A New Sealant System for Multilateral Junction Geometries
Authors: Xenakis Harry , Vijn Pieter , Covington Rick 
Published: December 2007

This paper describes the design, development and application of a new sealant system for multilateral well junctions. The sealant system has been successfully applied in offshore multilateral well installations for North Sea operators.

In recent years, the oilfield industry has shown an increasing interest in multilateral wells. Multilateral wells are an economically viable solution for many offshore operators because multilateral wells increase reservoir drainage and extend the wellhead slot coverage on offshore platforms. Multilateral wells can range from simple openhole completions to sophisticated cased-hole completions that have selective access to all laterals. In contrast to the openhole completions and slotted-liner completions, cased-hole completions enable the operator to have more control over production because cased-hole completions allow re-entry for perforating and production logging operations. However, zonal isolation is difficult to achieve at a multilateral junction where the hydraulic seal between the formation and the casing interior is maintained by the sealant only. The sealant system must be resistant to many destructive forces during completion and production.

Mathematical models were used to quantify stresses that are exerted on the sealant during completion and operational processes in a multilateral well. A procedure was established to scale the stresses to laboratory conditions. Based on the procedures, it became apparent that the multilateral sealant required high elasticity and high impact resistance, as well as standard oilfield cementing properties.

This paper presents how the sealant system was specifically designed and developed to meet the requirements of a cased-hole multilateral completions. Field jobs in which the sealant system was successfully placed are presented. These jobs show that the sealant system withstood stresses at the junction in North Sea multilateral completions.

Introduction

Multilateral wells have recently been developed to increase production. Multilateral wells can be vertical or deviated (including horizontal) wellbores connected to one or more subordinate laterals. Drilling and completion equipment have been developed that allow multiple laterals to be drilled from a main cased and cemented wellbore. Each of the lateral wellbores can include a cemented liner that is connected to the principal wellbore (Figure 1).

Multilateral wells have been successfully drilled and operated; however, one operational problem involves zonal isolation of the multilateral junction. The casing and liners are cemented in the principal and lateral wellbores, respectively, by introducing cement slurries in the annular clearances between the walls of the wellbores and the casing and liners. In the past, conventional well cement slurries were used. Although cement is a strong material, it cannot withstand repetitive impacts and stresses that occur during drilling, milling and other well operations in the laterals. Once the set cement is shattered, it may allow leakage of fluid through some portions of the wellbores.

Improved methods of cementing multilateral wells were necessary. A sealant research project was initiated to determine testing methods and fluid compositions. A number of test methods were used including API fluid testing, impact resistance tests and chemical resistance tests. The multilateral sealant required high elasticity and impact resistance. Therefore, a highly elastic sealant and a brittle sealant were chosen as extremes to guide the development of the multilateral sealant. Elastomer-based slurries were chosen for testing because they are known to provide high elasticity to set materials.

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