Horizontal drilling of multilateral wells within the last five years has rapidly developed the Saih Rawl Shuaiba reservoir. These wells consist of up to 7 branches, and expose more than 10 km openhole of reservoir per well. Current production of the reservoir is around 8,000 m3/d of oil with an average watercut of 75%. A "produce-the-ultimate"-initiative was followed to systematically analyze, debottleneck and optimize every detail of the production chain: from water injection manifold, water injectors, to oil producers with ESP, including the waterflood efficiency. This has resulted in a range of optimization activities such as ESP changeout, flowline looping, water injector tubing resizing, automation of chokes and an improved water injection distribution system. Initial gains from part of these activities amount to more than 1,000 m3 of oil per day within the first six months. Introduction The Saih Rawl oilfield situated in Central Oman, was discovered in 1971 by SR-2 and came on stream in 1975 (see figure 1). The field is oil-bearing in Shuaiba, Mafraq, Gharif and Al Khlata reservoirs, while gas and condensate are produced from the deeper Amin and Barik intervals. Production started from the most productive Gharif reservoirs. Attempts to produce the Shuaiba reservoir with vertical wells were made from 1984 onwards with three drainholes (SR-16, 20 and 22). However, the tight nature of the reservoir and the thin oil column underlain by water have jeopardized this approach, due to early water production and low initial rates. The development using the conventional vertical well design of that time was not economical. The current Shuaiba reservoir development has started with bare-foot horizontal multilateral wells from mid-1993 onwards. Due to very limited aquifer support, water injection was implemented from end-1994 with horizontal, multilateral barefoot injection wells located below the OWC. To date 50 production and 20 injection wells have been drilled at lateral spacing from initially 250 m to now 60 m. This adds up to more than 172 km of producing and 104 km of injecting drainhole. The current typical well design is a 4 to 7-legged multilateral well with a cased 7"-backbone and 6 1/8" openhole legs. Production wells are completed with ESP and deliver between 1,000 to 3,000 m3/d gross. Apart from drilling new wells, development activities have included side-tracking single horizontal to multilateral producers, restimulating existing wells, isolating a water-cycling leg (in injector well) and converting gas-lifted wells to ESP. The current oil production is around 8,000 m3/d, for a total gross rate of 35,000 m3/d; this represents a field water-cut of 75%. Pressure support is performed at voidage replacement and the reservoir pressure is currently balanced at around 9,000 – 10,000 kPa. Today almost 11 Million m3 of oil have been produced. Currently 38.5 Million m3 are booked as ultimate recovery out of a STOIIP of around 90 Million m3. All Saih Rawl Shuaiba production is routed to the Saih Rawl production station, where it is commingled with gross production from a number of other reservoirs and fields prior to dehydration. The net oil is exported via the Qarn Alam Main Pumping station into the Main Oil Line to the terminal at the coast. The separated gas is compressed; some is recycled as gaslift, the balance being exported. All produced water is currently re-injected into the Shuaiba reservoir, together with additional water from local water supply wells, as required. The separation and dehydration facilities at Saih Rawl Production Station are going to be expanded during the third development phase in Q3-2001. This will result in a "no constraints" facility.

The Shuaiba limestone reservoir of the Saih Rawl field (Oman) is a nearly unfaulted, low-relief structure of large areal extent. Permeability is matrix-dominated and relatively low (1 to 10 mD). The reservoir contains approximately 90 106 m3 of light oil (35 API) in column heights of typically 15 to 30 m. Economical production only started in the early 1990's, 20 years after field discovery, when horizontal well technology could be effectively introduced. Today, multi-lateral wells of up to 7 legs are used in a pattern water-flood development. Total open-hole lengths drilled in the reservoir have reached 11 km for a single well. To date, 166 km of producing and 107 km of injecting open-hole have been drilled, representing 167 horizontal legs, and yielding an oil production level of up to 9,000 m3/d (60,000 bbl/d). The original producer to injector leg spacing of 250 m has gradually been decreased to 60 m and still fits the economic criteria. In 2000–2001, a field development review was carried out to identify further development opportunities, both for the short and long term. A series of new reservoir management tools were put in place to predict the response of the reservoir to various development options. A pilot area was defined in order to test these concepts and perform extensive data gathering. Finally, new lab and feasibility studies were initiated. As a result from this review, a portfolio of short and medium term activities was identified, including infill drilling to 40 m spacing, and drilling additional injection legs in existing injectors. Also, upsides for developing the flank areas of the field (sub-15 m oil columns) were identified. Later, fishbone legs drilling and reversing water injection into existing producers will be piloted, while sweep optimization methods such as water shut-off and re-stimulation are envisaged to target recovery factors in excess of 50%. Introduction The Saih Rawl field, situated in the Ghaba Salt Basin of Central Oman, was discovered in 1971 and came on stream in 1975. The field is hydrocarbon bearing in several formations. The most productive intervals, the Gharif and Al Khlata sandstones, were developed first. The gas in deeper Barik and Amin formations was only put on stream over the past few years. The first attempts to produce the shallower Shuaiba carbonate reservoir date from 1984. It is not until 10 years later, however, that this formation started producing to its full potential. This paper presents the history of a development that owed its success to horizontal and multilateral well technology, to geosteering of producer legs at the top of the reservoir, to pattern water-flooding of a low-permeability matrix, and to a systematic approach at maximizing production offtake, while mitigating risks. A field development review of the Shuaiba reservoir was recently carried out, to gather the learning and experience of the past. It was also time to put in place the tools and methodology necessary to fully manage a reservoir, where already 70 wells had been drilled, representing more than 270 km of open-hole. This review could identify further attractive activities, as well as improvements and upsides in the existing approach. This paper covers this more recent analysis and its implications for the future of the field. The Multilateral Waterflood Concept The Shuaiba reservoir is a simple, salt induced, low relief anticline elongated roughly N-S, and covering a surface of 40km2. It is truncated on its southern side by a NW-SE trending fault (Figure 1). The formation is a relatively homogeneous wackestone, of 26% average porosity. It contains light (35 API) under-saturated oil of 1.9 cP initial viscosity. The bubble point is approximately 10,000 kPa below initial pressure. Vertical depth is around 1,400 m. Fractures are present in the reservoir, as detected on FMI/FMS logs acquired from 13 horizontal wells. However, fractures seem to have no detectable influence apart from productivity improvements (negative skin): no early water breakthrough is observed in horizontal producers, despite the under-laying aquifer, thin oil columns (15 to 30m) and low permeability matrix (1 to 10mD). The long term productivity and injectivity of drilled wells are all in line with matrix permeability.