Focussed Hydraulic Energy - A New Look At Two-Nozzle Drilling
Authors: Barmby R.P. 
Published: March 2010

Drilling with a tri-cone bit equipped with two equal-sized nozzles and one blank nozzle has previously been used in an attempt to increase penetration rates through improved bottom-hole cleaning. A new concept uses two unequal-sized nozzles and higher bit hydraulic power to create maximum hydraulic energy at the bit and to transmit a larger fraction of that energy to the hole bottom in a pattern more conducive to bottom-hole cleaning. Applications of this technology in the Valhalla Field of Alberta have increased the average rate of penetration by 42% and reduced drilling cost by 27%.

Introduction

The rate of penetration (ROP) achieved in conventional rotary drilling with a tri-cone bit is controlled by a dynamic equilibrium between the generation of cuttings and the removal of cuttings from the drilling surface, Ineffective cuttings removal often limits the actual ROP to much less than what would be mathematically expected if perfect bottom-hole cleaning conditions had xisted. In such cases, improved bottom-hole cleaning results in increased ROP.

When drilling with three equal-sized nozzles each jet impacts on the drilling surface and lifts drill cuttings off of the rock face by turbulent scouring. The fluid then reverses against itself in the confined area between the cones to transport the cuttings up the hole. There is no fluid flow underneath the cones where cuttings generation takes place. Cuttings removal can only take place after the cones have rolled over them and they have been lifted off of the rock surface sufficiently to be entrained in the returning fluid flow.

The original two equal-sized nozzle hydraulics design attempted, with varying degrees of success, to improve cuttings removal by initiating crossflow under the bit and providing a flow path for the returning fluid unhampered by a downward jet, This design failed to relate the effect of nozzle diameter and jet impact pressure distribution on the cuttings removal process.

A new two-nozzle design was proposed which incorporates unequal nozzle diameters to focus hydraulic energy on the hole bottom in a pattern which improves bottom-hole cleaning, It has been field-tested with consistent success in hundreds of bit runs in northern, central and eastern Alberta, and in over 100 bit runs in the Australian Outback.

The field test summarized in this paper was conducted in the Valhalla Field of Alberta, 50 km northwest of Grande Prairie (Fig. 1). The reader should be cautioned that the study population is small, and that the tests were conducted to minimize, but could not eliminate all the variables which could impact on the findings. These constraints prevent the author from establishing conclusive proof of the effectiveness of focussed hydraulic energy. The author intends only to provide sufficient evidence to encourage thoughtful discussion.

Discussion

Literature Review

Laboratory bottom-hole pressure surveys by Sutko and Myers(1) demonstrated that as the number of nozzles on a tri-cone bit is progressively reduced from three to two to one, the maximum impact pressure under the remaining jets is increased. Concurrently, the minimum bottom-hole pessure is reduced.