Thin Oil Columns: Most People Think Horizontal Wells, Obagi Field Case Suggests the Contrary

Abstract

Abstract The OBAGI Field is situated onshore NIGERIA. Among the numerous productive layers, one is a thin 36 ft thick disk-shaped oil column. This unconsolidated deltaic sand is produced, making the best possible use of the natural geological features to possible use of the natural geological features to limit gas and water coning. Permeability is 2 darcys and bottom hole oil viscosity 0.8 cp. In that area, such an oil disk is considered as almost impossible to produce economically, because of immediate coning problems. In fact, in this type of sand, it is our experience that shaly streaks are always present. If they can be accurately located it is always possible to position the perforations so that they are adequately protected from early invasion by water and gas. Some wells can produce up to 3500 bbl/day, that is 10 times more than the theoritical critical rate derived from Darcy's law in a supposed homogeneous sand. This shows that although shale interbeds cannot be correlated from one well to the other, their abundance is such that as one disappears laterally, another one develops not far above or below it this results in a low vertical permeability and limits coming. Shale lamination is difficult to pick up on the logs, and even the Gamma ray can be misleading as a shale indicator. Fortunately, some other tools have a good vertical resolution and proved very useful in detecting shale streaks. An interesting feature of this field is that a horizontal well, mow expected by some people in the industry to be the best way to produce a thin layer of unconsolidated sand with a coning problem, would probably not be as successful in OBAGI because of probably not be as successful in OBAGI because of the low vertical permeability. In fact, ELF NIGERIA has already recovered 21 × 10 bbl of oil (that is 19 % of the original oil in place), and expects to obtain a final recovery of 30%. Introduction The OBAGI Field is situated onshore NIGERIA. The reservoir of interest (Level IX) is at depth of 2500m (8200 ft). It is a 75m (246 ft) thick sand with a large gas-cap, an 11m (36 ft) oil column, and an active aquifer. The sand is barely consolidated. A pilot well was put into production at the end of 1980, after which development continued in successive stages. By mid 1989, there were 17 wells and development is considered to be virtually complete. On 30 June, 1989, cumulative oil production was 3.3 × 10(6) m3 (21 × 10(6) STD bbl) i.e. 19% of the original oil in place. Reservoir offtake at this date, was 1500 m3/day. The GOR is still moderate (150 m3/m3 compared to a solution gas ratio of 95 m3/m3). The BSW is about 30 %. Recovery is mainly by aquifer drive and a final recovery of about 30% is expected. P. 297