Solving the Salt Challenge: Unique Drill Bit Philosophy Delivers Breakthrough Performance in the Gulf of Mexico
Authors: Barton Steven Paul , Weeden Ryan William , Mensa-Wilmot Graham , Hajardi Yoseph 
Published: July 2010

Salt formations present unique drilling challenges relative to other formation types of comparable compressive strength and abrasivity. Due to its unique properties, many detrimental issues can occur when employing bit designs which are not optimized for salt. These issues include severe stick-slip, lateral vibration, and poor directional control. In addition to high rig rates, Gulf of Mexico (GOM) projects typically use expensive BHA's. Consequently, improved drilling performance is seen as one of the enablers that can help reduce operational costs. Many considerations must be made relating to the design techniques employed in the development of Fixed Cutter (FC) bits for drilling salt intervals.

This paper will focus on various design techniques developed for drilling a wide variety of salt sections in the GOM. Aspects such as load balancing, unique blade geometry and materials, gauge design, and utilization of secondary components will be covered. In addition, cutting structure design (cutter layout aimed at achieving specific performance objectives), as well as the analysis and adaptation of bit profile for improved performance will also be discussed. In summary, the paper will focus on a novel design philosophy that was developed and incorporated into an ultra-large diameter (26") FC drill bit. This concept was successfully utilized to extend the capability of drilling the 26" salt section with Rotary Steerable Systems (RSS) in the GOM. This was achieved while significantly improving penetration rates and lowering operating cost.

Case studies are presented from the GOM that demonstrate true solutions to drilling salt in deepwater applications with FC drill bits.

Introduction

There are three primary forms of salt:Halite, or rock salt - Formed in either thin or massive layers. Occurs as a relatively soft white rock, with red or yellow coloring caused by impuritiesAnhydrite - This occurs extensively in beds associated with halite deposits. It is harder and less soluble than halite, and harder and denser than gypsum. It often occurs as fibrous, granular, or more compact masses.Gypsum - This is basically the hydrated version of anhydrite. It generally occurs in beds associated with layers of halite and dolomite.

Salt is notably less dense than other sedimentary rocks. As a result, salt deposits tend to flow under pressure, as compared to either folding or faulting which occurs with lithologies such as sandstones and shales. Salt mobility occurs due to the difference in density between the salt and any surrounding sedimentary formations. Due to the lower specific gravity of the salt, it will tend to move upwards, in a similar manner as a lighter fluid would rise through an overlaying denser fluid. The actual rate of movement is mainly affected by temperature. This plastic flow (or deformation) allows salt to concentrate into large, domed-shaped masses1, where it can help to form oil traps. Salt deposits are found in a number of locations around the world, including Colorado and Utah (in the United States), Mexico, Spain, United Kingdom, Russia, Saudi Arabia, India, and the Gulf Coast region of the U.S. The latter is the focus of this paper.

Extensive subsurface salt structures exist throughout the Lower Tertiary trend2 within the deepwater area of the GOM. These structures range from 2000 to 12000' in thickness and have trapped considerable amounts of hydrocarbons. The high purity of the salt in this region, combined with relatively low sub-surface temperatures, has resulted in slow-creeping salt bodies.