The invention relates to a rotary bit for use in drilling boreholes or wells in underground formations. In particular, the invention relates to a rotary bit provided for cutting elements and a plurality of nozzles for discharging liquid, at least some of the nozzles being arranged to have liquid under pressure supplied thereto, thereby forming pressurized liquid jets. These jets either break up the bottom of the hole to deepen the hole (which manner of drilling is often indicated by the expression "hydraulic drilling") or remove the drilling flour or fine earth particles from the cutting elements and/or from the bottom of the hole, which drilling flour results from the mechanical drilling action of cutting elements carried by the bit. Such cutting elements may be mounted on roller cones and consists of cutting teeth, cutting rings, etc., or be mounted directly on the body of the bit and consist of diamonds, abrasive bodies such as the bodies made of materials like those known by the tradename Stratapax, and like elements.
The most effective action of the liquid jets will be obtained by jets originating from liquid nozzles having a relatively small internal diameter (in the order of 2-4 millimeter), over which nozzles a relatively high-fluid pressure difference exists (in the order of 30-150 bar). It will be appreciated that such nozzles that are made of an erosion-resistant material, will be liable to become plugged by particles present in the drilling mud that is being pumped down through the drilling equipment (such as the drill string) to the drill bit.
These particles may be lumps of solid material that has been mixed in powder form at the surface with a liquid for making up the drilling mud. Incomplete mixing will result in the formation of lumps that may partly be broken up by the mud pumps via which the mud is passed down the well, but the remaining lumps will be sieved off from the mud by jet nozzles in the bit, which nozzles will be plugged thereby decreasing the mud flow through the bit and jeopardizing the drilling action. Other particles may be constituted by fragments of the formation, which fragments have passed through damaged parts of the screening trays on the drilling floor, via which trays the drilling mud is returned to the borehole after appropriate treatment thereof. Also, dirt or corrosion products may be detached from the inner wall of the drill string by the mud flow passing therethrough, and be caught at the entrances of the nozzles when the mud passes therethrough.
Further, lost circulation material may be added to the mud flow to fight circulation losses occurring during drilling. These materials sometimes contain lumps or chunks of solid material that cannot pass through the small passages of the fluid nozzles present in the drilling bit.
Clogging of the fluid nozzles of the drilling bit described in U.S. Pat. No. 3,175,629 to D. S. Rowley is prevented by a screening element mounted in the shank of the bit, which screening element prevents relatively large particles present in the mud stream to reach the three fluid nozzles that debouch in the face of the bit. Only part of the mud stream is passed through the screening element, whereas the remaining part passes through a central channel in the shank and the body of the bit to a large-diameter nozzle situated in the centre of the lower end of the bit body. A plurality of choke plates provided with a single central passage of a diameter equal to the diameter of the central nozzle is arranged in the central channel to restrict the flow through the large-diameter central channel in order that a sufficient volume of drilling fluid will pass through the screening element to the small-diameter nozzles. The screening element is self-cleaning, and the particles caught by the screening element are subsequently discharged by the fluid flow passing through the central channel.
A similar arrangement, apart from the choke plates in the central channel, is known from the U.S. Pat. No. 2,293,259 to C. D. Johnson. Part of the mud flow has the relatively large-sized particles filtered therefrom, and is supplied to six nozzles, whereas the remaining unfiltered part of the mud flow is passed through a nozzle of a diameter larger than the diameters of the six nozzles.
Tests carried out with drilling bits have made clear that drilling efficiency will considerably be promoted by the use of liquid jets of extremely high pressure and high velocity. Unfortunately, when raising the pressure of the mud supply to the jet nozzles in the abovementioned prior art bits, large volumes of fluid will pass through the central channel of the bits to the centrally arranged large-sized nozzle without effectively supporting the drilling action of the bit. In the bit of U.S. Pat. No. 3,175,629, this situation might be improved by increasing the number of choke plates in the central channel, but this will lead to a complicated and costly structure of the bit.