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Abstract

This investigation was focused on studying the complex problem of stability of multi-branch horizontal openholes in a complete three-dimensional anisotropic in-situ stress field simulating drilling, completion, and production conditions.

Previous work on this subject consisted of parametric analyses placing the junction in the overburden. Whenever reservoir management requires the construction of a specific multilateral scenario such as a multibranch horizontal inside a producing formation, a complex analysis is required. In this work, stress-displacement analysis in steady-state was coupled with transient phenomena to compute stress behaviors and changes in pore pressure due to disturbances created during simulated drilling and production conditions. This coupled analysis allowed for the inclusion of time dependent non-linear processes that influence behavior of the system compounded by rock, fluids contained in the rock, and in-situ stress field.

Introduction

Several authors1,2,3 have addressed the topic of the variables that deeply impact the construction and production performance of multilateral wells. In general, they agree that placement of the junction of the lateral, lateral length, stability of the junction and the branches, and pressure gradients strongly affect the effectiveness of the drilling, completion, and deliverability of multilateral wells.

Design of placement of the junction and laterals must take into account stability, orientation of the lateral branches, and wellpath design, among other critera. It has been documented that dogleg severities during construction of the wellbore impair the effectiveness of the completion systems¹; for this reason, installation of completion and production equipment must also be taken into account for planning and design.

Drilling and production problems often result from mechanical failure of wellbores. Mechanical stability of the wellbore depends on the magnitude and orientation of in-situ stress field, rock strength, fluids in the wellbore and those contained in formation, and the orientation of the wellbore itself. Junction stability problems increase as the number of junctions increase¹. Collapse of the junction is one of the two major causes of multilateral failure. Despite this well recognized problem it is not common finding information regarding the negative aspects of costly failure related to high technology applications, such as multilateral technology. In this respect, Brister4 presents a review of the problems encountered in the drilling and completion of a multilateral well in East Wilmington Field in California. He concluded that six critical problem areas were detected: hole cleaning, cementing and junction stability, whipstock orientation, tubular size, re-entry, and risk management. Thus, the importance of pointing out and studying stability behavior of junctions in different multilateral scenarios becomes a very important issue in planning and design.