Abstract

Many enhancements have recently been made to multilateral completion systems, not only as a result of advances in technology but also because of changes in developmental concepts that have been instrumental in achieving many of the system successes. Although numerous multilateral wells have been drilled, particularly in the Austin Chalk, until now completion methods have been limited in providing features to manage borehole stability, flow control, and well maintenance.

This paper will discuss several examples of the advances in drilling technology that have made these multilateral well development strategies possible. Information will also be provided on several simple multilateral well completion techniques as well as a multilateral system that will 1) mechanically connect the lateral liner to the parent casing, 2) allow through tubing re-entry access to selected laterals, and 3) seal the junction between the lateral and parent bore so that flow control capability can be maintained. In the latest system developed, the hole size of the lateral has been maximized in comparison to the parent wellbore casing, and full bore access to the lateral liner drift ID without restriction has been provided. Information will be presented concerning the project methods undertaken to develop this system, the bench tests that were performed for the components of the system and results of the full scale system test conducted. Finally, case histories of the completion experiences of the two wells in which the new system has been successfully implemented as well as several earlier successful completions using other systems will be presented. Other field installations in progress will also be discussed.

An "integrated development" approach with a multi-disciplinary design team and direct producing company involvement was used in the development of the full featured multilateral completion system and proved to be an instrumental factor in the achievement of its success.

Introduction

Multilateral oil and gas well concepts have been in existence since the early 1900's, but advances in directional and horizontal drilling have recently made this reservoir development approach more feasible. A primary factor in the increased interest in multilateral completion technology has been its enhancing impact on the economics of field development. Multilateral systems can reduce field development costs by:

  1. Providing more productivity per well with more zone exposure feasible in each well.

  2. Reducing slots required to reach a given number of formation targets, thereby reducing platform size, weight, cost and surface footprint.

  3. Reducing total footage drilled per foot of zone exposed, thereby reducing drilling time and expense.

  4. Reducing cuttings and mud disposal costs and surface and intermediate casing expense through the use of a common parent borehole from which multiple laterals are drilled.

In some Austin Chalk area wells, the changes from vertical well development to horizontal and then to multilateral systems has lead to reductions of developmental costs from $12/barrel of oil equivalent (BOE) to $5.75/BOE to $4.65/BOE respectively. Similarly, some North Sea operators see reductions in development capital expense of 23% and 44% respectively when horizontal and multilateral approaches are compared to vertical well-development schemes. With such potential, the drivers for multilateral technology are certainly clear. These key reductions in developmental cost structures can increase economic feasibility of producible reserves in marginal fields, fringe areas or compartmentalized reservoirs, lenticular reservoirs, vertically segregated thin sands, attic oil accumulations, etc. (See Fig. 1). Reservoir applications of multilateral wells have been addressed in several papers, and specific opportunities continue to be studied.

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