Comparison of Multilateral Completion Scenarios and Their Application
Authors: Hogg Cliff 
Published: December 2007

The evolution of multilateral technology has created a large variation in completion styles for multilateral wells. Many of the methods are simple and basic while others are much more complex. These complex completions allow the use of multilaterals in a much wider range of well scenarios but they also create a new array of obstacles, concerns and risks. This paper will outline and categorize the various multilateral options that have emerged and the advantages and disadvantages of each. In addition, the reservoir and production parameters that influence the final multilateral completion choice will be discussed.

Introduction

Multilateral wells in their most simple form have been utilized in the oil and gas industry since the 1950's. These early multilateral systems, however, were only suitable in their application to a small segment of wells. Fortunately, over the last few years, drilling and completion techniques have improved to the point that an ever increasing variety of wells can now be completed as a multilateral. The challenge is becoming a question not of whether a multilateral system is available, but rather a question of what type of multilateral, if any, is best suited to the reservoir and production demands.

When considering a multilateral completion, five parameters merit particular attention:

Reservoir Suitability. The goal of the multilateral system is to maximize production from the reservoir with a minimum increase in drilling and completion costs. This requirement can be satisfied in one of two ways:The multilateral can be constructed with all production bores located in a single producing formation. This allows an optimized drainage pattern, greater fracture exposure, and a decreased probability of water or gas coning due to drawdown.The multilateral can be completed with the production bores located in separate producing formations. This allows marginal formations to be produced that otherwise could not be economically completed.

In most cases, a multilateral well will cost more to construct than a single vertical or horizontal bore. Economic benefits will be derived primarily from increased production and/or reserves. To ensure such benefits, it is vitally important to have a thorough knowledge and understanding of the reservoir mechanics, and to use that knowledge and understanding to design multilateral completions from the reservoir up.

Formation Characteristics at the Lateral Bore Kick-Off Junctions. As with conventional wells, the wellbore stability must be considered when choosing whether or not to case the hole. In addition, with a multilateral system, the geology at the junction of the lateral bores must also be closely scrutinized. The most flexible multilateral completions are those designed with the junction kick-off point located in a strong, competent, consolidated formation. However, if geology or other downhole conditions preclude this ideal scenario, mechanical support and, perhaps, hydraulic isolation must be included as part of the completion design. Differential Pressure at the Junction. Even if the lateral junction is initially competent, the completion design must take into consideration how the formation will respond as the well is produced and pressure drawdown occurs. It is not enough to just provide support during the initial few months of the well production; multilaterals must be designed for the life of the well. If the junction formation cannot retain its integrity as pressure drawdown occurs, hydraulic isolation of the junction may need to be considered. Production Mechanics. Production mechanics, as well as regulatory and environmental requirements, exert strong influence on multilateral completion design, particularly as regards zonal isolation. Any of these factors, either individually or in combination, may necessitate isolated, dual-string production to surface when the production is from multiple reservoirs. On the other hand, casing and tubular sizing and uphole equipment needs often dictate that production be commingled at the lateral junction and produced up a single string.

P. 215^