Sergey KABLASH, NOV Fidmash

 

Radial drilling operations currently involve capillary coiled tubing units, actually being smaller copies of usual CT units, the so called “small coiled tubing”. The units are meant for working with coiled tube of stainless steel. The diameters of the tubing are ½ inch (12.7 mm), ¾ inch (19.05 mm) or 1 inch (25.4 mm). As in “big” coiled tubing, the tube is reeled on spool, equipped with a levelwind swivel and a manifold for supplying fluid into the tubing.

 

A differential characteristic of the capillary CT units is a possibility of running low depth (2,000- 2,500 m) round-trip operations without injectors. In such cases the spool acts as a winch of logging truck hoist. In case of deep operations small CT units are necessarily equipped with an injector with the pulling force of up to 5,000 kg. The unit managed from the control desk in the cabin. The unit drive is made from hydraulic actuation station, powered by diesel or electric engine. In order to make the units more self-sustainable they also equipped with high pressure triplex pump for supplying technological fluid into the well and power generator. Besides, many units have a positioning crane for easer installation of the equipment on the well. It is located in the back side of the unit. Small size of the unit allows locating it on one chassis, semi-trailer or skid.

 

Yet, radial drilling is just a new field of application for these units. Initially, they were meant for installing capillary columns into the well and injecting various chemicals (corrosion inhibitors, resolvents and so on). In such case the efficiency is achieved by precise proportioning of air-foams and other chemicals, possibility of automatic continuous injection of substances on the certain depth. Light weight of the unit and easy transportation, low exploitation expenses and high efficiency provide for short pay-off period of small coiled tubing in this function.

 

The term “small coiled tubing” also includes CT units for working in the annular space. Such units have a steel tube with the diameter of ¾ inch (19.05 mm) or 1 inch (25.4 mm), the length of 3,000 m and an injector with the pulling force of up to 10,000 kg. The construction of the injector is adapted for working in wells, equipped with beam-pumping units.

 

The units with polymer tubing or umbilicals, available at the market, are also called in Russia “small coiled tubing” by the producers and positioned for radial drilling. Yet, low working pressure and low running depth don’t allow taking this offer seriously. Today these units are nothing but a usual logging truck hoist. It limits the depth of the wells, where their application is possible. In order to be operational at large depth, these units will have to be fitted up injector, increasing the power of the drive and making the hydraulic system more complicated, which whittles all its advantages.

 

The polymer armored tube ТГ-20/37-90, according to producer Pskovgeokabel, with the outside diameter of 37 mm (analogue of steel CT of the diameter of 38.1 mm) has lower yield load (9 tons, compare to 18,6 tons of the usually applied steel tube (QT-800) with the wall thickness 3 mm and maximal working pressure (18 MPa, compare to 55 MPa of the steel tubing). By the way, the steel tubes applied in practice can boast of much better characteristics. The radius of the bend of the polymer armored tube is practically equal to the minimum acceptable radius for steel tubes (740 mm, compare to 800 mm). It means that we won’t have advantages in spool assembly size. A narrower passage (about 20 mm) also limits the sphere of application of umbilicals (for example, the use of trip ball for downhole tool orientation) and increases losses of

pressure during fluid flow. The only positive effect is little advantage in mass (1.6 kg/m, compare to 2.1 kg/m).