Patent ID: 8167058

Claim:
Method for operating an earth formation drilling device arranged to supply a jet of abrasive fluid for the purpose of providing a borehole by removing earth formation material through abrasion, comprising a drill string and a drilling assembly connected to the drill string, said drilling assembly comprising a jetting device comprising a mixing space, a drilling fluid inlet for feeding a drilling fluid into the mixing space, a particle inlet for feeding magnetic particles into the mixing space, an abrasive fluid outlet for discharging a mixture of drilling fluid and magnetic particles from the mixing space and onto the earth formation material, and a magnetic particle circulation system comprising a supporting surface which is exposed to a return stream along the drilling assembly after abrading the earth formation material, a magnetic device for attracting the magnetic particles onto the supporting surface and for feeding said particles to the particle inlet, said supporting surface sloping radially inwardly and having at least one entrance connected to the particle inlet, comprising the steps of: fixing the magnetic device with respect to the supporting surface, selecting a magnetic field density that increases along the sloping supporting surface towards the entrance, attracting magnetic particles onto the supporting surface under the influence of the fixed magnetic device, making the magnetic particles move over the sloping supporting surface under the influence of the magnetic field of the magnetic device.