Increased Penetration Rates Achieved With New Extended Nozzle Bits
Authors: Pratt C.A. 
Published: December 2007

Field testing more than 90 extended-nozzle/center-jet bits has shown this combination to increase penetration rates from 15 to 40 %. Design modification has eliminated the jet washout problem that occurred with earlier models. Extended nozzle bits now should be considered operational because problems with nozzle integrity have been eliminated. Cast extender tubes will withstand normal rig wear.

Introduction

The importance of hydraulic effect on penetration rate was observed during the late 1940's. Eckel and Bielstein noted that the total fluid impact force is inversely proportional to the distance from the nozzle. During the early proportional to the distance from the nozzle. During the early 1960's, the first extended nozzle bits were designed and built. These first long nozzles were constructed of tubular material with fins or welded wrap-around support lips. Although these early extended nozzle bits experienced excessive mechanical failures, there were indications that further testing was warranted.

In 1970, research began that culminated in an improved extended nozzle design that cleaned the hole more effectively and improved the efficiency of the hydraulic influence on the formation removed. Extended-nozzle/center-jet (EN/CJ) bits were field tested with milled-tooth cutter bits at 19 locations. These test results showed an average increase of 28% in penetration rate (Fig. 1). High operating costs offshore make cost penetration rate (Fig. 1). High operating costs offshore make cost per-foot calculations overemphasize savings in less per-foot calculations overemphasize savings in less expensive operations. Therefore, the performance of EN/CJ bits is evaluated by considering the average increase in penetration rate. penetration rate. Original Work With Extended Nozzle Bits

Many authors have shown, in both laboratory experiments and field tests, that drilling rate depends on the hydraulic effect at the bit. The hydraulic jet system performs two functions at the bit: (1) prevention of bit performs two functions at the bit:prevention of bit balling--keeping the bit clean so that it does not lock up when the area between the teeth becomes clogged with cuttings, andbottom cleaning--removing the formation chip from bottom before the next cone gets to it, so the teeth do not have to redrill this chip.

Jet impact, bit hydraulic horsepower, and Reynolds number are related to the fluid velocity at the nozzle. The fluid flow is accelerated and a pressure drop occurs at the nozzle. This energy conversion provides the hydraulic influence that cleans the bit and the bottom of the hole. It has been shown that jet impact, fluid velocities on bottom, bottom-hole cleaning, and pressure distribution under the bit all depend on the distance of the jet from the bottom of the hole. Most of the hydraulic force with normal jet-bit configuration is required to transfer the energy from the nozzle to the bottom of the hole. As the distance between the nozzle and the bottom of the hole is reduced, the fluid jet passes through less fluid, less energy is required to move the fluid, and dispersion of the jet stream and mixing are reduced. By reducing the distance between the nozzle and the bottom of the hole, extended nozzles increase the hydraulic effect for better penetration rate. penetration rate. Previous attempts to build extended nozzles for rotary rock bits experienced three main problems:mechanical failures of the extender tubes,tube extensions too short to influence the bit performance, andlong lead times required to manufacture some designs.

JPT

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