A Unified Model for Shale Barriers

Abstract

AbstractCement as barrier material have weaknesses because of inherent shrinkage and lack of robustness over time. In recent years shale barriers have become a likely alternative to cement in many instances. Shale is a natural material with an indefinite lifetime. There are, however, many aspects around using shale as well barriers, related for example to activation and qualification.This paper presents a unified approach from an operational perspective. Firstly the relevant models for shale barrier formation are established. Then the loading required to establish a shale barrier will be defined both from a well pressure perspective and a thermal heating perspective. A mechanistic elastic-plastic model for shale deformation will be derived and coupled to the wellbore situation. Hydration of the shale also help establishing the barrier.Several field cases are presented starting at defining potential shale barrier intervals from UCS logs coupled with an elastoplastic model to define barrier extent. The result is presented as a log showing radius to virgin formation. High MSE and drillability is another parameter identifying weak rocks. A drillability log will also be shown and correlated with the shale barrier extent from the UCS. Finally, after casing is set high end cement bond logs are used to confirm the barriers. The paper presents the field application, defining the optimal log and wellbore pressure conditions to establish an effective shale barrier.