Patent EA-002542-B1 (EA)
Title: Abrasive jet drilling assembly
Inventor: BLANGE JAN JETTE
IPC: E21B21/00
Filed: 2000-04-27
Abstract: 1. A drilling assembly for drilling a borehole into an earth formation, comprising a drill string (1) extending into the borehole (2) and a jetting device (5) arranged at a lower part of the drill string, the jetting device being provided with a mixing chamber (10) having a first inlet (12) in fluid communication with a drilling fluid supply conduit (9.9a), a second inlet (14) for abrasive particles and an outlet (15) which is in fluid communication with a jetting nozzle arranged to jet a stream of abrasive particles and drilling fluid against at least one of the borehole bottom (7) and the borehole wall, the jetting device further being provided with an abrasive particles recirculation system for separating the abrasive particles from the drilling fluid, characterized in that the jetting device is provided with the mixing chamber (10) and the abrasive particles recirculation system and that the abrasive particles recirculation system is designed so as to provide separating the abrasive particles from the drilling fluid at a selected location where the stream flows from said at least one of the borehole bottom (7) and the borehole wall towards the upper end of the borehole and for supplying the separated abrasive particles to the second inlet (14). 2. The drilling assembly of claim 1, wherein the recirculation system includes means for creating a magnetic field in the stream, and the abrasive particles include a material subjected to magnetic forces induced by the magnetic field, the magnetic field being oriented such that the abrasive particles are separated from the drilling fluid by said magnetic forces. 3. The drilling assembly of claim 2, wherein the recirculation system includes a recirculation surface (44) extending from said selected location to the second inlet, and the means for creating the magnetic field is arranged to create a moving magnetic field which induces the abrasive particles to move along the recirculation surface to the second inlet. 4. The drilling assembly of claim 2 or 3, wherein the means for creating the magnetic field comprises at least one magnet (26,27,28,29). 5. The drilling assembly of claim 4, wherein each magnet (26,27,28,29) is provided at a rotatable member (16) having an outer surface extending between said selected location and the second inlet (14), the axis of rotation (20) of the member (16) being arranged so that during rotation of the member each magnet pole moves in the direction from said selected location to the second inlet (14), and wherein the recirculation system further includes means for rotating the rotatable member. 6. The drilling assembly of claim 5, wherein the means for rotating the rotatable member includes a nozzle (12) formed by the first inlet (12). 7. The drilling assembly of claim 5 or 6, wherein the jetting device (5) is provided with at least one guide element (22,24) extending along the outer surface of the rotatable member (16) and at a selected angle to the axis of rotation (20) of the rotatable member (16) so as to guide abrasive particles adhered to said outer surface to the second inlet (14). 8. The drilling assembly of any one of claims 5-7, wherein the poles of each magnet (26,27,28,29) extend substantially parallel to the axis of rotation (20) of the rotatable member (16). 9. The drilling assembly of any one of claims 5-8, wherein an annular space (8) is formed between the drilling assembly and the borehole wall, and wherein said selected location where the abrasive particles are separated from the drilling fluid is in the annular space (8). 10. The drilling assembly of claim 9, wherein the shape of the rotatable member (16) is selected from a cylindrical shape and a convex shape conforming to the curvature of the borehole wall in the vicinity of the rotatable member (16). 11. The drilling assembly of any of claims 2-10, wherein said material subjected to magnetic forces comprises at least one of a ferromagnetic, a ferrimagnetic and a paramagnetic material. 12. The drilling assembly of any one of claims 1-11, wherein the recirculation system includes means for separating the abrasive particles from the drilling fluid by centrifugal forces exerted to the particles. 13. The drilling assembly of any one of claims 1-12, wherein the drill string is at the lower end thereof provided with a drill bit, and the jetting nozzle is arranged to jet the stream of abrasive particles and drilling fluid against the wall of the borehole as drilled by the drill bit so as to enlarge the borehole diameter to a diameter significantly larger than the diameter of the drill bit. 14. The drilling assembly of claim 13, wherein the drill string has an inner diameter larger than the outer diameter of the drill bit, the drill bit being detachable from the drill string and being provided with means for detaching the drill bit from the drill string and for retrieving the drill bit through the drill string to surface.