Drainhole drilling assembly with oriented elliptic drill collar

Abstract

In deep drainhole drilling, a specially oriented drill collar of generally elliptic cross section is combined with a drill bit, a downhole motor and means adapted to deflect the bit to drill the build section of the drainhole curving at a medium angle of 2.degree. to 50.degree. per hundred feet. Preferably, the drill collar is at least twenty feet long. The elliptic drill collar is oriented in a manner such that the minor axis of the ellipse lies generally in the direction of the intended curve of the build interval. For the desired vertical build, this places the locus of the minimum axis of the ellipse toward the vertical and the locus of major axis toward the horizontal when the curved interval of the drainhole approaches horizontal. Preferably, the minor axis is at least twenty percent shorter than the major axis. An elliptic drill collar combined and oriented in this manner will produce significant torsional resistance because the bending moment required to deflect the collar about its minimum axis is significantly less than with a collar deflected about its major axis. This stabilizes the orientation of the assembly during drilling operations. The minor axis may be misaligned with direction of build to generate forces to counteract drill motor reactions. The deflecting means may be a bent motor housing, a bent sub, a bent elliptic drill collar or any other suitable deflecting means.