Expandable underreamer is designed for gaging and milling of the interior of casing string and for reaming of open holes before setting profile liner.(Fig. 1) It also can be used for drilling with simultaneous reaming, straightening collapsed casing string, cleaning out clay coating asw. Expandable underreamer also can be used for remedial operations in casing. In this case it is necessary to replace the rock cutting elements with metalcutting.

The operator bodies of the underreamer consists of inserts installed in dovetail type slots. The inserts are equipped with pivot bolts rigidly coupled milling cutter hardmetal teeth. The teeth on each milling cutter are placed with different step. The insert design provides reliable gaging of the wellbore wall. The milling cutter design, the quantity of the hardmetal teeth and their disposition are defined by means of calculations.

For improvement (cost-)performance characteristics jointly with “UDOL” close corporation were developed new designs of rock cutting elements underreamer in which the special inserts are performing reaming and gaging activities of the wellbore wall. (Fig. 2) The inserts of new underreamer are equipped with two types rock cutting elements. These are chisels reinforced with synthetic diamonds (PDC) having plain type configuration and hardmetal teeth that used for the cone bits. The computer designing system Wear Graph and its wear calculation were used on developing of the inserts. This enables it to optimize the placement cutting and gaging elements of underreamer for increasing its operational capability as well as to minimize the dynamic imbalance and vibration of underreamer.

It makes the main difference of this inserts design that every chisel is backed up with additional working element that so - called “impreg” on terminology HYCALOG company. These teeth perform several activities.

First they protect chisels from shock loads that arise from vibration during drill string rotation. The lateral oscillations are the most destructive for PDC chisels. The instantaneous center drifts to the point of engagement of chisel and rock when the PDC chisel “bites” the rock.

In its turn this bring to shift in backwards and shock load on chisel PDC situated on opposite from The instantaneous center. The beats – caused damages usually happen owing to big influencing shock loads on chisels. While the chisels protected with “impregs” stabilize the operation of the underreamer and decrease their tendencies to the lateral oscillations. The “impreg” takes on itself the basic part directed from behind shock load thereby protecting PDC chisel against damages.

Secondly they generate the preliminary rock fragmentation thereby unloading on the nose of chisel. The pick forms the nucleus of preliminary rock fragmentation by means of generation ruptures, tension zones and other deformations in rock what promotes the decrease of resistance following after it PDC chisel. In addition operational efficiency of these inserts in comparison with roller-bit design consists in as well that slime particle formed owing to operation of the cutter tooth never stays on spot of its formation at any differential pressure as it is happened on using roller-bit design. At the same time the chisel works as destructing and cleaning tool.

Thirdly as a result of trim between PDC chisel and “impreg” is decreased the abrasive wear of cutting elements.

At fourth the hard metal bands are welded on gaging parts of the inserts that are executing the aligning and gaging activities.

Application sheet:

The underreamer is tripped-in on drilling tool to preset depth. Then the rotor is switched and the rotation is performed with the frequency 60 – 75 revolutions per minute. After that the pump is activated and the pressure is established in chamber of underreamer. Under pressure the inserts is pulled out to working position and penetration is performed during 5-7 minutes.

In order to make sure in this by means of smooth admission of drill string we define the landing of underreamer. By this the load should not exceed 50 kN.

If landing of tool is out then penetration of underreamer is continued. In case of presence of landing it is performed enlarging of hole diameter wit axial load 10 – 15 kN.

For termination of enlarging it is stopped supply of flush fluid and rotor rotation. It is performed smooth trip-out tool. After trip-out it is performed the examination and measurement of rock cutting elements.

Expandable underreamer is designed for gaging and milling of the interior of casing string and for reaming of open holes before setting profile liner.

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