Construction of a Large Field Simulator on a Vector Computer (1980)

Selected modeling and computer techniques are described that were developed during the construction of a simulator for the Prudhoe Bay field. The simulator contains 16,421 active subsurface blocks and is dimensioned for 1,000 wells. The finite difference equations for the reservoir model are solved iteratively by an areal application of the red-black algorithm, a technique ideally suited to the vector computer. Introduction Due to the importance of the Prudhoe Bay field, an unusually large reservoir simulation effort is devoted to the analysis of the field's performance and to planning its development. As part of this effort, Sohio Petroleum Co. has constructed a numerical simulator for the whole field or more precisely for the whole of the main Sadlerochit reservoir.The sand is about 550 ft (168 m) thick and has been subdivided into 12 layers to describe flow in the vertical direction. With an areal grid no finer than 160 acres (647 500 m2), a model of 16,421 active blocks results. The formidable computing requirement implied by this number of blocks happily coincides with the advent of vector computers equipped with large central core memories. In addition, calculations on several hundred wells must be carried out at each time step. The simulator is dimensioned for 1,000 wells.Wells are frequently situated in adjacent blocks. Two or more wells may even be placed in the same block. Although reservoir behavior must be adequately represented, a coarse grid model of this type is not suited to the analysis of reservoir mechanisms. The model's main purpose is its use as a scheduling tool. This is especially important in Prudhoe Bay where expenditures are large, lead times to construction extend several years, and ice restricts access to the field by sea to a few weeks in the year. It is, thus, not by accident that the title of this paper makes mention of a field simulator rather than a reservoir model. Indeed, despite the large number of blocks, the bulk of the computation is concerned not with reservoir flows but with hydraulics in wells and surface facilities.This paper describes some of the computer and modeling techniques that were developed during the construction of this simulator. A complete description of the model is not attempted. Various aspects of the subsurface reservoir simulation are discussed first. Details on the well calculations are then given, followed by some timings for the computational performance of the simulator. General Description of Subsurface Model The subsurface model is divided into two parts. An area covering the hydrocarbon-bearing portion of the Sadlerochit formation is modeled using 12 layers and an areal mesh of 160-acre (647 500-m) square blocks. This part of the simulator will be referred to as the reservoir model. In the reservoir model, the usual three-phase differential equations are approximated as finite difference equations that are fully implicit in pressures and saturations. Pseudo-implicit treatment is given to relative permeabilities and capillary pressures. Upstream weighting of mobilities is used for calculating interblock flows. JPT P. 2253＾