ConocoPhillips' Magnolia project will utilize a Tension Leg Platform (TLP) in a record water depth of 4674 feet (1425 m) in Garden Banks block 783 in the Gulf of Mexico. The wells will be predrilled and then completed with a platform rig. Production is expected from unconsolidated, fine grained and over-pressured Pleistocene and Pliocene formations. Reservoir compaction was identified as a drive mechanism for the field. Rock mechanics studies indicated that reservoir depletion in the low-strength and high-porosity reservoir rocks could result in significant reservoir compaction due to initial shear failure followed by pore collapse. This paper will describe our analysis of such rock failure mechanisms and projected strains, which served as the basis for deriving estimates for reservoir compaction and stretch of overlying strata.
The team faced a unique challenge to develop a casing design that would address reservoir compaction concerns. By itself, casing design was only one part to a solution that involved developing a comprehensive well construction and completion strategy that could deal with the reservoir compaction problems. The cost of casing failure would have significant negative impact on project economics.
A concerted effort was undertaken to study the geology and the rock characteristics of the formations in this field. The resulting casing design strategy conforms with, and in some aspects improves upon, ConocoPhillips' current casing design philosophy. This paper discusses two of the major design factors: 1) the effect of tension on the casing in the overburden above the reservoir and 2) the effect of compression on casing in the reservoir. Specific design criteria were developed for each of these factors. The field was sub-divided into different reservoir compartments, then the maximum stretch and compressive strain that is expected in each compartment was calculated for the reservoir and overburden zones.
The selected design strategy was applied to the casing design with a view to optimizing wellbore life and project economics of each well. The result is a unified approach to compaction that begins in geomechanical analysis and ends with the financial decision process.