Abstract
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.
Highlights
Weighting material is one of the main drilling fluid additives used to achieve the required density to overbalance formation pressure during drilling operations and prevent any loss of well control [1,2,3,4].In addition to well control, drilling fluids have many other functions, such as circulating the drilled cuttings to the surface and suspending the cuttings while drilling operation is stopped [5] and cooling and lubricating the drill bit to ease the drilling operation [6]
While most of the previous studies focused on adding a stability enhancer, polymers, or special additives to solve the sag issue, this study suggests playing with the weighting and adding a mixture of barite and ilmenite to be used as a combined weighting agent in oil-based drilling as a solution to the barite sag issue
Different drilling fluid samples were prepared by mixing ilmenite with barite in different Different drilling fluid samples were prepared by mixing ilmenite with barite in different concentrations (20, 40, 60, and 80 wt.%)
Summary
Weighting material is one of the main drilling fluid additives used to achieve the required density to overbalance formation pressure during drilling operations and prevent any loss of well control [1,2,3,4].In addition to well control, drilling fluids have many other functions, such as circulating the drilled cuttings to the surface and suspending the cuttings while drilling operation is stopped [5] and cooling and lubricating the drill bit to ease the drilling operation [6]. Weighting material is one of the main drilling fluid additives used to achieve the required density to overbalance formation pressure during drilling operations and prevent any loss of well control [1,2,3,4]. In high-pressure high-temperature applications, a special drilling fluid formulation is required to meet the critical downhole conditions. In such cases, drilling fluids should have a high density to suppress the high formation pressure with good stability at that elevated temperatures [7].
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