Abstract
Abstract
One hundred fractured reservoirs from around the world were evaluated to determine how ultimate recovery was affected by inherent reservoir and fluid properties, such as porosity, permeability, viscosity, mobility ratio, Sw, wettability, fracture distribution, and drive mechanism vs the choice of reservoir management strategy, e.g., optimization of production rate and type of EOR technique. Fractured oil reservoirs were divided into four groups. Type I reservoirs have little matrix porosity and permeability. Fractures provide both storage capacity and fluid-flow pathways. Type II reservoirs have low matrix porosity and permeability. Matrix provides some storage capacity and fractures provide the fluid-flow pathways. Type III (microporous) reservoirs have high matrix porosity and low matrix permeability. Matrix provides the storage capacity and fractures provide the fluid-flow pathways. Type IV (macroporous) reservoirs have high matrix porosity and permeability. Matrix provides both storage capacity and fluid-flow pathways, while fractures merely enhance permeability.
Previous fractured reservoir classifications did not distinguish between Type II and Type III reservoirs, instead combining all fractured reservoirs with low matrix permeability together into one group. By cross-plotting the various reservoir properties vs ultimate recovery factor and reviewing case studies of primary and enhanced recovery history for 26 Type II and 20 Type III reservoirs, it is demonstrated that recovery factor is controlled by different factors in these two reservoir types. Recovery factor in Type II reservoirs is sensitive to aquifer-drive strength and optimization of flow rate. Type II reservoirs are easily damaged by excessive production rates, but when managed properly, some achieve good recovery without the need for secondary or enhanced recovery programs. Recovery factor in Type III reservoirs is affected by inherent rock and fluid properties, particularly matrix permeability, API gravity, wettability, and fracture intensity. The choice of proper EOR technique is essential for optimum exploitation. No Type III reservoir is produced to final depletion without the aid of some type of secondary or EOR technique. Recognition of the differences between Type II and Type III fractured reservoirs should lead to better choices of exploitation strategy.
Introduction
Fractured petroleum reservoirs, although less common and more poorly understood than conventional sandstone and carbonate petroleum reservoirs, are very important contributors to world oil and gas reserves and production. The 100 fractured reservoirs examined in this study have a combined recoverable oil and gas reserve of 90 billion bbls of oil equivalent. Fractured reservoirs are often considered to be short-lived with high flow rates, rapid production declines, and low ultimate recovery factors. Engineers often look unfavorably on fractured reservoirs because they are difficult to characterize and recovery techniques must be carefully and judiciously applied in order to avoid production problems. This can drive up the time, cost and risk of developing a reservoir. However, many of these prejudices are unfounded. Often they derive from a bad experience early in the career of a person now in a decision-making position. Fractured reservoirs that have been properly developed have ultimate recoveries that compare favorably with many conventional sandstone and carbonate reservoirs.
In order to provide a more solid footing for understanding the controls on the recovery in various types of fractured reservoir, we have undertaken a systematic study of one hundred fractured reservoirs throughout the world. This paper examines 46 of these reservoirs in the Type II and Type III categories. Only reservoirs for which a comprehensive spectrum of parameters was available, were chosen for study. The effect on recovery of both inherent parameters and reservoir management techniques were examined, in order to achieve a thorough understanding of the relative importance of each variable.