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Freeze-hole drilling for the Gremyachinski shaft sinking project
In May 2008 EuroChem Volgakali awarded Thyssen Schachtbau GmbH the contract to sink a new mine shaft for the extraction of potash near the town of Kotelnikovo in the Volgograd administrative district of Russia. The geological and hydrological conditions prevailing at the site – the strata comprise loose rock, sands and clays traversed by a number of water bearing measures, some of which are used for the provision of the region’s drinking water – dictated that a stable freeze-wall would be required to strengthen the strata sufficiently for the planned sinking operation.
 Calculations indicated that the freeze-wall needed to sink the shaft with its planned internal diameter of 8.0 m would require 44 freeze holes set around an 18 m-diameter circle and extending to a depth of about 520 m, along with four temperature measurement holes drilled to a depth of 550 m. The holes would be fitted with a protective casing 339.7 mm in diameter to a depth of approximately 33.0 m and were to be supported by freely suspended freeze pipes 146.05 mm in diameter and extending to the bottom of the freeze-wall at a depth of about 520 m.
The uniform propagation of the freeze-wall was crucially dependent on maintaining the very tight tolerances for the deviation of the freeze pipes from the vertical over the entire length of the casing. A drilling diameter of 216 mm was specified for each of the holes.
Operating equipment
The operation was carried out using two new and highly manoeuvrable RB 50 universal drilling rigs with hydromechanical drives, each mounted on a trailer chassis and equipped with a mast extension. These machines, which boast a large hook load of 500 kN and can deliver 31.580 Nm of torque at the power swivel head, have the advantage of being very compact.
The mast extension allows each rig to deploy casings with unit lengths of up to 12 m. A flexibly mounted, 12 m-long deck with integral drill-rod carriage was used as a working/drill-rod platform. The modular design of the working platform meant that a safe and effective workplace was available for the drilling team to carry out the various tasks required during each stage of the operation.
Drill-fluid circulation was provided by four mud pumps that were permanently installed in 20-foot shipping containers along with their hydromechanical drives and all components required for transport and operation. The pumps were operated from the drilling-rig control console via a cable-linked remote control system. Centrifugal pumps were connected upstream as booster units in order to keep the triplex pumps working efficiently and troublefree.
 Vibrating dewaterers, desanders and three agitator tanks each of 26 m3 capacity were employed for cleaning and servicing the drilling mud. As the strata yielded a large quantity of sand during the drilling operation, which caused serious wear at the pumps, an extra centrifuge was subsequently leased from a Russian operator to provide additional mud cleaning. The continuous monitoring and conditioning of the drilling fluid was carried out in conjunction with a Russian mud servicing company, which also supplied the reactants needed for preparing the drilling fluid on site.
Transport
The entire site installation was assembled in Germany and in spite of all the logistic problems, which included having the equipment certified for the Russian market, cleared through customs and transported to Russia, the Thyssen Schachtbau shipping and logistics department succeeded in ensuring that everything was delivered to the drilling site with the help of an international haulage company. EuroChem, the Russian authorities and Thyssen Schachtbau worked closely together to ensure that the operating permit required for the entire drilling installation was obtained without delay.
Drilling system
Because of the extremely high demands on drilling accuracy the holes were drilled using MWD technology (Measurement While Drilling) combined with an external, stand-alone EMS (Electronic Multishot) system. This technology allows the course of each hole to be steered in any direction on the basis of the data being received on hole orientation and inclination, with the result that drilling tolerances can be maintained very precisely.
 As the drilling work had to be completed within a very narrow timeframe a total of six drill motors of 171.45 mm diameter were made available throughout the entire drilling phase, with two motor units usually being serviced at any one time. This rotation principle effectively eliminated drilling downtime caused by defective drilling motors. In addition, service engineers from the operating companies were on site around the clock to ensure that the MWD equipment and EMS system were incident-free at all times.
Not all the geological data required for the final shaft design were available when the project commenced. Test cores of 102 mm diameter were therefore taken from various parts of the drilling zone. A special core pipe fitting with a plastics liner was used to minimise the stress acting on the core samples during the drilling operation. This system proved successful in allowing the samples to be safely extracted from depths of between 150 m and 550 m.
Summary
In spite of the adverse climatic conditions the 48 holes were successfully drilled and cased in a mere six months and the work was completed in August 2009. The project had involved the installation of nearly 26,000 m of freeze pipes of 146.05 mm diameter. Each of the 48 holes drilled met the very stringent specifications for directional accuracy and maximum drilling performances of more than 200 m a day were repeatedly achieved by each drilling machine. During this period the drilling equipment underwent about 60 rebuilds. The rigorous implementation of the certified safety standards normally applied by Thyssen Schachtbau meant that not a single accident was recorded on site during the entire drilling phase.
R. Krause, May 2010
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