Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.
The principal producing reservoir of Algeria's second largest oil field, Zarzaitine, has been studied using a three-dimensional reservoir simulator. When the study was conducted, the reservoir had been producing for over 10 years, the last 5 of which had been with peripheral water injection. The primary objective of the study was to determine how to maximize current production without sacrificing a significant quantity of ultimate recovery. To accomplish this objective required, first, attaining a reasonable match of past performance and then investigating future field operational techniques. A five-layer, three-dimensional, three-phase model was used to accomplish this objective.
A one-dimensional model was used extensively to first evaluate overall reservoir volumetrics, then nine 3-D runs through various periods of performance history were required to obtain an acceptable match of gas- and water-oil ratios and reported bottomhole pressures. To develop the match it was pressures. To develop the match it was necessary to adopt a multiple kg/ko correlation and, since bubble-point pressure varied with depth, it was necessary to account for depthwise changes in fluid properties.
Predictions of future reservoir performance under three plans of operation were performance under three plans of operation were studied with the model. An elaborate prediction routine which provided for the effects prediction routine which provided for the effects of tubing size and gathering-system pressure on productivity and injectivity was used in the study. In the course of these predictions, additional drilling, workover of predictions, additional drilling, workover of existing wells, cycling of the reservoir's gas cap, and various water-injection programs were considered in arriving at a recommended optimum plan of operation.
It was concluded that the 1-D and 3-D model combination provided a practical and accurate means of evaluating the Zarzaitine F4 reservoir.
The Zarzaitine field is located in the Sahara Desert of southeast Algeria and is one of that country's major fields (Figure 1). Discovered in 1957, it has a productive area of approximately 35 square miles and produces from several horizons. The major reservoir is a series of Devonian sandstone members designated as the F4 which initially contained about 2 billion stock tank barrels in place.