Drilcorp was recently commissioned to carry out an investigation to pinpoint locations that would intersect open mine workings for the construction of large diameter mine-water abstraction and reinjection boreholes.

The remit of this project was to ensure that the boreholes must be both vertical and fully cored in order to identify all strata changes and possible mined seams that would be intersected before reaching the target seam. Using the detailed information gathered, it would be possible for the team to precisely design the construction of production boreholes.

Cores showing a section of mine workings 1

The target mine workings were determined to lie around 50 metres below ground level for the recharge borehole and 150 metres below ground level for the abstraction boreholes. The mines were worked over 100 years ago and, although there were plans available, the methods of surveying at the time meant that the accuracy of the plans was questionable and the boreholes might deviate from the plans by several metres.

To achieve the required deliverables of no deviation and 100% core recovery Drilcorp management team decided to use a Geobor S wireline coring system with water/mud flush.

Geobor inner tube ready to launch 1

The client sited the borehole positions initially using the mine plans as a guide. However, after several “misses”, drilling locations were moved slightly off-plan, advancing our knowledge with every attempt as to where the workings were actually located. Some of the boreholes found positions where the coal had been mined but due to the age of the workings, it was identified that the strata had closed up with little or no void left.

Twelve exploration boreholes were drilled in total before the exploration phase of the work was completed and three locations were chosen for the construction of production boreholes.

Core recovery in all boreholes was excellent and the only losses were in zones where coal had been mined. Verticality surveys were carried out every 10 metres during the advancement of the boreholes using Drilcorp’s Flexit Multishot tool. No borehole had deviated more than one metre from vertical at 150mtr depth proving that Drilcorp had made the right choice of drilling technique to meet the client’s requirements.

The exploration phase enabled the client and team to determine optimum depths for the various casings to be placed in the production boreholes. If this critical phase of the project had not been carried out, it would have been necessary to proceed with the more costly approach of drilling larger diameter boreholes with additional casings. Not only would this solution have been significantly more expensive, but it would also have come without any guarantee that the borehole would hit the target workings as required.

To find out more about Geobor S coring services provided by Drilcorp, please get in touch via info@drilcorp.vm or by calling 0191 5273970.

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