Reservoir Compaction Well Design for the Ekofisk Field
Authors: Schwall G.H. , Slack M.W. , Kaiser T.M.V. 
Published: December 2007

Reservoir Compaction Well Design for the Ekofisk Field G.H. Schwall, SPE, Phillips Petroleum Company Norway, M.W. Slack and T.M.V. Kaiser, Center For Engineering Research Inc. (C-FER) Copyright 1996, Society of Petroleum Engineers, Inc.

Compaction strains in porous chalk reservoirs can be many times larger than the elastic strain capacity of well liners. Even completions with high plastic strain capacity allow deformations impairing well access. This paper describes a new elastic well design, tolerant of high compaction strain (on the order of 10%), enabling completions with reduced risk and extended life. Supporting the new design are new failure criteria, test results and specifications for slip devices to control axial loads, and an assessment of the drag load parameters governing slip device implementation. A new friction-based approach to drag load (cement bond) is detailed, including physical and numerical testing components.

Introduction

The Ekofisk Field is in the Norwegian sector of the North Sea. The reservoir consists of multi-layered, high porosity chalk that is nominally 1000 feet in total thickness and lies at a vertical depth of 10,000 feet. High compaction strains in some chalk layers have led to reservoir casing deformations. Surface subsidence and associated overburden formation movements have resulted in overburden casing deformations. Overburden deformations involving a loss of production casing pressure integrity have impacted operations. Successful mitigation measures through remedial operations or alternative completion techniques have been implemented and are ongoing.

The consequences of reservoir casing deformation on operations have been relatively minor; negligible impact on production rates and restricted access below deformations. Future wellbore access is of greater value, partially due to continuing waterflood operations. Access allows remedial zonal isolation measures, addition of perforations, selective restimulation, and logging to characterize production profiles. These activities are essential for optimum reservoir management.

Since discovery in 1969, the field has produced more than 1290 MM bbl oil and 4640 BSCF natural gas. Production from Ekofisk will continue for approximately 30 years, facilitated in part by redevelopment of the field. The redevelopment, titled Ekofisk II, includes the installation of a 50 slot wellhead platform (2/4X) in late 1996. The 2/4X installation presents a unique opportunity for examining the completion of conventional directional wells.

Reviews of industry experience, Ekofisk field evidence and studies, and the identification of future completion requirements led to the conception of a design proposal for the Ekofisk II conventional wells. To validate the proposal, a detailed analytical modeling and physical testing program was devised and carried out through joint efforts by Phillips Norway and the independent research contractor, C-FER.

The validation efforts consisted of; parametric analysis of the proposed completion with and without slip devices to investigate the effectiveness and the range of applicable operating conditions; chalk compaction modeling to evaluate nonuniform loading effects and frictional drag loads at various well inclinations; full scale experiments on pipe samples with different coatings to determine casing-cement shear and frictional behavior as a function of normal or confining stress; and full scale laboratory experiments to investigate practical concerns regarding the use of mechanical slip devices.

Design Premise

Industry Experience. Deformation of casings and liners installed opposite compacting formations is documented. At Wilmington 40 ft long casing joints have been measured to be as much as one foot shorter using precision collar counting equipment. Kennedy reported widespread casing failures in the Bolivar Coastal Fields of Venezuela.

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