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).