Searching in the dark - drilling systems in the Chilean miners' rescue
During the Chilean mining incident late last year, different drilling survey techniques were employed to locate and free the trapped miners. Martin Cox, Head of Technical Business Development at Aberdeen Drilling Management Ltd, UK, talks to Michael Forrest about directional drilling and underground guidance systems.
Drilling is the definitive tool in minerals and oil/gas exploration and production. It is the reality check on all the preceding exploration technologies that defined the target. Both in mining and in oil and gas drilling, technology has relied on a variety of drill bits and rigs to penetrate the sub-surface strata to locate and define resources. In principle the techniques are similar, with the drill bit breaking the rock, while the annular drill string provides the mechanical power and a route to bring cuttings to the surface.
In mineral/metal mining, the drill can, in most cases, be located over any part of the target zone. In oil and gas, the rig’s location may be constrained by the platform from which it operates, undoubtedly the case in offshore operations. Mining exploration rigs use diamond-impregnated bits to cut and recover core. In oil and gas, a large range of bit designs and materials are available, with cutting fluids (referred to as ‘drilling fluid’) used to recover rock chips for lithology identification.
There is also a difference in the length of the drill bore. Mineral exploration drill depths are measured in a few hundred metres, while in production drilling a few tens of metres are the norm. Oil and gas drilling wells can extend to several kilometres. Although there are many exceptions, oil and gas drilling is in softer sedimentary horizons and most metal ore is in hard competent rock.
‘Drilling to a significant offset or target has been pioneered by the oil and gas sector, developed in tandem with downhole surveying technology,’ says Cox. ‘The traditional magnetic compass and free-directional gyroscopes used on simple well configurations proved to be lacking in accuracy when drilling in more complex geological regions, such as the diverse grabens of the North Sea.
‘For these environments, new devices capable of greater accuracy were needed to reduce the effect of cumulative error as borehole targets were extended to targets at increasing outsteps and distances from their surface location and origin.’
Developed from advanced aerospace guidance systems, gyroscopes that maintain a fixed reference point have been manufactured, while any deflections are measured by accelerometers. This horizontal component, together with true north, gives an accurate location based on the earth’s rotation adjusted for latitude. Unlike a magnetic survey tool, a gyroscopic surveyor can measure azimuth without using the earth’s magnetic field, so it can survey wells where casing has been set or where nearby wellbores create magnetic interference.
‘The outcome has allowed the reach of drilling to extend while maintaining target accuracy within industry accepted limits. It has also increased the density of wells that can be placed together at a surface location due to the reduction in collision risk as the well bore location (together with its potential error box) is more closely controlled, reducing the risk of a well being drilled into an existing well,’ comments Cox.
Initially, drilling would be halted at points along the borehole, and accurate measurements taken and transmitted to the surface by mud pulse technology. This encodes measurements into a sequence of pressure pulses, which travel in the mud column back to the surface, where they are detected, picked up, decoded and transmit data that was collected in the bottom hole assembly. This technology has now been developed for continuous data collection and transmission simultaneous to the drilling process.
Advanced gyroscopic survey equipment enables accurate measurement down the hole and, with new battery-powered rate, gyroscopes can survey without the normal wire guideline to surface. Compared with magnetic and free gyroscopes, lateral errors have improved from 20m per km drilled to 2m per km. Gyroprobes can now fit in a 4.5cm diameter drill pipe, and by using the drilling fluid contained within, pump the probe to the bottom of the hole.
In tandem with these technical advances in navigation has come advanced sensor design. This can provide lithological profiles in real time as the probe descends the borehole, including both horizontal and vertical trajectories at high speed. Probes have also been ‘ruggedised’ to allow them to sustain shocks, temperatures from freezing to 1,500°C, and to operate while sitting in the drill string for weeks and still deliver data from many kilometres underground. The ability for this type of equipment to operate in the downhole environment has resulted in measurement while drilling, more commonly known in the industry as Logging While Drilling.
Saving the day
Modern technology for oil and gas directional drilling has revolutionised production. It has also offered a number of solutions when things go wrong. Oil and gas reservoirs are often under intense pressure (reservoir pressure in deep formations that can exceed 15,000psi are in present day production), and the drilling of production wells can, as a result of loss of containment, result in a blow out.
Occasionally, the capping equipment and pressure valves can fail, as in Deepwater Horizon in the Gulf of Mexico in 2010. To relieve that pressure and stem the flow, a relief well had to penetrate the reservoir at a depth of over 7,000m without damaging existing infrastructure. This process has been likened by directional drillers to pushing a knitting needle from Paris’s Charles de Gaulle airport between the legs of the Eiffel tower.
A more human story provided the background to the Chilean mine rescue in October 2010. Initially, a number of boreholes were drilled to the underground drives where miners were calculated to be in refuge areas according to mine plans. Not all were successful, but one (Poloma 1) reached a drive near a refuge area and when withdrawn a note was attached indicating all the miners were alive.
The rescue plan was to drill a shaft wide enough to bring miners to the surface in an escape capsule. Gyroscopic equipment was used to accurately map the trajectory of the Poloma 1 wellbore. Three escape bores were initially drilled (known as Plans A, B and C), the first two being raise bore systems that rely on a narrow target wellbore to guide a much larger raise bore cutting head. The raise bores allowed the cutting debris (estimated to be around 700t) to fall down the guide hole for removal by the miners underground during the five and half weeks of drilling.
However, it was Plan C that eventually provided a route of egress by drilling a hole of sufficient diameter for the escape capsule to be installed and bring the miners back to the surface one at a time. This used a large Canadian oil drill rig that was guided by the gyroscopic system, and cut a shaft large enough for the escape capsule. It also brought drilling rubble up to the surface as it penetrated. It accurately reached the refuge area some two weeks after the bore Plan B had reached the miners (as Plan C had been initiated two weeks after the Plan B bore, which had used percussion hammer drilling to create a 70cm diameter shaft).
After being trapped underground for 69 days, all of the 33 miners were rescued on 11 October 2010, in a globally televised operation that saw them transported to the surface, one at a time in the rescue capsule. The success of the rescue project had brought together directional drilling technology, whose main application and development over the last 20 years has been focused on oil and gas exploitation, with the skills of the hardrock miner. The mine survey data gave the rescue teams target co-ordinates to drill towards to intersect the roadways inside the mine. The refuge to which the miners had retreated was about 700m below the surface.
Aberdeen Drilling Management Limited, 403 Holburn Street, Aberdeen, AB10 7GS, Scotland, UK. Tel: +44 (0) 1224 574482. Email: email@example.com. Websites: www.drillingmanagement.com and www.gyrodata.com
Martin Cox is the Chairman of the International Mining and Minerals Association (IMMa) at IOM3.