Exploratory Drilling of Nickel Laterites by Air Circulation Coring
Biankouma - Sipilou - The Ivory Coast
The Background
RADS's team completed a major drilling programme exceeding 20,000 metres at Biankouma Sipilou in the Ivory Coast using air circulation coring systems.
The air coring system which is being used to achieve very high core recoveries from soft, weathered and oxidised formations was first developed in the United Kingdom coal fields nearly 40 years ago.
Present management personnel were involved in the development of the system and initiated the first metalliferous application of air coring over 30 years ago when 100% core recoveries were achieved from the lead-zinc rich ''black mud'' of Tynagh in the Irish Republic, this formation had proven to be impossible to recover using the standard fluid circulation coring equipment of the day.
The success at Tynagh attracted the attention of Falconbridge in the early 1960's when improved coring systems for evaluating the Dominican Republic nickel laterites were urgently required.
Successful early trials led to the introduction in the Dominican Republic of a long term air coring operation, which over many years became the primary source of sampling and evaluation information.
In the early 1980's 10 000m metres of air coring were successfully completed for Falconbridge at Sipilou in the Ivory Coast and the system was also introduced at the Soroako property of Inco at Sulawesi, Indonesia.
RADS returned to The Ivory Coast in the last 90’s and carried out a core drilling programme at Biankouma for Falconbridge. Subsequently a new contract was agreed for the continuation of drilling in the Biankouma area.
The Drilling System
In essence, compressed air replaces the normal water or mud circulation.
Because the expanding air requires relatively larger spaces within the core barrel, bit kerf is wider than for fluid circulation core barrels, which combined with the reduced cooling effect of air tends to limit the economic use of the equipment to soft to medium hard formations.
Air circulation tools generally achieve higher rates of penetration due to faster bottom hole clearance of cuttings and this effect when used with face discharge core bits is an important contributory factor in achieving high core recoveries. Drill Sure's usual core barrel produces cores of 3"e; diameter from a hole of 4 1/8"e; diameter, this is non standard but a standard H. size core barrel is available for use where the hole has to be cased.
Compressors delivering 750 cfm at pressures up to 170 psi and are suitable for coring to depths of around 200 metres, covering most normal open pit mining depths.
When formations are moderately damp the drying effect of the air circulation is beneficial otherwise air mist or foam injection is used to deal with very sticky zones.
As air has little or no wall stabilising properties and the core barrels are of the conventional type unstable ground frequently require casing.
The standard core barrels, although of the double tube type, when used by experienced drillers, are capable of producing core recoveries close to 100% from the very soft formations at Biankouma.
Triple tube barrels are very rarely required but to improve core presentation although not necessarily recovery, mylar inner sleeves can be used, either in the form of film or as a rigid barrel liner.
Cores are extruded from the core barrel by mean of a hydraulic cylinder and at Biankouma it has been found to be more economic to receive the cores into polythene sleeves which can be sealed to retain in-situ moisture content than to use core liners.
Core in barrel
Air Core PCD Bit
The Drilling Rigs
At Biankouma hydraulic top drive rigs are being used which have very long, in excess of six metres, feed strokes which when used with three metres long corebarrels aids the achievement of high core recoveries, by allowing the barrel to be filled quickly without interruption for chucking.
The long feed stroke is also an advantage when reaming casing through unstable ground.
The drill rigs were built onto compact 4 x 4 tractors which have proven to be a very practical compromise between truck and track carriers.
Similar 4 x 4 tractors incorporating 12 ton timber winches are used to move auxilliary plant, including the compressors which are mounted on large wheel goose neck trailers and to recover rigs should they become bogged.
In very soft ground conditions the tractors which have slewing capability can be fitted with tracks to increase their mobility.
The companies drillers have spent long years, working with air circulation for coring, non coring an R.C. and their particular experience of coring soft friable coal seams is a factor underlying their successful performance at Biankouma.