Openhole Horizontal Completions in Niger Delta

Abstract

The Niger Delta is a highly activity region in open hole applications. A wide variety of completion types have been deployed in open hole in the region. A major challenge in these completions is to deliver quality wells through the choice of an optimal sand control system. A locally developed database tracks the performance of the sand face completions, their reliability, observed failures and their causes. The oil reservoirs have used barefoot, standalone screens, slotted liners, gravel packs and expandables while pre-drilled liners have been used for borehole support. The expandable technology is relatively the most recent technique but of the three hundred and forty run so far globally as at September 2005, more than 3/4 has been deployed in Niger Delta alone. For the non-associated gas reservoirs, most of which have been conventional wells, based on fair understanding of the risks and uncertainties associated with two main sand control options namely, gravel packing and the expandable sand screens, work has been done to show adequate demonstration of the process of selection. This structured approach considers design options based on several factors such as possible problem with under-reaming relatively deep sections, previous performance in the existing high rate gas wells, productivity and better life cycle design perspectives. Within the Darcy permeability regime of the sandstone gas reservoirs, productivity with respect to horizontal versus conventional completion was compared and a matrix capturing the risks impact of a horizontal completion is attempted. The work concludes by suggesting future concerns in open hole horizontal sand control to include cost of installation failure, accurate modelling upfront and remedial zone isolation after gas/water breakthough. Introduction Conventional wells have been applied to drain reservoirs in Niger Delta extensively. In recent years, horizontal wells have started to gain acceptance as a proven reservoir management and well completion method. The major purpose of a horizontal well is to enhance reservoir contact and thereby enhance well productivity. Within the Niger Delta environment, horizontal well completions have been widely used with success. Productivity improvement factors (compared to conventional wells) of two or higher is not uncommon. Even in recovery by waterflooding, driving a water front between two horizontal wells in a thin reservoir may enhance oil recovery when compared to a waterflood between vertical wells.[6] Field development plans for most Niger Delta reservoirs that target thin oil rims overlain by gas and underlain by an active aquifer, therefore now also rely almost entirely on horizontal wells. Vertical wells are not suited in this situation due to the rapid coning of water or gas at reasonable production rates.[2] Horizontal wells, correctly placed in the oil column lead to high flow rates with very little drawdown. The small drawdown delays premature breakdown of water or gas at flow rates much higher than those possible from conventional vertical wells. Multilateral wells drilled in the region have been mainly horizontal in the drainhole sections. The purpose of this paper is to review sand control systems in horizontal open hole applications in the region. The Niger Delta is truly a high activity region in the area and experience gained should provide a helpful baseline in terms of expenditure and well productivity.