How to Choose the Best Tool in the New Toolbox of Reservoir Engineers: Complex Well Architectures
Authors: Delamaide E. , Renard G. , Kalaydjian F. , Lobey O. , Duquerroix J.P. 
Published: December 2007

Abstract
The recent developments in drilling technology now provide reservoir engineers with several new well architectures for the production of oil and gas fields. Horizontal wells are becoming the rule rather than the exception and may in a not so far future be replaced by more complex well geometries. These include the various types of multi-lateral wells: either drilled from the same vertical section (multi-stacked or vertical multi-laterals) or from the same horizontal section ("fishbone-type" wells), as well as snaky wells. Although completing such wells may still remain the main issue in some cases, the problem now seems more to determine the most adapted strategy in terms of well design and completion than to actually drill the well.
The advantages of the multi-drain wells are enormous: increased drainage areas, reduced drilling costs per well for the same length drilled in the reservoir, and in swamp or offshore environments, reduced number of pads or slots. On the other hand, they may be restricted in terms of completion and may suffer from increased formation damage. They may not be the best suited to each type of reservoir and their use requires careful analysis.
This paper addresses these issues and presents a screening of well architectures, depending on the reservoir and oil characteristics as well as the recovery methods to be used. In each case, the advantages and drawbacks of the various architectures are considered on a reservoir engineering standpoint and drilling/completion aspects are reviewed. The synergy between complex architectures and EOR (thermal, chemical) is specially emphasized.

Introduction
Since the end of the eighties, horizontal drilling has become a major tool in the toolbox of reservoir engineers. Horizontal wells have to date been used widely worldwide, especially in Canada, in a number of situations. Their most frequent uses are: reduction of water and/or gas coning, production of thin reservoirs, production of fractured reservoir by well trajectories intercepting fractures, or simply improvement of productivity in cases where vertical wells would not be economic (for production of viscous oil for instance). The increase in production compared to vertical wells is usually much larger than the increase in cost incurred for the drilling of the horizontal section. In fact, the advances of drilling and completion techniques have been such in the past few years that horizontal wells have become the rule rather than the exception.
The beginning of the nineties has seen the apparition of a new generation of more complex types of wells, the multilateral wells. They basically consist of horizontal drains drilled from the same vertical or horizontal borehole. The economic interest of these wells is obvious, especially for offshore operations, because of the reduction of the number of slots required to drain a given portion of the reservoir.
An increasing number of such wells is now being drilled, for instance in the Austin Chalks, and research has started on the forecast of their performances such as their Productivity Index (PI). However to date there has been few clear attempts to compare the new advanced well architectures to horizontal wells. There is yet no way to select the optimum well for a given type of formation and heterogeneity. It is the purpose of this paper to provide some answers to these questions, by using a simplified statistical approach.
The work focuses on heterogeneous reservoirs because they are the most common case and also because analytical methods cannot provide answers in this type of formation. For a reservoir characterized by a given heterogeneity, the PI of various well architectures are compared to determine the best adapted architecture. The study's objective is more to demonstrate the usefulness of the methodology than to provide definite ranking between the production strategies.
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