Multilateral Waterflood Development of a Low-Permeability Carbonate Reservoir

Abstract

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.

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