A Pragmatic Approach to Injection Management of a Large Water Flood in The Largest Oil Field in Oman

Abstract

Abstract The Yibal Oil Field in the Shuaiba Formation in North Oman is the largest oil filed in Oman, contributing about 15% of the country’s production. The heavily faulted and fractured 70 km2 carbonate anticlinal structure was brought on stream in 1969 initially under depletion drive. This was followed by oil-leg injection directly into the oil column in 1972, then later by acquifer injection in 1980. Since then, more vertical infill drilling was carried out and almost all oil-leg injectors were converted to acquifer injectors with additional new injectors. The associated oil-gain is found empirically to be about 10% to 15% of the injected water (typically, 1000 m3/d injected water can result in about 120 m3/d oil gain). However, the injected water volumes varies substantially from injector to injector through out the field, giving rise to uneven water injection distribution which ranges from 50 m3/d on the low side to about 4500 m3/d on the high side. Furthermore, the subsurface water movement exhibit various mechanisms such as bottom water rise, bottom water coning, fault plane channeling, fault plane line drive, radial flooding, fingering and edge water encroachment. The intense horizontal drilling campaign in 1993 and improved higher off-take horizontal producers after simulation studies in 1997 resulted in direct impact on localised pressure drop and early water break-through. In addition, injected water was seen to have direct impact on the performance of some producers but not others combined with the difficulty of optimising the injectivity in some low-pressure areas due to surface or subsurface constraints. Thus, the need to develop a more pragmatic approach is innevitable. This paper discusses the development of injection management methodology of the very wide Yibal Water flood. In particular, the discussion focuses on the improvements in the water injection scheme to sustain the field with more than 300 producing wells (horizontal and vertical) coupled with intense drilling campaign (about 50 wells per year) in the last four years. The paper will address the in-house developed tools in addition to the associated voidage and pressure mapping to manage the oil displacement in this complicated mature field. With the aid of these tools, areas with anomalies in the static pressure and voidage maps are identified and stability is re-established through combined off-take control, water shut-off, closing high water-cut or gas-oil-ratio wells, converting watered-out producers to injectors, optimising existing injectors or drilling new injectors.