Overcoming a Difficult Salt Drilling Environment in the Gulf of Mexico: A Case Study
- Crispin Chatar (Schlumberger) | Mark D. Imler (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- IADC/SPE Drilling Conference and Exhibition, 2-4 February, New Orleans, Louisiana, USA
- Publication Date
- Document Type
- Conference Paper
- 2010. IADC/SPE Drilling Conference and Exhibition
- 5.3.4 Integration of geomechanics in models, 1.6.6 Directional Drilling, 1.10 Drilling Equipment, 5.1.2 Faults and Fracture Characterisation, 1.12.6 Drilling Data Management and Standards, 5.1.1 Exploration, Development, Structural Geology, 1.6 Drilling Operations, 4.1.5 Processing Equipment, 1.2.2 Drilling Optimisation, 1.6.1 Drilling Operation Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.2.2 Geomechanics, 4.1.2 Separation and Treating, 2 Well Completion, 1.6.3 Drilling Optimisation, 4.3.4 Scale, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.7.5 Well Control, 2.4.3 Sand/Solids Control, 1.6.1 Drilling Operation Management
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More than a decade ago few companies intentionally penetrated and successfully drilled the large salt sections in the Gulf of Mexico (GoM). Problems common to drilling in salt led to excessive nonproductive time (NPT): hole instability, stuck pipe, tools lost in hole, among others. Although techniques are now available that enable successful drilling of many of these large salt structures, there still are some formations with characteristics that make drilling challenging. This paper focuses on one such ultradeep subsalt prospect and the strategies used to overcome the key issues of nonhomogeneous salt formations, shock and vibration, and salt inclusions. Application of new and emerging technologies and techniques for drilling in similar environments focused on drilling parameter optimization, bottomhole assembly (BHA) tendencies, and stuck-pipe analysis. This paper presents comparisons of the results expected prior to optimization measures and the actual results.
The subject of this case study is an ultradeep subsalt prospect in the GoM at a water depth of 5,765 ft. The suprasalt section is 2,080 ft thick; below that is a salt section 17,593 ft thick, then the subsalt section, which is 7,700 ft thick. The well was drilled directionally with its kickoff point in the salt section.
The rig used was a fifth-generation semisubmersible with a capacity to drill to depths of 37, 500 ft. Salt formations currently being drilled in the GoM are theorized to have originated from the Louann salts, originally deposited during the Jurassic period (Israel et al. 2008). Over time, the sedimentation process has resulted in the characteristic salt structures seen today including nappes, domes, sheets, and canopies. This salt is now referred to as allochthonous salt, meaning salt that has migrated from the original place of deposition. There is strong evidence that the salt penetrated while drilling in any one field may actually be the coalescence of various salt bodies that simply appear as one image on the seismic data.
Salt has a relatively low unconfined compressive strength (UCS), ranging from 3,000 to 5,000 psi (Israel et al 2008; Willson and Fredrich 2005). However, significantly higher weight on bit (WOB) and torque are required when drilling salt than when drilling other formations with similar UCS. This need for additional energy is attributed to the plastic nature of the salt, and the ability of the salt to creep into a newly drilled wellbore presents some of the biggest challenges to drillers. Salt's ability to creep and deform is one of its unique and problematic characteristics. An explanation for this characteristic is that, unlike other clastic sediments, salt does not increase in density with depth of burial. As layers of sediment are deposited over salt and begin to compact, a critical depth is reached where the density of the salt equals the density of the overlying overburden, at which point the salt begins to flow (Barker et al. 1994). The rate at which salt moves depends on burial depth, formation temperature, mineralogical composition, water content, impurities such as clay, and the extent to which differential stresses are applied to the salt body (Israel et al. 2008; Fossum and Fredrich 2002).
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