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Abstract The Windalia reservoir is a 22,000 acre, 600 well oil reservoir with a 25 year waterflood history, complicated by stress induced fracturing and a major pattern realignment. The reservoir recovery performance was studied by an integrated combination of geostatistical, finite difference, and stream tube models. The methodology is an extension of previously published work' to larger and more complex reservoir problems. The basis of the approach is a geostatistical reservoir description derived from well logs and core data. Fine grid 2-D and 3-D models are developed to correlate sweep efficiency as a function of characteristic reservoir stratigraphics. These correlations are translated into fractional flow functions and pseudo relative permeabilities which are used in full field stream tube models and 2-D areal finite difference models. The stream tubes provide recovery calculation and front tracking and the areal model provides a calculation for pressure and well productivity. The methodology provided a reasonable representation of the reservoir and its well histories for an efficient expenditure of labor and computer time. Model construction, well history matching and forecasts were completed in four months, a significant improvement over previous attempts to model this reservoir. Refinement, update and sensitivities continue as a reservoir management process. Introduction The Windalia Sand Reservoir was discovered in 1964 on Barrow Island, which lies about 40 miles off the coast of western Australia. The reservoir is a heterogeneous sandstone, generally subdivided into nine layers. Six lithofacies have been identified from core: flat laminated mudstones, bioturbated sandy mudstones, bioturbated muddy sandstones, bioturbated sandstones, carbonate cemented beds, and carbonate concretions. These lithofacies are complexly interbedded and occur throughout the nine layers. The Windalia was initially developed on 40 acre, 5- and 9-spot waterflood patterns in the late 1960's. There was a general conversion to line drive in 1974 with various infill and realignment projects since that time. There are currently 650 production and injection wells, as shown on Fig. 1, producing 14,000 B/D of oil and 33,000 B/D of water. The reservoir has now produced 240 million barrels of oil from an OOIP estimated to be in the 750 million - 1 billion barrel range. STUDY OBJECTIVE The objective of this project was to derive a history matched model of the reservoir which could be used to forecast infill drilling and workover response and to assist in general reservoir management. During the past 15 years there had been several attempts to build simulation models for the Windalia, all of them unsatisfactory because of the heterogeneity of the formation, the large number of wells, and the various realignment and infill projects which characterize its history. In the current study, the technique of hybrid simulation was used as a means of accommodating the size and complexity of the reservoir, and its history, for a reasonable expenditure of time and cost. P. 627^