A Method for Handling Spatially Varying Fluid Properties in a Simulation Model for a Fissured Reservoir
Abstract The high relief, fractured carbonate reservoirs of the Asmari formation in Iran have extremely thick oil columns, with a large vertical change of reservoir temperature. This large change results in a significant effect on reservoir fluid properties. In contrast, there is a remarkable uniformity of PVT properties on a horizontal plane. Therefore, to properties on a horizontal plane. Therefore, to obtain meaningful results from reservoir engineering studies, PVT properties must be carefully weighted vertically. Intensive study of the known oil recovery mechanisms within these highly fissured systems resulted in a sophisticated reservoir simulation model. The model is programmed to include these recovery processes, which occur essentially in horizontal layers or zones. It also includes a technique for volumetrically weighting the large vertical variation of the PVT data. A description of this weighting process is the primary purpose of this paper. Introduction Oilfield structures found in Iran have been described as very long, high relief, assymmetrical anticlines that contain unusually thick oil columns (see Fig. 1). The oil reservoirs generally are capped with large primary gas accumulations and are often affected by natural water drive. Producing formations include the Asmari, Bangestan, and Khami carbonates, with the Asmari being by far the most common and prolific. Although some Asmari reservoirs have been discovered that contain sandstones interbedded within the limestones and dolomites, most reservoirs in Iran contain the bulk of the oil in compact carbonates that have been contorted and highly fractured during structural deformation. The resulting anticlinal oil accumulations are produced mainly from complex fracture systems, which apparently exist quite uniformly throughout the dense matrix. Study of surface rocks, cores and well producibility show that well developed fissure producibility show that well developed fissure systems are responsible for the excellent fluid communication. Because the fissures contain relatively little oil, maintenance of such prolific rates is dependent upon rate of oil replenishment from the adjacent matrix. The various productive mechanisms determine the rate of replenishment and duration. RESERVOIR AND MODEL CONSIDERATIONS Engineering studies of the fissured Iranian reservoirs have led to a mathematical model that is even more sophisticated than an earlier one. Many complex features have been included in the current model, making it a useful aid for advising management. The model consists of a master digital computer program that encompasses 54 subprograms and runs on any of the IBM 7040, 7090 and 360/65 computers. The known oil recovery mechanisms included in the model are based on consideration of practical oil recovery observations. For example, oil recovery from the expanding gas cap is predicted by using recognized gravity drainage formations, but if necessary, retrograde condensation recovery from the gas cap can also be included. Fluid and rock expansion, solution gas drive, and water displacement are also included, and vary according to the reservoir in question. But the manner in which gas production differs from what is usually observed in a solution gas drive must be known to include a proper simulation of behavior. Experience has shown that the producing gas-oil ratio continues to decline during the entire producing life of a well unless the proximity of the gas cap results in gas coning, at which time the well is shut in. Material balance calculations show that the initial gas-oil separation singe actually occurs in the reservoir, with the fissure system readily enabling the liberated gas to segregate toward the gas cap. SPEJ P. 25
