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The Drilling of a Cased Borehole

Author: John Wianwright, P. Eng.

During the summer of 1997, ECI CanCross ULC of Calgary, Alberta completed a very unusual drilling project for a pipeline rig and spread. This was the drilling of a 1,000-meter (approximate length) cased borehole to stop an underground river from flowing into a subsurface potash mine in Sussex, New Brunswick.

In June 1997, ECI CanCross was contracted by the Potacan Mining Company to drill a directionally deviated borehole that would terminate at a subsurface cavern located immediately above their existing potash mine at Sussex. This potash mine had operated since the mid-80's and was considered to be one of the richest potash deposits in the world. The potash deposits and the sub-surface mine were situated about 700 meters below ground. The material above the the potash deposits consisted of clay and soft shale with an underground stream located at about 300 meters depth. Below this underground river, a continuous layer of bedrock served as a very effective barrier in preventing any water from entering the potash the potash deposit. As long as this layer remained intact, the potash mining could proceed despite the great quantity of water flowing above. Unfortunately the "caprock" was disturbed and water did begin to enter the potash mine.

Initially, this ingress of water commenced as a few drops, but over a few months escalated to an estimated 9,000 cubic meters of water entering the mine each day. The difficulty with this water ingress was that the water would react with the potash, placing the mineral in solution. The Potacan Mine in Sussex was approximately eight kilometers in length and five kilometers in width. As the potash was removed from this deposit, columns of potash were left in place to support the material above. With water entering the mine, it reacts with the potash column, eroding the material and eventually causing the column to fail and the mine to collapse. For a few months, the Potacan Mining Company had been able to deal with the water ingress by simple containment and pumping methods, but eventually the flow of water reached a point where the pumps simply could not keep up. As a consequence, certain sections of the mine had to be closed due to the potential of the columns failing and the mine collapsing.

By June 1997, the rate of water ingress had caused virtually all operations in the mine to be terminated. The Potacan Mining Company soon concluded that their best recourse was to attempt to stop the flow of water into the mine altogether. Seismic surveys conducted at the site had identified the existence of a subsurface "cavern" immediately above the potash deposit. The Potacan engineers believed that the water was entering the mine through this cavern. They also concluded that if this cavern could be filled with material and the caprock sealed, then the flow of water into the mine could be stopped. Potacan decided to proceed with an attempt to stem the water ingress in this manner.

Potacan employed ECO Grouting Specialists Ltd. of Orangeville, Ontario to engineer the project. ECO concluded that the most appropriate method of stemming the flow of water was to drill two boreholes into the cavern. One borehole would be used to flush hot bitumen into the cavern while the second hole would be utilized to pump cement.

The process consisted of initially pumping the hot bitumen into the cavern. The bitumen would contact the cold water and immediately "set". It was imperative that the bitumen set very quickly, otherwise it would simply be carried through the cavern by the rushing water. Once the bitumen had set, it would serve as a base for the cement which would be pumped through the second borehole soon after. This procedure of pumping hot bitumen followed by a cement plug would continue until the cavern had been filled, the "caprock" sealed, and flow of water in this area stopped. This attempt to stem the flow of water was to cost millions of dollars but was justified by the value of the mining assets and unmined potash totaling well into the billions.

The plan to stem the flow of water could only be successful if the boreholes could be drilled the estimated 980 meter length into the cavern, which required an accuracy of two meters. The drilling of the boreholes was further complicated by the fact that the cavern was located immediately below a tailings pile which could not easily be relocated. As a consequence, the two boreholes would have to be drilled by positioning the drill rigs alongside the tailings pile and drilling the borehole by directionally deviating into the cavern. The second borehole was complicated by the extent of the tailings heap. Potacan designed this second borehole to spud at an angle of 18 degrees from the horizontal. This borehole would then be directionally deviated to enter the cavern at about 75 degrees from the horizontal.

In June 1997, the Potacan Mining Company contracted ECI CanCross ULC to drill this latter borehole. ECI CanCross ULC is owned jointly and equally by CanCross Ltd. of Calgary, Alberta and Environmental Crossings Inc. of Traverse City, Michigan. The companies own and operate a variety of drilling rigs designed specifically for the installation of pipelines and cables using directional drilling techniques.

