Written by J. Allan, and S. Q. Sun
Published by the Society of Petroleum Engineers
Sourced from the SPE Annual Technical Conference and Exhibition, 5-8 October, Denver, Colorado, 2003
Copyright: 2003. Society of Petroleum Engineers
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 optimisation 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.