Challenges of Directional Drilling Through Salt in Deepwater Gulf of Mexico

Abstract

As an increasing number of 6000-ft plus deepwater developments come on stream in the Gulf of Mexico (GoM), project economics dictate that fewer subsurface drill centers be used to develop these fields. This in turn requires longer step-out wells, pushing kickoff points higher up the wellbore, often occurring within extensive salt bodies. Salt drilling is still a relatively new practice and presents operators with many drilling challenges that are still not totally understood. Adding a directional component to drilling through salt not only magnifies the issues of traditional salt drilling, but introduces new challenges that require different approaches to ensure successful delivery. This paper will discuss the challenges faced and the lessons learned by two major deepwater GoM operators along with the directional service company in drilling directionally through the salt. Together, these companies have drilled over 100,000-ft of salt in the GoM and are considered pioneers in deepwater salt drilling. They have encountered and managed many of the challenges that extend past the traditional predrill and real-time directional issues, into the post drilling phase with issues such as casing and cement design for managing salt loading and ensuring long term wellbore viability. This paper presents several case studies that investigate and discuss directional drilling through salt, comparing variables such as hole size, bottomhole assembly (BHA) configuration, under-reamer selection, wellbore trajectory and directional control. The importance of geomechanics in the predrill planning of these directional salt wells is also discussed, and its link to casing design and cementing issues will be examined. The paper concludes by identifying critical areas for success in drilling directionally through salt, and will attempt to identify current technical drilling limits for pushing this envelope even further. Introduction The Gulf of Mexico (GoM) is well known for its extensive subsurface salt structures that have aided in trapping much of the hydrocarbons found here. Figure 1 illustrates the extent of the salt coverage, in relation to multiple deepwater discovery wells. As deepwater exploration successes progress into the development drilling phase, and with the increasingly recognized potential of the GoM's Lower Tertiary trend, (much of the 33,000sq mi trend is covered by a thick salt canopy, Figure 2) the requirement for deepwater wells to penetrate salt have become almost mandatory. More information on GoM salt coverage is presented in the OCS Report 2007–021 Deepwater Gulf of Mexico (2007). Over the next several years, successful and efficient drilling of salt will play an increasingly major role in achieving many of the area's deepwater drilling objectives. In order to meet this challenge, the ability to directionally drill through salt and to understand and manage the issues this introduces will be a key factor for deepwater operators. This paper will explore in further detail, the drivers for directional drilling in salt, the challenges that it introduces and will discuss the enabling and emerging technology required to execute this relatively new aspect of deepwater drilling. Three case studies from different deepwater GoM operators will be reviewed, and the lessons and recommendations derived from these presented. The paper will draw upon these and other GoM salt drilling experiences to formulate a comprehensive package of the requirements for the planning and drilling directionally through salt.

Drilling Deep in Deep Water: What It Takes to Drill Past 30,000 ft

“…If the formations are interbedded, it may mean that a secondary service such as real-time shock and vibration monitoring should be investigated. The best place to start any analysis is the offset data where available (Israel et al, 2008).…”

“…As deepwater exploration successes progress into the development drilling phase, and with the increasingly recognized potential of the GoM's Lower Tertiary trend (much of the 33,000 sq mi trend is covered by a thick salt canopy; see Fig. 14), the requirement for deepwater wells to penetrate salt has become almost mandatory (Interim Report of 2006Highlights, 2007Israel et al, 2008).…”

“…Some of the items for consideration include: poor subsalt imaging, geomechanical risks, pore pressure uncertainties, directional drilling through the salt, salt entry strategy, salt exit strategy, inclusions, salt creep, tar, and drilling vibrations. The risks and challenges of drilling through salt in the Gulf of Mexico are extensively covered by Israel et al (2008), but a summary of the typical concerns and management strategies for each of these is presented below.…”

Efficient and Reliable Vertical Drilling of Top Holes with RSS in Deepwater GOM

“…In drilling riserless, ROP is typically not an issue in the soft sediment just below the mudline, but once salt is encountered, bit selection and BHA design become crucial to being able to drill at an optimum ROP. As drilling salt in the GOM is almost mandatory when the target is the Lower Tertiary reservoirs, it is sometimes preferred to drill salt in the riserless tophole as salt provides fracture strength of up to 3 ppg greater equivalent than the sediments above (Israel et al 2008). However, drilling salt with large PDC bits and mud motors results in an inefficient ROP due to the high torque generated by the interaction of the bit with the highly reactive formation and the insufficient torque delivered by the motor.…”

“…The more salt that must be drilled, the greater the build and walk potential (Israel et al 2008). Therefore, RSS, with their continuous steering system, have been used to minimize unwanted high doglegs that would result in casing wear and higher torque and drag issues by well total depth (TD).…”