Evaluating the Performance of Multi-Lateral Producing Wells: Cost Benefits and Potential Risks
Authors: Smith S.J. , Tweedie A.A.P. , Gallivan J.D. 
Published: December 2007

Abstract
Advances in drilling and completion techniques have allowed drainage of reservoirs via tile use of multi-lateral well arrangements in a variety of configurations. The most important aspects to consider regarding the application of multi-lateral wells are the potential cost benefits and potential risks of utilising the technology. Difficulties exist in predicting the performance of multi-lateral well arrangements. Many of the performance prediction difficulties are being addressed via a current project sponsored by several North Sea operating companies. Recent work on the project has focussed upon the application of multi-lateral wells in fields operated by the sponsor companies. The particular fields studied were selected to have a good spread of generic characteristics. Examples include a thin oil rim with a large overlying gas cap and underlying aquifer, a heavy oil reservoir, a layered system, a gas condensate reservoir, and a reservoir containing volatile oil. This paper is based on the results of these real field studies and indicates how analytical techniques, coupled to reservoir simulation methods, have been applied in order to predict the productivity and drainage efficiency of multi-lateral well arrangements. The paper concludes with an overall assessment of the potential economic benefits of multi-lateral wells in a range of generic reservoir types commonly encountered in the North Sea. An element of risk assessment, incorporating both reservoir and completion risk, has also been included, in order to provide a realistic evaluation of multi-lateral performance.

Introduction
The desire within the petroleum industry to improve profitability through cost reduction and the advent of advanced drilling and completion technology have combined to raise the awareness of multi-lateral technology. Multilateral wells are defined as wells having one or more branches (laterals) tied back to a mother wellbore which conveys fluid to or from surface. The technology is not new, the first wells were drilled in the former Soviet Union in the 1950's. These however encompassed simplistic completions unlike the complex completion configurations on offer at the present time. In some cases the completion system is able to offer full hydraulic isolation at the junction point coupled with selective re-entry capability for each individual lateral.
Recent successes in the USA, Western Canada and the North Sea have provided increased publicity for multi-lateral technology with an increasing number of companies offering a wide variety of drilling and completion systems. The current enthusiasm for multi-lateral wells is often based on perception of the associated benefits and not on a rigorous evaluation. The decision to drill a multi-lateral well must, like any other project, make good economic sense. The analysis preceding this decision must incorporate robust cost models, accurate prediction of well performance, and a realistic risk assessment evaluation.
To help arrive at the drilling decision, a certain process must be followed. Asset screening should first occur to assess the applicability of well types for the particular project in question. Performance prediction must be evaluated and can be done via an integration of numerical and analytical techniques. Risk assessment should be carried out, in terms of both reservoir risk and completion risk. Finally the economics of the project must be studied in order to quantify the viability of multi-lateral wells. This paper aims to present the logical progression of the decision-making process, in particular focussing on recent evaluation exercises carried out using real field data sets.