Reservoir Characterization Of Fractured Reservoirs In Western Canada

Abstract

The author has worked on a number of fractured reservoirs in Western Canada, which show common characteristics. Production performance, pressure transient responses and stimulation results are discussed. A reservoir characterization is presented which is consistent with observed production performance, pressure transient responses, production logging results, core analysis and well stimulation. A key component is structural geological style. This description has been applied to a number of different reservoir situations. It has application to stimulation design, predicting reservoir performance, numerical simulation and pressure transient analysis. An example is also highlighted from a gas condensate reservoir. FIGURE 1: Core porosity vs. core permeability. Alberta Central Foothills Carbonate Reservoir all samples. Foothills reservoir. The Brazeau River Elkton/Shunda is located about 50 km to the east of the preceding example. The pool is located on a subcrop edge and is also a type of fractured reservoir. However the fracturing is derived from a different source—Karst development. Core permeability vs. core porosity from this field is shown in Figure 3. It is immediately obvious that the data does not resemble the “shotgun blast” or “triangular” appearance of the foothills reservoir. The average permeability and porosity are 56.54 mD and 9.25% (574 points). The Foothills reservoir core averaged 22.392 mD with a porosity of 4.11%. It is interesting to note that at an average porosity of 4.11%, the non foothills core has an average permeability of 0.3 mD to air (at surface conditions). Another reservoir that shows the Foothills type of core permeability vs. core porosity was plotted for a Wabamun D-1 reservoir, as shown in Figure 4. The core data shows a classic triangular distribution of core permeability vs. core porosity. In the author’s opinion this is diagnostic of fracturing derived from structural (Foothills type) deformation. Structural Style Effect Figure 5 shows a cross-section through a Western Canadian sandstone reservoir. As part of an economic evaluation the author plotted well AOF and type curve reservoir interpretation. This diagram shows the basic building block of structural style in Western Canada, which is a thrust fault. Well performance is strongly affected, depending on where in the structure wells are completed: 1. Along the top of the overthrust sheet leading edge, well deliverabilities are very high. Type curves, for this reservoir were typically single porosity radial responses. 2. Underneath the thrust sheet, and adjacent to the shear zone, long slivers of reservoir rock are dragged up or broken off. In this area a number of bi-linear test results were obtained. 3. Further ahead, there are a series of faults, that are of a much smaller scale than the thrust sheet. In this area, type curves indicate systems with concentric permeabilities. 4. Behind the thrust sheet on the “back limb,” well tests indicated hydraulic fractures and mixed medium deliverabilities. One thing that was immediately obvious was that there was good permeability across the front of thrust sheet. 2 Journal of Canadian Petroleum Technology FIGURE 2: Core porosity vs. core permeability. Alberta Central Foothills Carbonate Reservoir fractured samples only. FIGURE 3: Core porosity vs. core porosity. Alberta Central Foothills Brazeau River (Subcrop) reservoir. FIGURE 4: Core porosity vs. core permability. FIGURE 5: Alberta Sandstone Reservoir, thrust sheet.