Antistall Tool Reduces Risk in Drilling Difficult Formations
Authors: Reimers Nils 
Published: February 2015

Technology Update
A new method is now in use to reduce the risk and increase efficiency when drilling difficult formations. The antistall tool makes it possible to drill highly mixed and laminated formations with less risk of severe vibrations and downhole equipment failures. By reducing vibrations, the tool simultaneously improves the rate of penetration (ROP).
Drill bit-induced vibrations that occur during rapid transitions in subsurface formations are hard to predict and therefore equally hard to avoid through the preselection of bits and drilling parameters. The antistall technology is based on a dynamic, self-supported downhole mechanical system that actively controls the bit’s depth of cut (DOC) by manipulating the weight on bit (WOB). The system uses the rotary torque as an input control parameter to actively counteract the torsional peak loads and stalls that are common through sharp transitions and contrasts in the underground. Because of its relatively simple functional principle, the method has proved highly versatile. It is capable of reducing vibrations in a wide range of applications that, in addition to regular rotary drilling, includes reaming and through-tubing milling (Dagestad et al. 2006).
Continuous Prevention of Bit Stalling
Because it is part of the lower bottomhole assembly (BHA), the antistall tool can quickly and continuously prevent the bit from stalling and thereby limit the development of severe stick-slip vibrations. The mechanical function of the downhole tool is based on converting the rise in the drilling torque that precedes a stall into an axial contraction that will immediately cut back the WOB. The fast reduction of weight on the bit reduces the DOC sufficiently to keep the bit rotating. The conversion to the axial contraction takes place through an internal helix and appears as a reduction in the length of the polished, telescopic section seen in Fig. 1. With the contraction, a strong spring and absorber is simultaneously compressed internally in the tool body above the telescopic section. The energy absorbed in the spring is fed back through the system to maintain a steady torsional load. This capability of absorbing and releasing energy makes the system work continuously and with no need for reset.
Fig. 2 shows a downhole recording of torsional (stick-slip) vibrations in a 17½-in. hole where the antistall tool was run in at 2567 m to improve drilling, after an unexpected, difficult formation had caused several bit trips. Except for the antistall tool and a new bit, the BHA was practically identical throughout, and the data showed how the stick-slip vibrations were reduced and the reduction was achieved without much manipulation of surface parameters. The actual stroke needed to keep the bit rotating is a fraction of the telescopic capacity. The purpose of the extensive capacity is to provide the desired antistall effect with a wide range of loads and without the need for specific configurations or time spent on changing surface parameters. Because of its flexibility and ease of operation, antistall technology does not require any service personnel in the field.