Improved Hydraulics For Rock Bits
Authors: Cholet Henri , Crausse Rolland 
Published: December 2007

A new bit design, in which two nozzles of a three-cone bit are extended downward and the third nozzle is directed upward, provides better cleaning at the bit's face, resulting in increased penetration rates mad longer bit life. Results of laboratory research and field tests in different areas and formations are discussed.

Introduction

Drilling rate increases of up to 20%; plus longer bit runs can result in substantial savings, especially for deep wells in which large amounts of tripping time are required to run in a new bit. Extensive research on bit hydraulics has led to a new bit design that can consistently produce performances and increases of at least 15% and often more.

During the past 25 years, most improvements in rock bits have focused on teeth, metallurgical treatments, sealed bearings, and jet technology. Little effort has been made to achieve better bottom-hole scavenging, a major factor affecting drilling at great depths.

Therefore, some research has to be done. Garner and Millar have presented a review in rotary drilling bit development. Maurer gives R.W.N. relationships as a function of cleaning. Feenstra and Van Leeuwen have studied the effect of lets on bit penetration. Crown has developed a theory to explain the cutting action. Sifferman et al. have done research on drill cutting transport. Sutko et ad. have studied efficiency when using two nozzles instead of three, and especially when the nozzle angle is increased.

New drilling bits with increased performance have been designed and tested as part of research sponsored by ARTEP (Association de Recherche sur les Techniques d'Exploitation du Petrole), composed of Compagnie France des Petroles, Societe National Elf-Aquitaine, and institut Francais du Petrole (IFP). SMF international contributed to this research within the framework of an agreement ARTEP.

By applying new hydraulic concepts, checked out initially on scale models, particularly by means of ultrahigh-velocity motion pictures, IFP's drilling lab designed and subsequently field tested a new type of bit (Fig. 1) to compare its performances with standard bits.

Efficient bottom-hole and bit scavenging must accomplish at least three functions:washing at the mud-rock interface,controlling crossflow and preventing the recycling of cuttings, andfast preventing the recycling of cuttings, andfast lifting of cuttings around the bit.

This research is very important for large-diameter rock bits because annular velocities eve always low.

Laboratory Research

Laboratory research concentrated on two points: capacity for stripping off cuttings, and capacity for scavenging out cuttings.

Capacity for Stripping Off Cutting

The cuttings formed by bit teeth are subjected to mud pressure that holds them in place (Fig. 2). Therefore, a certain amount of energy, E, must be available to strip off the cuttings as a function of the square of the velocity:

Since it is possible in the laboratory to reproduce flow rates of the same order of magnitude as those used in actual drilling, tests were performed with model bits and, for each geometric and hydraulic parameter, a tentative similarity relationship between parameter, a tentative similarity relationship between the model and the actual bit was established. Similarity rules, based on observation of the flow pattern defined by the Reynolds number, led the pattern defined by the Reynolds number, led the laboratory investigation of parameters in Fig. 3.