Pressure control — products from the Bernic Lake pegmite in Canada
The Bernic Lake mine is located under its namesake in Manitoba, Canada, approximately 180km northeast of Winnipeg. The first survey crews were led by Harry Beresford and James Nicol, and in 1914 they located their camp on the shores of the lake that now bears their names.
Cabot Corporation bought the mine in 1993 and started negotiations with Shell to consider the feasibility of producing kilotonne quantities of cesium formate to create a new type of drilling fluid (see box p36). Full-scale production of cesium formate brine (82% w/w cesium formate in water) began in 1997 and currently averages around 3,000 metric tonnes per annum.
Today the mine is operated by the Tantalum Mining Corporation of Canada, a wholly owned subsidiary of Cabot.
A pegmatite is a granitic-related intrusion that cooled slowly enough to allow differentiation of the minerals. A characteristic of pegmatites is the formation of large individual crystals within the matrix. In the 1920s, the Bernic Lake pegmatite was referred to as ‘a geologist’s paradise’ and today over 50 different minerals have been identified, as well as a new mineral, named Cernyite after the discoverer, Dr Petr Cerny of the University of Manitoba. This was found in Bernic Lake ore.
The Bernic Lake pegmatite is located in the Bird River greenstone belt within the Superior geological province in the Canadian Shield, and is composed of metavolcanic and derived metasedimentary rocks and synvolcanic to late tectonic intrusive rocks. The Bernic Lake pegmatite is one of a number of sub-horizontal pegmatite sheets, which make up the Bernic Lake pegmatite group and is hosted by a synvolcanic metagrabbro intrusive. Since its discovery, the deposit has been the subject of many studies because of its uniqueness. Internally, the pegmatite is composed of eight discrete mineralogical zones with different ores of economic interest – those of tantalum, spodumene, cesium and rubidium – each essentially occurring in different zones.
The pegmatite was formed during the Kenoran Orogeny of the late Archean Age, and is approximately 2.55-2.65bln years old. It is highly fractionated, and the separate minerals had time to amalgamate into distinct zones within the pegmatite. The result is an ore body with zones containing concentrations of tantalum, lithium, and cesium minerals, as well as sub zones of varying degrees of fractionation within the main zones of the ore.
The pegmatite is 60m below Bernic Lake, and is accessed by both a shaft and a 20º decline from the surface. Mining is carried out using the room and pillar method. Originally rooms were 16m2, however, rock mechanics studies have shown that the rooms could be increased to 22m2, by shaving the pillars. The roof averages 20m above the current working levels, and in places reaches 30m. Due to the nature of the ore and the mining method, rock bolting is rarely required. However, the back is carefully monitored from custom-designed Giraffes (aerial lifting devices) and present geological pressures are pushing up.
Drilling and blasting of 12ft horizontal benches with electric-hydraulic jumbos is the primary mining method. Where practical or necessary, longhole methods are used, with vertical cuts of up to 70ft taken using a Simba longhole drill. Ore is moved from the blasted headings to the ore passes using trackless diesel-powered haulage equipment. Hydraulic rock breakers at each ore pass reduce the blasted ore to an easier to handle size on cast steel grizzlies, before it is trammed to the shaft with a diesel locomotive and four-ton ore cars. At the shaft, the ore is hoisted to the surface in two four-ton skips using a double-drum electric hoist. Three distinct ores are mined, which must be handled separately. This necessitates regular cleaning of the system to avoid contamination, as there are only two skips and two coarse ore bins.
Due to land constraints, the concentrator is constructed on a peninsula formed by two inlets on Bernic Lake. The building is multi-floored, with equipment on six levels. The first stage of processing, common to all four mineral products, is crushing, where the coarse ore from underground (~300mm in size) is broken down to 12mm. Tantalum and spodumene ores are crushed into separate fine ore storage bins, while pollucite and rubidium ores are crushed into covered stockpiles. A dry grinding plant supplies ground pollucite for the cesium formate chemical plant. The ground pollucite is subjected to acid leaching and other chemical processing to produce cesium chemicals.
Applications in drilling
The first application of cesium formate drilling mud was in the Shearwater field in the UK sector of the North Sea. This was followed by Total and BP in the Elgin/Franklin and Marnock fields, and later by Statoil Hydro in Norway. Nowadays cesium formate brine is the high-pressure, high-temperature (HPHT) drilling and completion fluid of choice in many parts of the world. It has been used by many of the major oil companies in over 120 HPHT well construction operations in 28 different fields.
Cesium formate brine improves HPHT project economics and significantly reduces the operational risk of developing these challenging oil and gas reservoirs. Not a bad outcome for a mine first developed over 80 years ago.
History of the mine
During the gold rush in the early 1920s, a Manitoba prospector named Jack Nutt relocated the workings to north shore of the lake. Here, a narrow pegmatite outcrop in the form of a flat dipping dyke, about five feet thick, was located. In 1929, work started on the Jack Nutt shaft to develop the dyke. The timber is still in place and the old shaft is now used to circulate fresh air into the mine.
In September 1929, exploratory diamond drilling at the lake discovered a massive dyke, 250 ft thick. Almost by accident the now famous Bernic Lake pegmatite dyke was discovered, as nowhere does it appear on the surface. In an attempt to develop the dyke, Nutt’s company was reformed into the Consolidated Tin Corporation Ltd. The venture failed, however, and the property was abandoned in 1932 with the claim reverting to the Crown.
In the mid 1950s further attempts were made to develop the mine as a source of lithium. Other metals, including cesium, rubidium, gallium, beryllium, tantalum and columbian were also found to occur. A new shaft was collared and sunk over 300ft into the bedrock, and development began at the 285ft horizon. Work progressed rapidly, but was stopped due to financial difficulties. By 1961 over US$2.5 million had been spent on developing the mine, with no returns. The mine was closed once again, and in 1962 the pumps were pulled and the mine flooded.
In 1966 new uses for tantalum created a market for the product, and the property was re-evaluated as a tantalum mine. Funds were obtained from the Goldfield Corporation, and a joint venture was arranged to determine the possibility of exploiting the tantalum ore. In 1967, the mine was re-opened by the Tantalum Mining Corporation of Canada Ltd and production commenced, in 1969, peaking at around 700t of ore per day. Minor amounts of other ores were produced including cesium. By the end of 1977 about 1,200t of tantalum oxide had been produced from 1.25Mt of ore, just over two pounds for each tonne processed.
Cesium formate in drilling
Drilling for oil and gas requires the well to be filled with a dense fluid or drilling mud to prevent highly-pressured fluids and gases entering the well bore and flowing to the surface.
Traditional drilling muds are made from slurries of micronised barite. To increase the density, heavy brines made by dissolving zinc bromide salts in water are added to the mix. The problem faced by the industry in the late-1980s was that wells were being drilled into deeper and hotter reservoirs, so called high-pressure, high-temperature (HPHT) reservoirs, where conventional muds become thermally unstable, adversely affecting both density and fluid rheology. The high temperature also increases corrosion rates in brine-based systems, to the extent that it was becoming uneconomic to drill these wells.
Shell was one of the first companies to consider using alkali metal formate salts to create very dense brines as an alternative to bromide (see graph below). Formate brines are thermally-stable, contain no barite and are essentially non-corrosive, so they provide the ideal medium for hydrostatically controlling the pressures and temperatures often encountered in HPHT wells (>10,000psi, >300ºF). While developing the concept was relatively easy, the challenge was finding a source of large volumes of cesium formate, the key ingredient in very dense brines.
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