Polycrystalline diamond composite (PDC) requires low cutting forces and has excellent wear resistant properties, making it ideally suited for drilling in Sydney sandstone, which is typically a soft to medium strength formation but highly abrasive.
A brief history
The first hole openers were fixed-wing and similar to others already being used in the market. The primary method of manufacture was fabrication and while they were effective and relatively inexpensive, maintaining accuracy of axial and radial run-out of the cutter segments always presented a challenge.
They were also limited being fixed-diameter, requiring a new tool built for even minor changes in bore diameter. Repairs and modifications could only be undertaken in a select number of engineering facilities throughout the country, often leading to drilling delays.
Customers wanted field replaceable wings of varying diameter to fit onto a standard body similar to the principal utilised on a roller cone body that was imported from the US. Engineering carrier arms to suit this US roller cone body, Hard Metals produced its first PDC hole opener with field replaceable wings.
On smaller diameters and over short distances these were very successful generating high ROP and good bore quality. Difficulties were encountered when attempting to upsize over 13 inch and reaming over 200 m. The reasons for this were not initially clear until it was discovered the fasteners were not fit for purpose. This necessitated a review and it was decided to fit a stabilising ring that would tie the wings together and improve structural stability of the reamers.
An immediate performance improvement became apparent when an 18 m reamer completed a forward ream of 530 m off a 6 å_ inch pilot bore in a single pass in September 2008.
This style of reamer became standard issue over the next two years. There was further development of the concept to increase the number of blades from three to six for better performance. While it ticked most of the boxes it could not be regarded as being easily field serviceable. Hard Metals Australia Director Stephen Ainge was not satisfied and set about creating a totally new product, which would become the Wombat Hole Opener.
Wombat Hole Opener created
The Series 1 Wombat Reamer was a development of the roller cone body concept. What changed was a complete review of the fastening system to achieve greater security of the cutter wings. Incorporating the stabilising ring presented some engineering obstacles and so it was decided to stabilise off the body. This necessitated a large diameter central body.
While this concept had merit, it also had some problems. Much of this was related to weight. While it was a critical development step, it was literally weighed down and it was not long before a replacement was on its way.
The Series 2 was shelved before it left the drawing board. The Series 3 concept of the body as a stabiliser was discarded. The stabiliser was to be connected directly in line fore and aft of the reamer and the body was to have the single purpose of carrying the wings and as many as possible for high ROP.
The series 3 discarded weight but was more robust in design. It was more easily field serviced and delivered excellent ROP. However, it still suffered from insufficient stabilisation and as a consequence, PDC life was short and service costs were higher than anticipated.
Series 4 Wombat Hole Opener
The series 4 was the culmination of all that been learnt and a quantum leap forward in every respect. It was very stable in operation, making it suitable in less favourable conditions. It was simple in design and yet it incorporated in-built stabilisation close to the cutters and all easily field serviceable.
Of all the new features, it was incorporation of near face stabilisation that was the most critical advance in design. This was especially apparent in the vastly improved life of the cutters over previous reamer concepts in difficult ground conditions.
The first real trial of the Series 4 Wombat reamer was on a crossing of the Nepean River in March 2012. This was a particularly difficult project due to the existence of large cobbles on entry and exit. Casing had been installed, however the conditions in the bore were highly unfavourable and it was decided to use less expensive tungsten carbide cutting wings through these sections as sacrificial cutters until the bore progressed into more favourable mudstones and sandstones.
The tungsten carbide cutters were changed out for PDC cutters while the body was on the drill string in under an hour. Reaming of the mudstone at
12.5 inches went successfully with only one chipped PDC. General ROP was 13 min/rod on 6.1 m rods at 50 RPM. The overall consensus was that the reaming was highly successful, enabling pipe pull within two weeks of starting the process.
Basic Strategy
While the results are limited to date, they challenge the long held belief that PDC are only applicable to soft sedimentary rock in trenchless applications. This belief is also at odds with the utilisation of PDC in oilfield and gas drilling operations where PDC is now the first choice for drilling a wide range of strata.
The basic strategy that has led to this has been largely dependant on quantum leaps in stabilising the PDC at the drill bit rock interface. While PDC is less impact resistant than conventional tungsten carbide cutters, PDC’s capacity for wear resistance can exceed that of tungsten by more than 30 times. By creating a more stable environment at the cutting face, PDC’s excellent wear characteristics can be exploited to the great benefit of the drilling operation.
Leading PDC bit manufacturers are aware of this and much of their research and development focuses on this very subject. In the Series 4 Wombat reamer Hard Metals have made good progress in following this strategy, yet Mr Ainge believes there is still much more that can be achieved.
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