experimentation, but knowledge of facts alone is not enough. There are also models and theories containing complex ideas and interrelationships that must be understood, and considerable attention has been devoted across a range of scientific disciplines to measuring understanding. This case study will focus on one of the main tools employed: the concept inventory and in particular the Force Concept Inventory (FCI). The success of concept inventories in physics has spawned concept inventories in chemistry, biology, astronomy, materials science and maths, to name a few. We focus here on the FCI and ask how useful concept inventories are for evaluating learning gains. Finally, we report on recent work by the authors to extend conceptual testing beyond the multiple-choice format.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe industry has developed standard methods to effectively evaluate the propping agents used in hydraulic fracturing operations. The understanding gained from the consistant application of the methods has greatly helped to optimize productivity.of hydraulicly fractured wells.This paper presents two mechanisms that may significantly increase the understanding of how to optimize fracture conductivity to sustain productivity. The standard methods use well "aged" (passive) proppant and simulated formation materials; whereas, in practice, formation faces are highly activated as an immediate result of mechanical fracturing, as is a significant portion of the pumped proppant.The chemistry that occurs at freshly exposed mineral surfaces is different than that of the aged surfaces used in the laboratory. For example, condensation of polymers on freshly generated surfaces may result in polymer chains becoming anchored to the surface. This anchoring prevents some polymer from being removed by normal gel-breaking mechanisms and forms points of attraction for the collection of broken polymer and fines debris, leading to significant permeability damage and reduced fluid recovery.After a treatment when pressure is relieved and well clean-up begins, temperature and stress gradients are high; significant crushing of proppant and formation material may occur as a packed fracture moves toward an equilibrium condition. The presence of hHigh-ionic-strength fracturing fluids, particularly at high pH, may promote a rapid mineral diagenesis-type reaction that leads to proppant compaction, embedment, and crystalline overgrowth permeability damage.This paper provides laboratory and field data supporting the conclusion that the application of a thin, highly dielectric, polymer film to coat proppant can result in a long-term reduction in mineral dissolution rate, compaction, embedment, and polymer anchoring sites. Field data shows this effect has a dramatic improvement in fracturing fluid recovery and well productivity.

Proppant production from hydraulically fractured wells can cause severe operational problems, increase safety concerns, and dramatically reduce economic returns on well-stimulation investments. Methods that have helped eliminate or minimize proppant flowback include modified completion designs, the use of controlled fracture closure for obtaining early closure on the proppant pack, and the use of materials designed to reduce proppant production. Curable resin-coated proppants, chopped fiberglass, thermoplastic strips, and chemicals that modify the surface of the proppant are all accepted methods for minimizing flowback. This paper presents the results of both physical and numerical modeling of proppant flowback recorded during the development of a chemical designed for modifying the proppant surface. The goal of this study was to develop an understanding of the mechanisms that control proppant flowback. Laboratory experiments performed in slot models with no closure stress helped establish the interaction of proppant size, proppant distribution, and fluid velocity. Additional studies of the impact of closure stress, fracture width, and fluid rate on proppant flowback were performed with modified API linear conductivity cells. Data obtained from the physical modeling were used to calibrate a numerical model that predicts proppant flowback. In this model, fluid flow in the proppant pack is described by Darcy's equation for flow through porous media. The resulting velocity distribution allowed local stability to be assessed along the free surface between the proppant pack and the continuous fluid phase. Repeating these steps allowed evaluation of the interface that develops over time.

Society of Petroleum Engineers Abstract A new liquid surface-modification system has been developed for coating proppant, dramatically increasing its surface friction and allowing it to interact instantaneously with surrounding particulates. High surface friction between the coated proppant grains allows them to withstand high flow rate, minimizing their flowback potential after fracture-stimulation treatments. According to field results, when this surface-modification material was used as a flowback-control agent after conventional fracturing treatments, it permitted more aggressive flowback procedures. This treatment did not impair conductivity, and in fact, increased proppant conductivity at closure stresses below 4,000 psi. Better vertical proppant distribution occurred in experiments that demonstrated increased hindered settling of proppant resulting from this surface modification. Fines that already existed within the proppant, fines generated from the formation, or fines derived from crushed proppant upon fracture closure all adhered to the treated proppant, which inhibited them from migrating and blocking the pore throats of the proppant pack. This unique coating technology further enhanced conductivity by improving frac-gel breaker action in certain fluids. This behavior results in faster, more effective well cleanup after stimulation. P. 101

Summary Rapid loss of fracture conductivity after hydraulic fracture stimulation has often been attributed to the migration of formation fines into the proppant pack or the generation of fines derived from proppant crushing. Generation of crystalline and amorphous porosity-filling minerals can occur within the proppant pack because of chemical compositional differences between the proppant and the formation, and the compaction of the proppant bed because of proppant pressure solution reactions. Findings presented in this paper suggest that diagenesis-type reactions that can occur between proppant and freshly fractured rock surfaces can lead to rapid loss of proppant-pack porosity and loss of conductivity. Introduction Lehman et al. (2003) reported that the use of surface-modification agents (SMA) to coat proppants used in propping hydraulic fractures resulted in sustained and more uniform production from wells. Fig. 1, taken from that publication, shows the production decline curves from some of their data, and it does appear to show a significant change in decline rate compared to the use of untreated proppant. This SMA was described as a nonhardening resin that is insoluble in water and oil. It is supplied in a solvent that is quickly extracted once it is introduced to aqueous-based frac fluids, leaving a tacky, hydrophobic coating on the proppant. Initial use of this type of SMA treatment (Dewprashad et al. 1999; Nguyen et al. 1998a, b) was promoted as a method to increase the conductivity of proppant owing to its capability to prevent close packing of the proppant, which can result in increased porosity and permeability of the pack, by rendering the proppant surface tacky. Subsequent studies indicated that its use provided proppant-pack protection from fines infiltration and migration. This mechanism has been employed to explain the observations that sustained production results from the use of SMA on proppants. This is further substantiated by long-term results obtained in a single field study known for fines production problems. That both mechanisms are active has been well established through laboratory studies, but they alone do not completely explain the reduction in production decline rate as reported. A field study of SMA-treated proppant was reported to the Arkansas Oil and Gas Commission 2004 CBM Workshop that disclosed long-term results on gas production. These were CBM wells in the San Juan basin that typically required refracturing each year to produce at an economical rate. With the SMA-treated proppant, no refracs have been required, and as shown in Fig. 2, production has remained essentially constant for 5 to 6 years. This longevity was initially attributed to prevention of fines invasion into the proppant pack; however, it is possible that there are additional mechanisms operational.