Salt Drilling While Underreaming Behaviors Illuminated by Distributed Downhole Measurements

Abstract

Many developments in the deepwater Gulf of Mexico target reservoirs are below salt sections. As well plans increasingly call for drilling through long salt sections; a common strategy in the Gulf of Mexico is to drill while simultaneously opening the hole with an under-reaming tool. This configuration presents unique dynamic behaviors to the drilling system. Understanding the drilling system is important in order to reduce vibrations and to meet desired run lengths. In the Gulf of Mexico dynamics have limited the effectiveness and life of hole opening tools. Therefore, the identification of drillstring dynamic behavior is critical to efficiently executing well plans that call for under-reaming while drilling. To study the vibration phenomenon unique to under-reaming while drilling bottomhole assembly dynamics through salt sections the operator employed a unique distributed downhole measurement system which included vibration sensors. The goals of this study were to evaluate the accountability of the real-time MWD and surface data, to illuminate complex dynamics and provide further stimulus for the development and adoption of real-time strategies. The distributed measurement system generated insights in all three areas. This paper presents examples of specific dynamic behaviors from several deepwater Gulf of Mexico wells, focusing on the drilling phenomenon occurring in salt. Key observations about the development of vibrations while opening the reamer, drilling, drilling transitions and back reaming are given. Attenuation of vibration to the surface is demonstrated. Predrill modeling of bottomhole assemblies was also cross-validated. The impact of cutting structure effectiveness and bottomhole assembly "neutral point management" is also highlighted with examples. Introduction Deepwater Gulf of Mexico wells often require drilling through the salt canopy. (see Figure 1.) In some plays, subsalt targets equate to the majority of the well being placed in salt. Similarly subsalt targets mean the wells may be very deep and therefore benefit from a large hole size to provide adequate clearances for deepening casing strings. Maintaining hole verticality is also important to limit casing wear due to hang down weight effects. Casing running in salt presents the added challenge of salt creep, often requiring oversized hole. (Zhang et al. 2008) A successful method to address the challenges of drilling deep wells through salt is simultaneous underreaming while drilling. The use of logging while drilling and rotary steering tools introduces constraints to underreamer placement and over all bottomhole assembly (BHA) design that may limit drilling performance. Chevron's (the operator) current developments in the deepwater Gulf of Mexico are successfully using rotary steerable tools and simultaneous underreaming. Distributed measurement in drilling assemblies is an established method of investigation for the operator; however this study is the first of its kind known for the deepwater Gulf of Mexico. (Marland et al. 2004) Many directional measurement schemes employ measurement at several locations. Distributed vibration measurement is often achievable in a BHA using multiple LWD tools with vibration logging capability. This ability is often marginalized or ignored as realtime telemetry and memory data prioritize formation evaluation data. Vibration measurement complements avoidance of destructive vibration modes by supplying symptomatic information about the downhole dynamic conditions. When vibrations are high, anecdotal evidence is used to diagnose a dynamic condition. Based on that diagnosis one may attempt to mitigate the phenomenon.

Dual RFID Activated Reamers Reduce Vibration in Deepwater Well

“…Because of the lack of permeability of the salt, the rate of penetration (ROP) in this interval could be maximized without monitoring the equivalent circulating density (ECD) for fracture breakdown pressure tolerance (Kull and Duff et al 2009). Later, the operator re-entered the well and sidetracked out of 14-in.…”