Analysis of Naturally Fractured Reservoirs With Bottomwater Drive: Nido A and B Fields, Offshore Northwest Palawan, Philippines

Abstract

Withjack, E.M., SPE, Cities Service Oil and Gas Corp.* Summary This paper presents results from computer simulations investigating the influence of reservoir characteristics on the performance of naturally fractured reservoirs with bottomwater drive. The study of reservoir characteristics uses available data from the Nido A field in a basic reservoir model. The Nido A and B fields are naturally fractured reservoirs operated by Cities Service Oil and Gas Corp. in the Philippines. When reasonable changes are made to the characteristics of the basic model, the simulations suggest how fracture porosity, matrix-block size, relative permeability, and absolute permeability influence reservoir performance. The simulations show that large fracture porosity results in a long duration of water-free production before breakthrough. After breakthrough, matrix-block size and absolute permeability strongly influence oil production. Smaller matrix permeability strongly influence oil production. Smaller matrix blocks provide higher oil-production rates. Higher absolute permeabilities cause oil-production rates to decline more rapidly after breakthrough. Performance matches were obtained for the Nido fields. Effective values were determined for characteristics that could not be measured, including fracture porosity and matrix-block size. Estimated fracture porosity and matrix-block size. Estimated fracture porosities were 0.9 and 0.5% for the Nido A and B porosities were 0.9 and 0.5% for the Nido A and B fields, respectively. An effective matrix-block size, equivalent to a 60-ft [18.3-m] cube, was determined for both fields. Oil-production forecasts were made using the effective reservoir characteristics. Introduction The performance of a fractured reservoir with bottom-water drive may differ from that of a nonfractured reservoir. Oil-production rates, initially high because of oil available from the fractures, decline rapidly after water breakthrough into the producing well. Following this rapid decline, oil production may continue for many years because of the slow process of imbibition that occurs in the reservoir rock between the fractures. Conventional reservoir simulators have difficulty modeling fractured reservoirs because oil comes from both the high-permeability fractures and the low-permeability reservoir rock. Previous investigators have modeled the performance of fractured reservoirs with a two-step approach. First, they acquired imbibition data from either laboratory tests or mathematical models. Second, they used the imbibition data to simulate matrix-block performance and analytically or numerically determined the reservoir performance by combining the production from the matrix blocks. Mattax and Kyte determined matrix-block performance from laboratory tests of small rock samples. They performance from laboratory tests of small rock samples. They modeled fractured-reservoir performance by summing the oil production from rows of matrix blocks. Menouar and Knapps suggested numerical simulation of matrix-block performance. They used simulation results to calculate reservoir performance by summing the oil production from both the matrix blocks and the fractures. production from both the matrix blocks and the fractures. Their approach divided the imbibition process into three periods based on the water level in the fractures. periods based on the water level in the fractures. Rossen modeled fractured reservoirs with a bottom-water drive and a gas cap. He suggested obtaining matrix-block performance data from either numerical simulations or laboratory experiments. Rossen's numerical approach modeled fluid movement in the fractures and treated fluid transfer to and from fractures with source and sink terms. deSwaan-O. analytically determined production from waterflooding a fractured oil reservoir. The first step of his approach required imbibition data to derive an exponential term for use in a second-step formula describing reservoir performance. The general two-step approach used by these investigators is similar to the approach described in this paper. This study differs from the previous studies in two paper. This study differs from the previous studies in two ways. First, a conventional black-oil simulator is used to determine matrix-block performance. Black-oil simulators, which are normally used for simulating non-fractured reservoirs, are accessible throughout the industry and provide a convenient means for obtaining imbibition data. Second, the two-step approach is applied to actual reservoirs, the Nido A and B fields, offshore northwest Palawan in the Philippines (Fig. 1). Some of the Nido A properties are used in a basic model for investigating the influence of reservoir characteristics on performance. This study includes determining how performance. This study includes determining how matrix-block size, relative and absolute permeability, matrix-block porosity, and fracture porosity influence oil production. production. JPT p. 1481