Controlled Flow-Drilling Induced by Gas Lift - an Economic Approach to the Improvement of Productivity in Offshore Scenarios
Authors: Vargas Angela C. , Rodrigano Roberto 
Published: October 2008

Abstract
By analyzing case histories of wells drilled successfully using conventional Underbalanced Technology (UBD), it can be observed that in many cases it is difficult to determine the short term productivity gains achieved. This often results in incomplete analysis of the drilling costs and results and budget considerations may lead to the conclusion that application of UBD in future wells may be only a marginally effective technique for development of a specific field. In general this is due to only the increased cost of the drilling phase being considered without factoring in the actual benefits of incremental productivity in the long term. Long term economic analysis is not the focus of this paper, however it is strongly recommended that it should be taken into account when reviewing the potential benefits of UBD technology in field development. Consideration of drilling costs offset against long term production returns is certainly appropriate. With prevention of formation damage and productivity improvements as the goals, decreasing the differential pressure window may result in decreasing the benefits that UBD can provide. A solution to this dilemma is the application of Controlled Flow Drilling (CFD). This allows the well to be drilled in the optimum differential window with a cheaper lightweight drilling fluid, the reduced hydrostatic head of which is induced by the use of natural gas injection. Gas injection is continued until the formation starts to flow, at which point CFD can continue under the wells own energy. In this paper the authors will provide basic guidelines for designing this type of drilling condition. In particular they will present the case for the use of CFD induced by gas lift as an economically viable approach to obtaining improvements in productivity in offshore environments, where space for equipment is at a premium.

Introduction
Underbalanced Technology is one of the proven emerging technologies that can be applied to obtain higher reservoir recovery. This can de done while avoiding pollution and environmental impact in surface shallow aquifers caused by lost circulation. It can also be used to achieve economic optimization of the drilling performance when applied to hard or consolidated formations. Underbalanced operations provide an enhanced safety environment, specially while drilling unknown or heterogeneous reservoirs, due to the surface returns control and design that allows drilling to continue even when producing zone influx is encountered. The track record of UBD with regard to safety is excellent due in part to the increased awareness induced in all drilling personnel when drilling in this manner. All of these reasons provide sufficient grounds for the operating company to at least evaluate this technique before deciding to drill in a conventional manner and thereby perhaps lose productivity, or create the need for additional remedial work in order to maximize the potential of the well.
Some of the main objectives that have allowed the application of CFD to be a reservoir success and an economical operation are summarized in this paper. When planning a UBD operation in mature offshore fields, it is important to consider the facts that the infrastructure, platform and pipeline systems are in place and have been amortized and that production is declining.