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Borehole Image Based Aperture Characterization to Identify Primary Versus Secondary Fracture Opening


In this paper we discuss a unique method of evaluating fracture aperture from borehole image log and its impact in reservoir characterization. Aperture calculation from image log relies on the resistivity contrast between fracture trace and the surrounding rock. Traditionally fracture aperture is represented by a single value which works well for idealized “parallel plate” fractures. This falls short in nature as fracture surfaces are characteristically rough and fracture aperture varies continuously along the surface of the fracture. Fracture aperture is the key parameter for evaluating porosity and/or permeability in type 1 or type 2 fractured reservoir. It is therefore critical to get a thorough characterization of this property along the entire surface. To address this issue, we have developed an algorithm that determines a continuous measurement of aperture along the trace of the fracture to capture the surface roughness (pinch and swell as opposed to parallel plate) of the fracture. The continuous data is then analyzed to identify the outliers in fracture aperture population. The outliers may be due to breakouts or some secondary processes. Based on the genetics of fracture the aperture variation along the trace can have a wide range with high standard deviation or a narrow range with a low standard deviation. Using this method on example reservoirs we worked on shows that typically the aperture variation can be related to process of formation, where primary aperture will have a low standard deviation whereas aperture created by secondary processes (like dissolution) or apertures impacted by breakouts have a high standard deviation. Based on the character of aperture the fractures can be classified in various groups that can in turn be related to facies hosting the fractures thus providing a more robust means to distribute fracture porosity/permeability by facies in static model. This method is especially beneficial in carbonate or mixed carbonate system where facies are more likely to be altered by later diagenetic processes causing destruction or enhancement of aperture due to dissolution, recrystallization or filling. Characterizing and classifying the aperture will enhance ability to better evaluate fracture porosity/permeability over the field. This will in turn lead to improved reservoir simulation reliability, robust history matching and optimized infill development in fractured fields.