Abstract

This paper presents the methodologies adopted to model a tight carbonate reservoir located in Abu Dhabi and to better predict its performance (completed with horizontal wells) under a water-alternating-gas (WAG) process. The model is built through integrating geological, geophysical, petrophysical, geomechanical, and geostatistical information.

The large-scale reservoir framework is built by integrating horizontal wells and 3D seismic data. Horizontal well results are used to improve the velocity modeling and depth conversion. The fine scale reservoir zonation is based on lithostratigraphic correlations derived from the porosity and micro-resistivity logs. Stylolitic intervals are used as stratigraphic markers to guide the reservoir zonation. The Porosity model is derived from a high resolution stochastic seismic inversion, and the permeability model is generated using cloud transforms with P-Fields applied by reservoir rock types. High-pressure mercury injection data is used to define reservoir rock types. Lorenz plots have been applied and found to be a useful technique for capturing the heterogeneity of the reservoir and determining the main flow units. Fracture analysis is conducted using cores and image logs (FMI). A geomechanical study is performed to assess the orientation of the horizontal wells in the field. A discussion on the orientation of the horizontal wells with respect to maximum principal stress versus productivity/injectivity is also addressed in this paper. A mechanistic compositional flow model is built to perform sensitivity analyses on various WAG schemes (cycle, ratio, etc). A full field compositional model is subsequently built to evaluate the field performance under various development scenarios. The field is scheduled to come on stream by December of 2005.

Field History

The field was discovered in 1969. Reflection seismic had defined a number of structure closures at several stratigraphic levels, and the discovery well, W-1, was drilled to test one of these structures. Oil and gas shows were recorded when the well penetrated the main reservoir interval. Subsequent tests proved the commercial viability of the structure. In 1995 a 3D seismic survey was acquired and a new re-interpretation performed. The combined evaluation of OH logs, well test and 3D seismic results provided some encouragement, and in 1999 the field was declared commercial.

Between 1994 and 1999 ADCO implemented an Early Production Scheme (EPS) to evaluate the well performance, with vertical and horizontal wells. Based on the successful results of horizontal well performance a field development plan was devised consisting of a line drive with 20 horizontal producers and 15 horizontal injectors under a WAG process (Fig 1). The development plan calls for 20,000 BOPD by end of 2005, maintaining the plateau for a specified period of time, and achieving a high recovery efficiency. Production forecasts were based on compositional models which are used to predict and monitor the reservoir performance during Phase-1 and also to assist the subsequent phases of development beyond 2015. The latter was based on detailed static (geological) modeling, the subject of this paper.

Seismic data

Three generations of seismic were acquired between 1962 and 1995, with the latest being a 3D survey recorded in 1994 and 1995 by Western Geophysical. This survey covers 1334 sq. km over the field area with a 25 × 25 m. bin size that translates into 2.1 million traces providing a dense grid of information of the subsurface.

The original processing was completed in 1997 by Western Geophysical and the seismic data quality was good enough to carry out accurate structural interpretation. However, despite favorable surface conditions, the seismic data suffered from stacking velocity dispersion that had a significant impact on amplitude values.

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