Hematite In Oil Drilling Applications
Hematite is a mineral used in the oil extraction industry as a component in drilling fluids. Drilling fluids are oftentimes water- or oil-based and the addition of hematite - which like manganese oxides and barite - contribute to an increasing density in the fluid.
This is particularly useful as the primary functions of drilling fluid are to maintain hydrostatic pressure at the drill site (to prevent formation fluids entering the bore), to carry out drill cuttings and to keep the drill bit cool and relatively contaminant-free during the drilling process. Drilling fluids prevent blowouts.
The most common drilling fluids across oil extraction operations are oil based mud (OBM) and water based mud (WBM). In these ‘muds’, the largest component is bentonite clay, mixed with either oil or water, supplemented with any desired additives(1). Hematite is such an additive, and is described as a weighting agent. ‘Mud’ is a commonly used term to describe drilling fluids, owing to clays being the largest component in a large proportion of drilling fluids.
Drilling fluids are essential in petrochemical exploration as they enable efficient drilling to the target depths to reach the oil and/or gas deposit. Modern oil extraction would be almost impossible without drilling fluids(2). During drilling, five key factors are measured in the mud to ensure ongoing success: rheology, density, fluid loss, solids content and chemical properties. Hematite, as a weighting agent, is primarily concerned with density and solids content; as well as rheology.
Introduction To Weighting Agents
Weighting agents are added to the drilling fluid to allow the modulation and optimisation of a drilling fluid for a particular application, for example through certain hard rock types, or at extreme depths. They often are used to maintain density levels in the fluid, alleviating high formation pressures and ensuring a consistency in displacement of mud and other debris. As a general rule of thumb, the deeper the boring, the more weighting agent is required.
Barite and Manganese
Barite, from the Greek word barus meaning ‘heavy’, is the most used additive in oil drilling as a weighting agent(3), and is a mineral consisting of barium sulfate. Owing to its high specific gravity of 3.9 to 4.4 g/cm3, it is a widely used weighing agent. The use of barite is not without its problems, however Barite sag is a phenomenon whereby solids in suspension in the fluid fall out of the suspension and settle at high temperature and pressures(4), causing problems with ongoing drilling and an eventual stoppage of operations. Barite sag can be partially alleviated by the addition of ilmenite and other metal oxide ores. Furthermore, demand for barite is far exceeding production. It is thus imperative that alternative weighting materials are identified.
Manganese compounds, manganese(ii) and (ii) oxide, derived from hausmannite ore are also used widely as weighting agents. Because of its ability to form small, spherical particles, oxides of manganese represent the challenge of higher particle loss at high temperatures, which requires additional steps to negate(5).
Hematite (red iron oxide) is an oxide and ore of iron, bearing the chemical formula Fe2O3, and a hardness of 5.5 to 6.5 on the Mohs scale. Its major use is as a source of iron for the production of that metal and of steel. Due to its hardness, high natural purity, abundance and low price, hematite has found increasing uses in fields removed from smelting, such as oil and gas production and exploration, as part of drilling fluids.
In comparison to barite, hematite enjoys advantages such as a higher specific gravity, a greater solubility in acidic media and an overall lower attrition rate - the latter due to its hardness. One of hematite’s headline claims as an improvement over barite, aside from it’s plentiful nature, is that it has a higher specific gravity of 4.9 to 5.3 g/cm3(6)Because of this, drilling companies can use less hematite in their mud solutions than barite, and achieve the same results. This results in a lower cost to weight the mud, and fewer solid particles dispersed through the mud. Hematite is additionally regarded as a good choice for drilling uses as it is incredibly pure and contaminant free as mined. Furthermore, better performance in terms of a reduced need for dilution, improved rate of penetration and a higher capacity for solids tolerance are other qualities associated with hematite as a weighting agent for drilling applications(7). Hematite’s major use as a drilling agent in oil exploration is in deep drilling(8).
Hematite In Oil Based Mud (OBM)
OBMs are characterised by having an oil-based continuous phase, with water in a dispersed phase alongside other additives such as emulsifiers and gellants.
Wetting is a concern for OBM drilling. Wetting is where water adheres to the solids in the mud, either the weighting agents or otherwise. Materials that are prone to wetting may cause excess water to reach the drilling area, which can result in clumping of the solids, leading to severe inefficiencies in the drilling process and potential degradation of the drill bit. Hematite is characterised as a low-wetting material, and thus the use of this over other weighting materials will ensure less wetting(9).
Hematite In Water Based Mud (WBM)
In WBM applications, lubricants are often used to ensure high efficiency drilling and to reduce friction and wear caused by solids including weighing agents such as hematite. It has been found that a 1% by volume addition of surfactant to the mud mixture was sufficient to reduce friction coefficients by 60% - increasing the longevity of the drill bit(10). In WBM scenarios, hematite can be used in quantities of up to 20% by weight, with a notable decrease in sedimentation relative to borite due to the high iron oxide content(11).
Hematite And Rheology
Rheology refers to the flow of matter, and particularly in the drilling space, the plastic flow of solids (this should not be confused with plastics as a materia)l. Plastic flow is a movement proportional to an applied force, and specifically in petroleum exploration, a shape or phase change resulting from such a force. Rheological properties are profoundly affected by the presence of solid materials, and therefore adding hematite to a fluid will have an impact upon its rheological performance. Particle size is crucial (no more than 25 µm, but often far less)(12). Across multiple studies, hematite has produced higher density values, better gelling ability and higher plastic viscosity than barite(13), especially at deep boring depths. These qualities are continuous with successful long term depth drilling projects, affording stable bore holes.
Hematite, Drilling Fluids And The Environment
Oil drilling is not known as the most environmentally friendly activity. Hematite itself is non-toxic, yet care should be taken so as not to release too much drilling fluid, particularly in marine environments, particularly if it is of the OBM type. Barite is described as non-toxic only due to its insolubility in water. As with all industrial processes, reduction in the quantity of any material used is advantageous - drilling waste is the second largest waste stream originating from exploration(14). Hematite use instead of barite or manganese can aid in this due to the aforementioned higher density thus lower mass requirement.
Considerations When Using Hematite In Drilling Fluids
Hematite is a significantly harder material than barite. As such, hematite can itself cause wear on the drill bit and column under long term drilling situations, in addition to at high pressures. Research has shown that decreasing hematite particle size, and ensuring a narrow range of particle sizes, causes a positive effect on drilling bit erosion, i.e. less erosion over time, with comparable timeframes to barite(15). It has been proposed that hematite is only a sufficient replacement for barite if it is finely milled to a size of no more than 25 µm(16).
Because of the advantageous properties of hematite, barite and manganese, in some situations a combination of these materials are used as weighting materials. For example, a drilling operation may wish to take advantage of hematite’s longevity and hardness, barite’s wide inertness and manganese tetroxide’s oxidation properties(17) to provide a complimentary weighting phase in the mud suited to the local rock/sediment composition(18).
- Hematite is fast becoming a popular choice as a weighting agent in drilling fluids for production applications in the oil and gas sector
- Used in both oil- and water- based mud fluids, hematite provides enhanced rheological properties, and a higher specific gravity meaning less of it can be used relative to its competitors to achieve the same result
- Hematite is valued for being highly pure at the point of being mined and very hard, in addition to being economically attractive
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18 US Patent US6248698B1, 1999, expired