This procedure entails positioning the specifically constructed drilling equipment on one side of the surface obstruction (typically a road, river or environmentally sensitive area) where there is a desire not to employ conventional surface construction techniques. From this location a borehole is drilled below the surface obstruction to eventually rise to the surface on the opposite side. With this technique, boreholes over 1,800 meters long or diameters of over 1,500 mm may be drilled.

The boreholes are typically very shallow; they are seldom more than 30-40 meters below ground surface, and usually occur in very remote areas where access is challenging. As consequence, the drilling equipment employed for pipeline/cable installations differs from conventional oilfield drilling in many ways. The pipeline cable drills are designed to spud at a very low angle from the horizontal; typically at 10-18 degrees. The pipeline drilling rigs are designed with significant push and pull since this equipment must have the capability of placing weight-on-bit with the rig only, and cannot rely on the weight of the drill string to do so. This is a result of the very shallow drilling depths. The rig pull is necessary to have equipment to pull the product pipeline through the borehole once the drilling is complete. The equipment is lightweight, modular and compact, relative to similar oilfield equipment, due to the necessity to access extremely remote and poorly prepared sites.

In early July 1997, ECI CanCross ULC's Drill Rig and Spread #2 arrived in Sussex, New Brunswick, at the Potacan Mine. This drill rig is a track-mounted unit providing over 100,000 pounds of push/pull. The drilling equipment was very quickly positioned on the site and readied for operation.

This particular borehole represented a very unusual application for this type of drilling equipment. Rarely is a pipeline rig used to directionally drill by deviating down, or to drill to a target located 650 meters below the ground surface. It is also rare that a borehole is drilled by a pipeline rig lined with a steel casing, as in the case of the Potacan borehole. The pipeline rigs are not designed for these operations and drilling accuracy is not usually as essential as it was with the drilling of this particular borehole.

Despite the many challenges, the borehole was drilled quickly, efficiently and accurately. The borehole was initially drilled a distance of 791 meters with a 251mm-diameter pilot hole, followed by a 311mm-diameter reaming pass. Once this portion of the borehole was drilled out, a 244mm-diameter threaded steel casing was run into the hole and cemented. The purpose of the steel casing throughout the borehole was to facilitate the future pumping of the bitumen slurry, but also to prevent the ingress of additional water into the mine. In drilling the borehole, a number of aquifers were intersected. The borehole through these aquifers had to be sealed by the casing string prior to punching into the cavern. As a further precaution, since the exact elevation of the of the cavern was unknown, Potacan decided to run casing at selected intervals to reduce the possibility of an early entry into the cavern which would cause additional water to enter the mine.

Once the 244mm-diameter steel casing had been cemented, a 222mm-diameter bit was run to the bottom and the pilot hole drilling continued. This portion of the borehole was drilled to 822 meters and then lined with 178mm-diameter threaded steel casing. The pilot hole was then continued, utilizing a 159mm-diameter bit to a total measured depth of 976 meters or approximately 10 meters above the expected depth of the cavern. A 76mm-diameter tubing was then run to the bottom of the borehole and cemented. The borehole was then ready for the final few meters of drilling to "punch" into the cavern. On August 22, a 51mm-diameter drill bit and string was run into the borehole and the final few meters drilled out. The drill was on target entering the top of the cavern as planned. The borehole was camera surveyed, confirming that the desired target had been achieved.

The role of ECI CanCross ULC in this project was, at this point, complete. On August 24, the equipment was rigged down and then demobilized the following day.

A few months after the completion of the ECI CanCross borehole, the pumping of the bitumen and cement slurries commenced. The flow of water into the potash mine was significantly slowed on a number of occasions, only to eventually continue at around the same rate. Eventually, the attempt to seal the "caprock" and stem the flow of water was abandoned. In October 1997, the Potacan Mining Company announced that, despite the success of the drilling done, they had determined their attempt to stop the inflow of water that the mine had permanently ceased operations.

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