Mitigating Salt and Sub-Salt Drilling Challenges Using Hybrid Bit Technology in Deepwater, Gulf of Mexico

Abstract

Abstract The deepwater Gulf of Mexico (DW GOM) poses a variety of drilling challenges including subsurface challenges related to drilling through salt, such as salt creep, wellbore instability, inclusions, sutures, and tar that add risks to drilling. The sub-salt challenges include drilling through interbedded formation triggering drillstring vibration, hard and abrasive sandstone and pressure transition. In most exploratory areas where operators are currently active, the water depth is greater than 4,000 ft. As a result, long drill strings are required which increase the likelihood for drilling dynamic issues. Torsional oscillations, lateral vibrations, and whirl pose the risk of bottom hole assembly (BHA) failure, resulting in an unplanned trip. Drilling through interbedded formations of variable hardness often initiates dynamic instability. If not managed, instability shortens the BHA run life and adversely impacts the success of the operation and exposes rig personnel to additional safety risks. The majority of deepwater drilling applications require the use of an expandable hole opener in the BHA. Drilling through an interbedded formation with an expandable hole opener increases the possibility of the pilot bit and reamer losing synchronization, causing the reamer to overload, which can result in a premature failure and unplanned trip. Alternatively, having an aggressive hole opener in the BHA with an unaggressive pilot bit can result in whirl and the resultant loss of drilling efficiency or eventual failure. Therefore, it is important to use a bit with managed aggressiveness to ensure the bit and reamers remain synchronized. The use of polycrystalline diamond compact (PDC) bits often aggravates the drilling dynamic instability because of inherent shearing action and resultant downhole torque fluctuations. The instability compromises drilling efficiency and makes it difficult to synchronize the expandable reamer and pilot bit. In this case study, the operator addressed the challenges of drilling dynamics, expandable reamer and pilot bit matching issues by utilizing a hybrid drill bit in the 18⅛" x 21" hole sections. The hybrid bit, because of its dual-cutting mechanics, addresses drilling dynamics and reamer synchronization challenges through better torque management and controlled aggressiveness. This paper presents the successful application of a hybrid bit to address drilling challenges and improve efficiency and penetration rates. It also compares the downhole torque produced by PDC and hybrid bits and demonstrates consistent, linear torque response of a hybrid bit for managing aggressiveness.