Abstract

Many oil and gas reservoirs have their associated aquifers, which provide them with some pressure support and form a hydraulic system. When material balance calculations are applied to such reservoirs, aquifer behavior models are required. In practice, the most common procedure is to assume an aquifer model, which has an analytical solution that can readily be implemented. The simplest methods are based on the Steady State (Schilthuis), Modified Steady State (Hurst), and the Unsteady State (Van Everdingen and Hurst), models. For correct description of the hydraulic system using the available techniques, it is required to have a minimum set of information about the reservoir and aquifer, such as: viscosities, compressibilities, permeabilities, volumetric factors, and geometry of aquifer and reservoir. To overcome possible lack of some of this information, the technology of aquifer influence function "AIF" has been developed and therefore employed.

Most of the previous work was mainly concerned with AIF models for gas reservoirs. This study has focused on devising a methodology for assessing water influx into undersaturated oil reservoirs based on AIF models. After developing the AIF model, it was validated using actual field data and then applied to four real field cases from the Gulf of Suez region. The results of this application have shown that, the developed model can generate an accurate aquifer influence function for the given reservoir that may directly be extended for future predictions of the reservoir and aquifer performances. The model results have indicated good agreement with actual field data. The model required some basic historical data to run without assumption of reservoir and aquifer rock and fluid properties or aquifer geometry.

The semi-analytical approach devised in this work, with Levenberg-Marquardt Non-Linear technique, provided a quick, easy and yet accurate procedure for calculating the aquifer influence function. Four field cases have been studied in this work, namely: October-Nubia, October "J"-Nubia, October "J"-Asl, and Ramadan-Nubia "C" reservoirs. Confirmed by the actual field studies, for different reservoir- aquifer geometry and strengths, the model has proven to be capable of:

  1. evaluating the reservoir-aquifer performance,

  2. defining an appropriate AIF, and

  3. predicting reservoir pressures under varying production schemes.

Sensitivity cases have also shown that better description of the aquifer influence function model can always be achieved and consistently improved by using additional production data when available.

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