Three primary horizontal methane drainage holes totaling 2,428.4 meters (7,967 feet) have been completed in the Pittsburgh coalbed from a directional surface borehole. Directional control and sidetracking techniques developed during the project increased the horizontal drilling rate from 24.4 meters (80 feet) per day initially to 64.9 meters (213 feet) per day on the third horizontal hole. Drilling data indicate that horizontal holes can be drilled substantially longer than the maximum 977 meters (3,207 feet) achieved.
A project to drill horizontal coalbed methane drainage holes from a directional surface borehole was begun in September 1978 at the Emerald Mine near Waynesburg, Pa. Included in the project were a directionally drilled borehole intercepting the Pittsburgh coalbed, three long horizontal Pittsburgh coalbed, three long horizontal degasification holes drilled in the coal from the directional borehole, a vertical borehole for coalbed dewatering and seven vertical boreholes to monitor the progress and extent of degasification with time. The goals progress and extent of degasification with time. The goals of the project were to demonstrate that the technique of directional drilling can be used to degasify coalbeds, to obtain information to improve the technique and to determine the horizontal distance such a hole can be drilled.
The general plan for the directional borehole was to drill a 76-mm (3-inch) diameter pilot hole in a circular arc from the surface to intercept the coalbed horizontally (fig 1). The hole would then be overreamed (using BCQ drill rod as a guide string) to 222.25 mm (8-3/4 inches) in diameter and 139.7-mm (51/2-inch) casing would be cemented in place. Three 76-mm (3-inch) diameter horizontal degasification holes would then be drilled, each up to 950 meters (3,100 feet) in length, fanning out from the bottom of the casing. A vertical borehole would be drilled near the coalbed intercept and equipped with a downhole plunger pump for dewatering. The vertical borehole would be hydraulically stimulated to create a path for water to flow from the horizontal holes into path for water to flow from the horizontal holes into the vertical dewatering borehole. In addition, seven vertical boreholes (EM 21-27, fig 2) were planned for monitoring the progress of degasification by observing the changes in hydrostatic pressure by measuring the water column over the coalbed at each location.
The directional borehole (EM-19) was located at the top of a ridge to allow sufficient overburden above the coalbed for a circular arc of 0.3 to 0.35 radians per 100 meters (5 to 6 degrees per 100 feet) to be drilled to reach the coalbed horizontally. The vertical distance to the coalbed was 304.8 meters (1,000 feet). Of this vertical distance 289.25 meters (949 feet) was required to achieve the required arc to the coalbed. The actual drilled distance of the directional pilot hole well path was 503.5 meters (1,652 feet).
Directional drilling on the pilot hole was begun on November 9, 1978 at a vertical depth of 15.2 meters (50 feet). The directional drilling was accomplished using a 60.3-mm (2-3/8 inch) diameter Dyna-Drill downhole motor and BQ wireline drill rod as the drill pipe. The rate and direction of angle build were controlled by using various bent housings and standoff rings. By orienting the bend in the housing, the tool could be made to drill a hole in the desired direction. The four housing assemblies used to drill the EM-19 pilot hole were, in order of increasing angle build capacity, a 0.0087radian (30-minute) housing, a 0.013-radian (45 minute) housing, a 0.0087-radian (30-minute) housing with a standoff ring and a 0.013-radian (45-minute) housing with a standoff ring. Control was achieved by changing bent housings and standoff rings as required to keep the hole on the target well path.