Challenges of Underbalanced Drilling for West Africa
Authors: Oyeneyin M. Babs 
Published: December 2007

Abstract
Economic viability of the challenging deep-water environment of the offshore Niger Delta requires the depletion of the reservoirs with a minimum number of wells. To achieve this, there is the increasing trend towards the use of long horizontal and extended reach wells. Also to improve production, recovery and drill safely, underbalanced drilling and techniques need to be considered by operators as being a key part of future developments in the deep water environments which are susceptible to high pressure-high temperature, have very narrow pore pressure-fracture pressure window. The geology is generally characterised by high secondary porosity/permeability clastic reservoirs, many of which are faulted, friable and unconsolidated. These formations can be severely damaged by whole mud, filtrate and mud solid invasion penetrating beyond the range of conventional stimulation treatments when conventional overbalance drilling techniques are used. These concerns require optimum ECD[static + dynamic pressure] management to minimise severe losses, pipe sticking, among other expensive problems which makes UBD an attractive option. Additionally, the UBD is useful for picking TD especially from production rate and first water influx during the drilling process.
However, the technology is still seen as a costly niche technology because it is relatively more expensive than conventional drilling. To make it an attractive option, the higher expense must be offset by sustainable improvements in production capacity.
In order to justify UBD as a viable option during well planning therefore, it is crucial to be able to quantify the potential Return-On-Investment (ROI). Although there have been reported evidence of some productivity improvement during UBD the key questions however are: What is actually responsible for the productivity improvement? Is this phenomenon limited to certain reservoirs and why? If so, what are the reservoirs’ screening criteria? Is the productivity improvement transient and limited to the drilling time or will it be sustained beyond that? What is the payback time?
In this paper attempts have been made to present the key criteria for the applications of underbalanced drilling with focus on offshore West Africa. This is followed by the special design considerations necessary to mitigate the problems associated with UBD in order to maximise its efficiency and promote improved production. The paper is rounded off with key guidelines on the UBD of horizontal wells for optimum reservoir performance and minimum operational and hole problems. The guidelines presented are based on the results of the in-depth analysis of the UBD process validated with field data particularly on the multiphase wellbore hydraulics and the effect of the depressurisation whilst drilling on the reservoir quality index [RQI] and overall productivity. Example data from specific case studies have been used to illustrate how the concept can be used to screen UBD candidates and define a safe operating window during planning and actual drilling operation. This work is part of an on-going programme on the development of a comprehensive strategy for underbalanced drilling being carried out at RGU in collaboration with industry.