Reservoir Scale Fracture Modelling Formulae, Arab-D, Ghawar

By

 Mohammed S. Ameen¹

(1) Saudi Aramco, Dhahran, Saudi Arabia

 Delineating the controlling factors on fracture occurrence in the Arab-D is a challenging task, and an essential prelude to fracture modelling for reservoir management and development purposes. So far, this has been mostly based on hypothetical assumptions.

This paper formulates and ranks the main controlling factors of fracture distribution in the Arab-D, Ghawar. Multi-source data are integrated and used, including cores, borehole images (from vertical and horizontal wells) as well as seismic data. This study covers fractures spanning the whole scale from micro-fractures (microns-scale, visible in plugs/thin sections) to macro-fractures (detected from offset on seismic sections). The study shows that fracture distribution follows power-law irrespective of scale. This applies to both the fracture aspects (length and aperture) as well as the abundance of fractures in 3D volume (% volume of moderate to densely fractured rock in the reservoir). The power-law “fracture abundance” formula indicates that Arab-D is under-saturated with fractures. The fractures are dominated by ENE-striking, near vertical set. Preferential occurrence of the Arab-D fractures is systematically dictated by key geological “controlling” factors. According to their controlling factors the fractures are two equally abundant types: stylolite-related fractures, which cluster around bedding stylolites, and non-stylolite (tectonic) fractures. The latter include large faults detected from offset on seismic sections and subseismic fractures detected from core and borehole images. Subseismic fractures spatial distribution is not random but follows formula related mainly to matrix porosity (inverse power-law), bedding curvature (linear proportional) and lithology (average fracture density in dolomites is two folds that in limestones).