Incorporation of fluorine into proteins has long served as a means of probing structure and function, yet there are few studies that examine the impact of fluorine substitution, particularly at locations distant from the active sites of enzymes. The flexibility of isomeric fluorine incorporation at Phe is used to explore subtle substitution effects on enzyme activity and conformation. The unnatural amino acids o-, m-, and p-fluorophenylalanines were incorporated biosynthetically into the representative PvuII restriction endonuclease. Interestingly, m-fluoro-Phe-PvuII endonuclease exhibits very similar conformational stability to that of the native enzyme, but it exhibits a reproducible, 2-fold higher average specific activity. Given the level of incorporation and the distribution of species, the species of modified enzyme responsible for this increase in specific activity is most likely even faster. Further, moving the fluorine atom from the meta- to the para-position of Phe results in a 4-fold decrease in specific activity and a decrease in conformational stability of 1.5 kcal/mol. Since none of the Phe residues in PvuII endonuclease lies near the DNA recognition or catalytic sites, this differential behavior alludes to the impact of subtle changes in enzyme conformation on endonuclease activity and suggests novel ways to influence catalytic behavior.
The effects of acid solutions injected into hydraulic fractures created in carbonate formations can be assessed at the laboratory scale in acid fracture conductivity tests that mimic the conditions in an actual acid fracture treatment. We conducted a series of acid fracture conductivity tests using a protocol that mimics the fluxes in a hydraulic fracture, both in the main flow direction along the fracture, and in the fluid loss direction. In our tests, the injection rate into the fracture is much higher than in many previous tests, and the fluid loss flux is controlled to match field fluid loss rates. We studied three commonly used acid fracturing fluids---an acid viscosified with polymer, an emulsified acid system, and an acid viscosified with surfactants---at elevated temperatures of 200ºF and 275ºF. The acid fracture conductivity apparatus is similar to a standard API fracture conductivity cell, but with a capacity to hold core samples that are 3 in. long in the leakoff direction. The long cores allow for better control of leakoff as the acid creates wormholes into the core samples. In these tests, acid was pumped through the fracture for contact times ranging from 15 to 60 minutes. After the fracture surfaces were carefully characterized with a surface profilometer, the fracture conductivity was measured at increments of closure stress, up to a maximum closure stress of 6,000 psi. In this paper, we present the results obtained from a series of experiments with these fluids using Indiana limestone and dolomite core samples. Among the findings: Introduction Acid fracturing is a well stimulation process in which acid dissolution along the face of the hydraulically induced fracture is expected to create lasting conductivity after fracture closure. However, conductivity after fracture closure requires that the fracture face is non-uniformly etched by the acid while the strength of the rock is still maintained at high levels to withstand the closure stress. At low closure stress, the etched pattern of the fracture face should have a dominant influence on the resulting fracture conductivity as long as the strength of the rock can withstand the load. As the closure stress is increased, surface features along the fracture faces may be crushed and the fracture conductivity is more dependent on the rock strength than on the initial etching pattern. The success of the acid fracturing process depends highly on the resulting fracture conductivity which is very difficult to predict because it inherently depends on a stochastic process and is affected by a wide range of parameters. Most predictions of conductivity are made with the empirical correlation developed by Nierode and Kruk.1 This correlation was based on experiments using 1 inch diameter by 2 to 3 inch long fractured cores, with no fluid loss through the rock samples. The acid fluxes through the fracture in these experiments were much lower than expected in a field fracture. While there have been several other experimental studies done2–5, they were either done at conditions that did not scale to field conditions or did not consider rock weakening or the etching pattern on resulting fracture conductivity while the effect of only a few parameters were studied in a limited number of experiments.
TX 75083-3836, U.S.A., fax 01-972-952-9435. initial Rock Embedment Strength (RES) values or a lager reduction in RES after acid injection have lower conductivity after closure. Measurements and analyses of rock embedment strength provide understanding of the conductivity behavior in acid fracturing.The results of preliminary tests for a North Sea chalk are also presented and discussed in the paper.
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