Numbi Field: Adapting a Reservoir Management Strategy to Changing Reservoir Conditions
Authors: Schaaf Robert Paul , King Gregory R. 
Published: December 2007

The Numbi Field is located approximately 20 miles offshore of the Angolan enclave of Cabinda. Put on production in 1987, the Numbi 57-05 Vermelha reservoir has been a prolific oil producer. After water injection commenced in 1991 into 14 peripheral injectors, the 49 production wells had a peak production of over 85,000 barrels of oil per day (BOPD). However, after reaching peak production in 1992, the reservoir experienced a rapid production decline. In order to minimize the decline and thus maximize recovery, interim and long term strategic reservoir management changes were required. Interim solutions to problems have included running through tubing bridge plugs, adding through-tubing perforations, performing selective completions with rig workovers performing acid pre-pack gravel packs in injection wells, and shutting in high gas producing wells. The through-tubing work, intended to shut off excessive water or gas production and open unproduced intervals, has contributed 16,000 BOPD of incremental production. The selective completions have increased production by 14,000 BOPD The wells that have been gravel packed have shown an initial 75% increase in injection and have not experienced further sanding problems. The short term changes have been effective in slowing the current production decline. Although the recent well work has slowed the production decline, it has not arrested the depletion of reservoir pressure. A new longer term strategy is required to address this issue. In order to develop a long term strategy, a detailed geological model was assembled. This model was then used as a basis for a finely detailed (25 layers, 102,000 cell) simulation model. The simulation model showed that particular layers and certain areas of the field were under-injected. The model also showed that large quantities of oil (approximately 30 MM barrels to date) have been displaced into the gas cap. An aggressive repressurization strategy is required to prevent further pressure depletion and to minimize the volume of oil displaced into the gas cap. Initiating injection into up-dip injectors that penetrate the gas cap is anticipated to result in incremental recovery of 35.1 MM barrels by the year 2010.

Introduction

The Numbi Field was discovered in July 1982 by Well 57-05. The field is located in Area A of the Cabinda Block 0 Concession of Angola in water depths ranging from 120 feet to 150 feet (see Figure 1). The field is comprised of three Upper Cretaceous reservoirs: the 57-05 Vermelha reservoir, the 71-27 Vermelha reservoir (Southeast Numbi), and the 58-05 Vermelha reservoir (Northeast Numbi). The reservoirs are at a depth of 3,500 feet TVD subsea

Regional Geology

The Numbi field is part of a regional geologic system which is situated along the western coast of the Angolan province of Cabinda. This system contains several major producing fields including the Takula, Wamba, and N'Sano fields to the north and the Vuko and Kungulo fields to the southeast. This geologic system continues parallel to the African coast towards the Congo Republic in the northwest and the Republic of Angola in the southeast.

The early stratigraphy of the area consists of lacustrine carbonates and shales (Lucula, Toca, Bucomazi formations) deposited in rift valleys. The overlying Loeme Salt marks the initial opening of the Atlantic Ocean. As the Atlantic continued to open, the Albian age Pinda Formation was deposited as a marine transgressional sequence. The overlying Cenomanian age Vermelha formation was then deposited after a regressive event at the end of the Albian.

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