Functionality of chemically modified wild-type, partial waxy and waxy starches from tetraploid wheats
Partial waxy (reduced-amylose) and fully waxy (amylose-free) tetraploid durum wheats (Triticum turgidum L. var durum) were developed by introgression of null alleles at the Wx-A1 and Wx-B1 loci from common hexaploid wheat (Triticum aestivum L.). Purified starches were obtained from each genotype, and chemically modified by: 1) cross-linking with phosphorus oxychloride, 2) substitution with propylene oxide, and 3) sequential cross-linking with phosphorus oxychloride followed by substitution with propylene oxide. Functional properties were compared to blends of waxy and wild-type durum starches of known amylose contents. Significant differences in functionality were observed amongst the genotypes and blends after each modification. Waxy (0% amylose) and wild-type (30% amylose) typically were at the extremes of the observed ranges of functional properties. In general, the functional properties of the chemically modified starches were dependent upon amylose content. Starches from Wx-B1 null lines (24% amylose), were an exception. After substitution, such starches had the significantly highest value for RVA final viscosity, and generally performed in a manner similar to starch blends of 12–18% amylose.
In vitro digestibility of cross-linked starches: RS4
In vitro digestibility of cross-linked starches (RS4 type), which were prepared from wheat, corn, rice, potato, and high-amylose corn starches, was determined by Prosky AOAC and pancreatin gravimetric (P/G) methods. The RS levels of RS4 starches obtained by the AOAC method were higher than those by the P/G method. Pronase treatment before digestion with heat-stable α-amylase (AOAC method) gave similar RS levels as those by P/G method. The shapes of residues after digestion by the AOAC method were not different from untreated ones, but those after digestion by the P/G method showed irregular and fluffy forms. It was found that the P/G method provided more exhaustive digestion of RS4 starches than the AOAC method.
Prevention of Barite Sag in Oil-Based Drilling Fluids Using a Mixture of Barite and Ilmenite as Weighting Material
Drilling high-pressure high-temperature (HPHT) wells requires a special fluid formulation that is capable of controlling the high pressure and is stable under the high downhole temperature. Barite-weighted fluids are common for such purpose because of the good properties of barite, its low cost, and its availability. However, solids settlement is a major problem encountered with this type of fluids, especially at elevated downhole temperatures. This phenomenon is known as barite sag, and it is encountered in vertical and directional wells under static or dynamic conditions leading to serious well control issues. This study aims to evaluate the use of barite-ilmenite mixture as a weighting agent to prevent solids sag in oil-based muds at elevated temperatures. Sag test was conducted under static conditions (vertical and inclined) at 350 °F and under dynamic conditions at 120 °F to determine the optimum ilmenite concentration. Afterward, a complete evaluation of the drilling fluid was performed by monitoring density, electrical stability, rheological and viscoelastic properties, and filtration performance to study the impact of adding ilmenite on drilling fluid performance. The results of this study showed that adding ilmenite reduces sag tendency, and only 40 wt.% ilmenite (from the total weighting material) was adequate to eliminate barite sag under both static and dynamic conditions with a sag factor of around 0.51. Adding ilmenite enhanced the rheological and viscoelastic properties and the suspension of solid particles in the drilling fluid, which confirmed sag test results. Adding ilmenite slightly increased the density of the drilling fluid, with a slight decrease in the electrical stability within the acceptable range of field applications. Moreover, a minor improvement in the filtration performance of the drilling fluid and filter cake sealing properties was observed with the combined weighting agent. The findings of this study provide a practical solution to the barite sag issue in oil-based fluids using a combination of barite and ilmenite powder as a weighting agent to drill HPHT oil and gas wells safely and efficiently with such type of fluids.