No 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.

No abstract

Even in routine applications, the safety, economic and technical problems associated with lost circulation can severely impact drilling operations. The negative consequences are magnified greatly in the deepwater environment. Dramatic reduction in penetration rate and the downtime spent regaining circulation can further escalate already high operating costs. More importantly, the well control issues surrounding lost circulation pose critical safety concerns. In many of these wells, merely identifying the thief zone or zones is a major technical challenge. Furthermore, once identified, the vugs and fractures in the loss zones are at times too large to be bridged with conventional lost circulation material. This paper describes the development of a uniquely engineered lost circulation pill that when used in tandem with new real-time geomechanical analysis methods and pill formulation software, cured severe losses in the deepwater Gulf of Mexico. The authors will describe the development and laboratory modeling used in producing the specialized and chemically activated cross-linked pills, engineered to stop whole drilling fluid losses. As detailed in the paper, the pill proved to be far superior to conventional lost circulation material. Further, the paper will discuss its application in a well drilled in more than 2,800 ft of water in the Gulf of Mexico, which had encountered losses of up to 2,000 bbl of synthetic-base drilling fluid. The 100-bbl pill was formulated using a specialized software package. This was used in combination with a new process for analyzing the location, and extension pressure of the drilling induced hydraulic fracture using resistivity and annular pressure measurements. Once the pill was placed across the fracture zones, squeezed and later drilled out, normal drilling resumed with pre-fracture parameters resulting in penetration rates in excess of 50 ft/hr with no further losses. In addition to discussing the development and application, the authors will outline the lessons learned on the deepwater project featuring pre-planning issues geared toward ensuring circulation is maintained throughout the wellbore. Introduction The challenge of controlling fluid losses when drilling with synthetic-based drilling fluid systems in deepwater has been well documented1,2,3. The narrow operating window between pore pressure and fracture gradient often results in major fluid losses when drilling, running casing, and cementing. While drilling with synthetic-based mud delivers superior performance, the cost implications of massive losses often have forced operators to solve the problem rather than living with the consequences1. A technical and economic challenge associated with synthetic-based fluids is the fact losses are more difficult to cure, because of the tendency of fractures not to heal once closed. Consequently, the low fracture re-opening pressures allow for continual losses. Many products and techniques have been used in attempts to restore circulation while drilling. These include fibrous, flaky and granular materials2, as well as techniques such as gunk and reverse gunk squeezes, high fluid-loss squeezes and cement squeezes. More recently, a chemically activated cross-linked pill (CACP) has been developed and applied on challenging deepwater wells to cure and limit losses when drilling with synthetic-based fluids.

No abstract