The global oil and gas industry is being challenged to increase production to meet the rising world energy demand. One of the key areas now being explored and developed to meet this demand are reserves below massive salt formations.
To reach these reserves, it is necessary to drill through and case off the salt. Casing and cementing operations in such salt zones can pose particular challenges, ranging from the effect of salt dissolution on cement-slurry properties, to the potential dangers presented by salt creep to the integrity of the well and the need to plan for contingencies for potential zones of overpressure or lost circulation. This paper examines and explores the challenges inherent with cementing across salt zones in the global arena.
The current best practices for cementing casing strings across salt zones in some of the major subsalt basins of the world, including the North Sea and the Gulf of Mexico, are detailed and discussed.
This work should help assist those tasked with the construction of wells that have to penetrate significant salt formations to achieve their objectives safely and reliably.
Since the late 1940s, casing strings have been cemented across significant salt zones when wells were completed through salt domes in the Gulf of Mexico (Slagle and Smith 1963). In more recent times, the industry has identified enormous new oil and gas reserves below large salt sections in areas ranging from the Gulf of Mexico, Brazil, West Africa, the North Sea, and eastern Canada. Some of the reserves recently found below massive salt sections rank among the largest hydrocarbon finds in the world. In November 2007, for example, Brazil's Petrobras announced that the Tupi subsalt discovery in the Santos basin held recoverable reserves of up to 8 billion bbl of oil.
The construction of wells through large sections of salt formation can present operators with many challenges; a number of which are associated with the casing cementing process. This paper explores the issues associated with cementing casing strings across extensive salt sections and the solutions to these challenges that have been used with success in different areas around the world. Some key cementing technologies and methodologies are also highlighted.
The peculiar challenges associated with cementing operations in massive salt zones can be generally grouped into three major areas:
Formation and wellbore fluid interaction
Salt Movement—"Creep" and "Flow"
Massive salts can be mobile; they can move over time. The rate at which this movement will occur is largely dependent on three primary parameters: temperature, differential stress, and salt type or salt composition. The movement of the salt formation can be described as either salt "flow" or salt "creep." When considering salt movement, it is important to recognize the difference between these two types of movement.