The Shuaiba Formation at Saih Rawl, a tight carbonate reservoir in Central Oman, was not fully exploited after discovery for over 20 years with only three vertical wells producing at marginal rates. Exclusive use of horizontal and multilateral well technology has given the field a new life resulting in immediate full field development. A well lay out was chosen to accomplish an ideal line drive. This waterflood project involves the injection of produced water through multilateral horizontal injectors to provide pressure support. Two or three parallel legs are placed below the OWC to achieve bottom water sweep. The oil is produced through single or dual horizontal wells drilled along top reservoir equally spaced between the injectors at a maximum oil column height of 32 m. The wells are drilled at a spacing of 125 m lateral and some 30 m vertical distance between producers and injectors. Individual laterals are more than 2 km long. As a result of this development plan ultimate recovery is expected to rise tenfold from 3.9 (depletion only) to 39 mln m3.
The Saih Rawl field, situated in the Ghaba Salt Basin, was discovered in 1971. The deep but prolific Permian clastic reservoirs were brought on stream in 1975. Oil and associated gas are being produced from the Shuaiba, Mafraq, and Haushi reservoirs. The Shuaiba reservoir contains light, undersaturated oil (API 37 degrees) with an in-situ oil viscosity of 1.9 cP, an initial solution gas-oil ratio of 19 (m3/m3) and a bubble point pressure of 5,170 kPa. Maximum oil column in the reservoir is 32 m.
Production from the Shuaiba reservoir commenced in 1984 by vertical producer SR-20, followed by two additional vertical wells. The first successful horizontal producer, a sidetrack, was completed in 1991, and up to now (April 1996) 32 horizontal wells including five dual lateral injectors have been drilled. The horizontal producers have improved top structure definition significantly. Wells are drilled along the roof of the reservoir and thereby provide structural control points along the well path. Since start of horizontal drilling the STOIIP-estimate has doubled; largely as a result of the structural data provided by the horizontals.
Initial well rates from long horizontals were up to 750 m3/d. However, severe pressure depletion was observed in the area of the high productivity horizontal producers. A horizontal trial injector was drilled in 1994 to test the feasibility of water injection. Good reservoir response was observed and reservoir performance was successfully history matched in a simulation model. As a result a line drive waterflood development plan was formulated.
The current net offtake from the reservoir is approaching 5000 m3/d, with about equal amounts of water being injected. Present watercut is between 25 and 30%. At this early stage of the water flooding project, Saih Rawl production water is supplemented by import from the Qarn Alam production station (over a distance of about 40 km) with the advantage of reducing water disposal requirements in this area. The produced water from deeper reservoirs in Saih Rawl alone is insufficient to make up the injection requirements in the short term. The water is de-oiled and injected into the Saih Rawl Shuaiba reservoir. A future plateau production of 7000 m3/d is envisaged once upgraded facilities have been installed.