Abstract

In the oil bearing reservoirs of a mature field in the offshore Abu Dhabi, the understanding of the fracture network is essential with growing gas and water productions. Fracture characterization and modeling at the full field scale is the key to match the production profile and to optimize infill drilling.

The interpretation of fracture data from image logs and cores allows defining accurately the fracture pattern in terms of orientation, typology, density and relation with lithology and faulting. Two main fracture sets are chronologically observed:

  • NW-SE fractures are generally mineralized and clustered around faults of similar orientations;

  • NE-SW fractures are generally open, homogeneously distributed with their density controlled by the lithology and a few portion located in swarms.

The calibration of fracture data with PLT's in horizontal drains suggests that:

  1. the main flow comes from the matrix in porous layers;

  2. in areas without mineralized fractures, wells can be connected to water, either through matrix (permeable layers) or through open NE-SW fractures, particularly in zones of large curvature where a higher fracture density is expected;

  3. in areas with mineralized fractures, water in permeable layers is channelized and tight layers can be vertically connected to more permeable layers by open fractures whereas the lateral flow can be hampered.

Full field fracture modeling is done through an innovative geostatistical approach. First, discriminant analysis is used to derive, from geological attributes that are known to be physically related to fracturing, a (geological) component best correlated to well fracture data. Second, sequential indicator simulation generates equiprobable fracture density maps. Third, optimization of infill drilling is made through risk analysis.

The pertinence of this approach for the optimization of well planning is validated against recently drilled wells. Matching the production profile is made more difficult however by the different opening times of production wells and by their complex completions.

Introduction

The oil bearing field under study was discovered in 1969 and put on stream in 1973. In 1979 a water injection scheme was launched followed by a gas injection development; first pilot in 1991 then full field starting 1997. To date, more than 120 wells were drilled including several horizontal drains. Although fractures were identified in this field at an early stage, their contribution to field performance was not initially considered as important. However, with the decline of the field and the water and gas injection programs, well behaviors showed that this early statement should be revisited. Indeed, the impact of fracture systems on such mature carbonate reservoirs can range from very restricted to conduit for fluid flow and reservoir impairment. This impact does not depend only on the type of fracture filling but also on fracture geometry, connectivity and density. According to the full field distribution of the fracture pattern, the three above impacts are possible. Consequently, only the characterization of all fracture parameters can lead to a comprehensive fracture model. This fracture model needs to be evaluated against dynamic data and the spatial distribution of fractures at the entire field scale is required to improve field development plans. The model can be also evaluated against new data and revised if necessary. This is the purpose of this paper.

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