Casing Ultradeep, Ultralong Salt Sections in Deep Water

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 114273, "Casing Ultradeep, Ultralong Salt Sections in Deep Water: A Case Study for Failure Diagnosis and Risk Mitigation in Record-Depth Well," by Jincai Zhang, SPE, Shell Oil Company; William Standifird, SPE, Knowledge Systems; and Chris Lenamond, SPE, Nexen Petroleum U.S.A., originally prepared for the 2008 SPE Annual Technical Conference and Exhibition, Denver, 21-24 September. The paper has not been peer reviewed.

The deepest well drilled to date in the Gulf of Mexico (GOM), 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, the salt-creep mechanism and its implications for well design, and mitigation options for future well construction.

Introduction

Subsalt and near-salt formations are among the most attractive exploration prospects in many operating areas including the 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: (1) “creeping” salt masses can exert catastrophic stresses on casing, and (2) in areas near the salt/rock interface, salt movement can create unstable rubble zones that can make drilling difficult or impossible.

One of the key challenges for well construction in subsalt environments is maintaining casing integrity. Salt-induced casing collapse has occurred in a number of GOM wells, including Well 1. Well 1 was drilled in 2005 and, at the time of construction, was the deepest well in the GOM. The well was drilled through a salt body more than 15,000 ft thick and eventually reached a total depth of more than 28,000 ft true vertical depth. The casing collapsed near the base of the salt nearly 3 months after the salt section was cased. The wellbore was abandoned and was sidetracked successfully to reach 34,189 ft.

The objective of this study was to characterize the salt-creep mechanism and its implications for casing failure in Well 1 and to develop options for well design to avoid future failure events. Well 1 experienced drilling-fluid losses and hole ballooning in the subsalt formations. Given the extreme depths, fluid losses and hole ballooning could jeopardize the feasibility of future projects in the area.

Analysis of the geomechanical environment reveals many challenges; northern-GOM salt bodies are among the most varied and complex in the world. Some are active; some are passive. On a local geologic scale, the dynamics of salt creep are dependent on such factors as temperature, salt mineralogy, and overburden.