A coil tubing or jointed dual wall
drill string
assembly for subsurface drilling. The
drill string
assembly includes a metallic outer tube having an outer tube first end and an outer tube second end opposite the outer tube first end. The
assembly also includes a flexible, substantially non-metallic inner tube that is substantially enclosed within and generally coaxially aligned with the outer tube. The flexible, substantially non-metallic inner tube has an inner tube first end, an inner tube second end opposite the inner tube first end, and an inner tube inner
diameter. The inner tube and the outer tube define an annular channel therebetween. The
drill string assembly also includes a means for conveying fluid through the annular channel toward the inner tube first end. The annular channel is adapted to convey
drilling fluid under pressure toward the inner tube first end and the inner tube is adapted to convey cuttings toward the inner tube second end.