Quantifying fracture aspect ratios in reservoir-scale fractured carbonates
Mean aspect ratios (fracture length to height relationships) are an important formation-specific parameter for modelling of fractured reservoirs, particularly when predictions have to be made using sparse sub-surface data. In outcrop it is generally not feasible to measure directly the aspect ratio of a fracture, since at least one of its dimensions will be censored at the outcrop surface. Ascertaining fracture dimensions from borehole data is even more problematic. To overcome this limitation in direct measurement of aspect ratios, we compare the height-intensity and length-intensity distributions from large fracture populations derived from sampling multiple outcrops within the same reservoir unit. We co-plot the two relationships on a single multi-scale graph, and estimate a representative aspect ratio from the relative position of the two distributions.
In this study we present data collected using a combination of traditional and modern geospatial methods, including 1D outcrop transects, digital photogrammetry, Lidar (terrestrial laser-scanning), satellite imagery, and georeferenced, scaled outcrop photos. The study is based on well exposed outcrops of Cretaceous strata that are direct analogues for producing reservoir units in nearby fields, in the Kurdistan region of the Zagros fold and thrust belt, NE Iraq. Over 74,000 fracture lengths and 8,500 fracture heights are analysed and used to quantify the range of representative aspect ratios per formation. The scale range over which the aspect ratios are characterised is from ca. 0.1m to 3,000m in length and ca. 0.01m to 100m in height, and therefore mitigates the need to upscale aspect ratios measured at outcrop scale for use in reservoir modelling.
Ackermann at al. (2001) describe how fault aspect ratios increase when propagation of the fault becomes vertically confined by mechanical layers. We infer that this also occurs during the development of other types of fracture, including tensional, shear and hybrid fractures in fractured reservoirs. Mechanical layers exist at all levels within the Kurdistan stratigraphy (i.e. individual beds to entire formations), and we therefore expect confined fractures of all sizes to have greater aspect ratios than unconfined fractures (> 2:1). Ackermann at al. (2001) report that when the fracture systems in their experiments became fully saturated, aspect ratios of 4.5:1 were reached. In Kurdistan (and other fractured reservoirs we have studied elsewhere), mean aspect ratios for fractures can be significantly larger than 4.5:1 in sequences with strong mechanical contrasts.