Abstract

The deepest well drilled to date in the Gulf of Mexico, the Well #1 discovery well in Green Canyon, required setting casing in a 15,000 ft thick section of tectonically active salt. After casing collapsed in the initial wellbore, a comprehensive model was developed to characterize wellbore stability, salt creep mechanism and implications for well design, and mitigation options for future well construction.

To facilitate the construction of models characterizing the interaction of casing and salt, the regional pore pressure and insitu stress setting was analyzed. This analysis revealed that pore pressure in this area shows a significant regression in the subsalt formations. This pore pressure regression, if not accurately predicted, can lead to significant problems with the wellbore including wellbore breakouts and drilling fluid losses. The combined challenges of open-hole wellbore instability, salt creep, and casing damage made well design and construction very difficult.

In this paper, models are utilized to replicate wellbore instability, salt creep and the interaction of halokinesis, wellbore geometry, casing stress, and drilling fluid hydrodynamics. The modeling reveals that casing failure was caused primarily by non-uniform contact of salt on casing thereby generating stresses that exceeded the yield strength of the lightweight casing. The same model showed that heavier casing could withstand identical, non-uniform stresses if loading was diametric and not axial. The model was used to prepare an optimized drilling program that mitigates the risk of non-uniform application of stresses by salt and non-uniform application of slip loading. Recommendations included under-reaming in the slip zone, the use of drilling fluid of appropriate composition, and cementing practices that improve the stress distribution on the casing. Further, after setting casing through the salt, relatively high mud weights were required to avoid a large differential between internal and external loading on the casing. High mud weight had implications for wellbore stability in open-hole while drilling the subsalt sections to reach the reservoir. Modeling predicted accurate in-situ stresses and pore pressures that were utilized to drill a bypass well that successfully reached an unprecedented depth of 34,189 ft.

Both analytical and 3-D elasto-plastic finite element methods (FEM) were applied to analyze casing failure mechanisms, coupling the effects of in-situ stresses and mud pressures with the overburden. The methods were also applied to analyze mineralogic parameters that influence salt creep. The modeling process is applicable to essentially all Gulf of Mexico extended-reach wells to be drilled through salt sections.

Introduction

Subsalt and near-salt formations are among the most attractive exploration prospects in many operating areas including the Gulf of Mexico (GoM), offshore West Africa, Brazil, the Southern North Sea, Egypt, and the Middle East.

One of the characteristic features of the northern GoM salt trend is that the salt bodies are highly mobile. High mobility has two significant implications for wells drilled through salt: "creeping" salt masses can exert catastrophic stresses on casing; and in areas near the salt/rock interface, salt movement can create unstable rubble zones that can make drilling difficult or impossible.

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