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.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractUltra-deepwater drilling activity is at an all time high. Recently, there were only two rigs rated to, and actually working in, water depths greater than 6,000 ft. Soon there will be over twenty. This increase will involve new deepwater operators and bring a large number of new people to deepwater. This paper presents some of the problems that have occurred in deepwater operations under the assumption that an understanding of what can go wrong is the best way to avoid problems. It will identify these problems and discuss the critical topics of rig positioning, environmental considerations, BOP control, riser management, well control, wellbore construction, production problems, and personnel training and safety.

The Mad Dog field is located in Southern Green Canyon approximately 150-miles south of New Orleans, in 4400-ft to 6800-ft of water (Figure 1). Mad Dog wells are sub-salt and are drilled to the Lower Miocene at approximately 19000-ft true vertical depth (tvd). The discovery well for the field was drilled in 1998. Drilling in the field has been challenged by "mobile" tar. Tar has been encountered in other deepwater fields; however, other reported tar deposits have not had the mobility of the tar encountered in the Mad Dog area. Extensive work has been conducted in developing the Mad Dog tar mitigation plan and has included characterization of tar, laboratory plunger experiments with Mad Dog tar, tar shoe design optimization, investigation of solvents and catalysts to solidify tar, investigation of downhole pyrotechnic heating, investigation of coating agents and the review of drilling practices. This paper will document the theory on the origin of the tar deposits, steps taken to deal with the tar and experience with the tar. Introduction The Mad Dog spar platform was installed in Green Canyon Block 782 for producing the Mad Dog Field in 2005. To date, two grass roots wells have been drilled from the spar. Drilling operations have been conducted in the central part of the field which encompasses a graben structure, Figure 2. Mobile tar first became evident on the second appraisal well in the central part of the field. (Figure 3). The tar did not substantially affect drilling operations however it did result in lost time due to stuck wireline tools. One of the most severe mobile tar cases occurred when a Southern Flank exploration well encountered tar and was unable to meet the required total depth objective. Another severe mobile tar case occurred in the final pre-drill (pre-Spar) well drilled in the central graben. The well was side-tracked, encountered more tar and met its objective as a less-than-optimal borehole size due to the addition of several casing strings. Development of a detailed tar mitigation plan during installation of the spar rig proved fruitful as the first spar well also encountered tar and managed to deliver the required total depth and completion.

The advantages of a dual gradient mud column have been well documented. Significant work has been done on the riserless drilling systems by several different companies. This paper will propose two methods to achieve the same dual gradient.Nitrogen Injection: Building from proven air drilling procedures and underbalanced techniques nitrogen can be used to cut the mud weight back in the riser above the seafloor or the cut can be made deeper by combination with a concentric riser.Floating Mud Cap: A dual activity rig can use a casing riser linked to the adjacent marine drilling riser. A submersible pump in the casing riser regulates returns that will set the mud cap level in the drilling riser. A pressure sensor in the subsea BOP allows monitoring the effective hydrostatic pressure. This approach again combines field proven procedures and hardware in the deepwater environment. Both methods will be presented with emphasis on the hardware and operational procedures required to successfully implement them. Both of the methods employ existing equipment and procedures. Introduction One of the major challenges of drilling in deepwater in the Gulf of Mexico is due to low fracture gradients and shallow abnormal pressure. The margin between the fracture gradient and required mud is often less than 1 ppg.In young formations, fracture pressures are almost equal to the overburden1,2,3. Because much of the overburden is only due to the weight of the seawater the fracture pressures are only slightly greater than ‘normal pressure’. Unfortunately, there is not a corresponding drop in deepwater pore pressures due to the immaturity of the deposits typically encountered. The water confined in the rapidly deposited clay is in part pressurized by the weight of the overlying sediments trying to wring the water out, which creates a narrow margin between the pore pressures and fracture pressures. As a result the pore pressure and fracture gradients curves almost lie atop each other. A dual gradient drilling approach will result in the effective mud weight at the previous casing shoe being less than the effective mud weight at the drilling depth. The industry's effort for riserless drilling4 is designed to take advantage of this benefit. If casing points are not limited by the mud weight of the previous casing shoe, it is possible to eliminate casing points from the well program. Figure 1 shows a common goal of the dual gradient drilling. Previous efforts only brought the mud weight back to sea water equivalent at the seabed. Even better results can be achieved by cutting the mud to less than seawater at the seabed or at the shoe. By doing this it is possible to find a combination of the two mud weights that cause the heavier mud gradient to always lie between the pore pressure and fracture pressure lines. The pressures may be underbalanced inside the cased hole or riser but the open hole is overbalanced. This can be achieved by reducing the density in the upper mud section to less than seawater or by adjusting the cut point to below the seabed. This paper proposes to achieve the dual gradient by two methods. Both of which use existing equipment and employ known drilling techniques. The first proposal is nitrogen injection where normal gasification drilling methods are applied to the riser section. 1) Nitrogen injection option This paper will outline how nitrogen injected subsea can effectively create a dual gradient by gas lifting the mud in the riser. Another operator had considered a similar approach, but had proposed to sweep the entire 21" marine drilling riser with gas. This paper proposes to combine nitrogen injection with a high pressure concentric casing riser5. This reduces greatly the gas required to cut the mud. Figure 2 illustrates an example of the nitrogen being injected into a subsea BOP stack.

A 1999 SPE/IADC paper (52782) identified and documented solutions to a number of drilling problems encountered in Ultra-deepwater drilling. Since that time the industry has pushed the water depth record beyond 10,000' of water and drilling depths below 32,000'. A number of new problems have occurred in the last 8 years that have been caused by mechanical failures (equipment stressed to its limits) or human error. In the Gulf of Mexico recent drilling has encountered problems drilling salt and tar that the industry had not previously experienced. Three operators active in deepwater GOM have collaborated on this paper to document problems under the assumption that understanding what can go wrong is the best way to avoid problems. Introduction In 1999 the water depth drilling record was 7,520'. There were 56 rigs advertised as being capable of drilling in greater than 5,000' of water1. The water depth record is now 10,011' 2 and there are 74 rigs capable of drilling in greater than 5,000' of water3. In the 1990s there were 148 wells drilled in the Gulf of Mexico in greater than 4,000' of water. By the 2000s (mid 2006) there have been 524 wells drilled in greater than 4,000' of water. The authors' companies have drilled 291 of these wells in the Gulf of Mexico since 2000. The number of wells being drilled in deepwater has increased. The water depth, total depth and complexity of those wells drilled in deepwater GOM has also increased. The depth record is now 34,189' MD4.