NORTHWESTERN OHIO : CALCITE , DOLOMITE AND FLUORITE 1
Calcite is by far the most abundant mineral in the Devonian rocks at Pugh Quarry. The large crystals, from 5 mm to 10-15 cm long, are predominantly brown with wide variations in hue and intensity. The small crystals, smaller than 5 mm, range from colorless to gray-yellow to pale yellow. Both large and small crystals consist almost entirely of scalenohedral forms (dog-tooth spar habit). The most common and most easily recognized type of twinning in the large crystals is on the (0001) plane. Both dolomite and fluorite are minor minerals at Pugh Quarry, although fiuorite is found in all parts of the Quarry. Dolomite crystals occur as secondary encrusting material and are most common in vugs. OHIO J. SCI. 79(1): 24, 1979 In this third paper on the mineralogy of Pugh Quarry, calcite, dolomite and fluorite are described. The quarry is located in the SW3^, SW^, Sec. 6, T.4N, R.9E in Milton Township, Wood Co., Ohio. Its sulfide mineralization, including marcasite, pyrite, and sphalerite has been previously described (Parr and Change 1977, 1978).
BLUE JOHN FLUORSPAR
Summary The paper presents an account of the mineralogy, geological occurrence and history of the blue and white banded fluorspar found near Castleton in Derbyshire. Blue John is shown to be a fluorspar, coloured by bituminous inclusions, deposited as cavity-linings and replacements in Carboniferous Limestone of reef and massif facies. Alternative hypotheses of the cause of the colouring in Blue John are discussed. Lying more or less parallel to the bedding of the limestones, the deposits are in pipe-veins rising up the flanks of a pseudo-anticlinal structure formerly covered by impervious Namurian shales, which now outcrop only round the flanks of Treak Cliff. The relationships between Blue John and the associated minerals (calcite, barytes, galena, pyrites and quartz) are described, and an account is given of the history of the use of Blue John and the methods of manufacturing ornaments. Reasons are given for the conclusion that the story of the use of Blue John by the Romans is without foundation.
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.