Analytical Variability of Laser Diffraction Granulometry in Carbonate Fault Core Rocks: Implications for Fault Sealing Predictions

Storti, Fabrizio ¹; Balsamo, Fabrizio ^1
1 University "Roma Tre", Roma, Italy.

Faulting in poorly layered carbonate rocks can provide dual permeability systems consisting of intensely fractured conduits in damage zones, and strongly comminuted cataclastic rocks forming hydraulic barriers in fault cores. A fundamental parameter controlling the sealing potential of such fault zones is fault displacement that, in turn, determines the intensity of rock comminution in fault cores and their particle size distributions and, eventually, their permeability. Empirical relationships between fault displacement, particle size distributions, and permeability drops in fault cores are commonly derived from field analogue studies. Particle size determination is central for establishing predictive relationships to be implemented in reservoir modelling tools. Laser diffraction granulometry provides the possibility of fast particle size data acquisition over a wide size range by using a variety of analytical methods. They ensure the appropriate flexibility for analysing very different granular materials, but deserve accurate investigations and systematic testing for the possible influence that different analytical methods may exert on results obtained by the same or different instruments. We present results from specific tests on poorly coherent carbonate platform fault core rocks by using two different instruments with different sample dispersion and pumping systems. Most of our analyses were performed with a Malvern Mastersizer 2000 laser diffraction granulometer that allowed us to apply several wet and dry analytical procedures at different pump speeds, which included measure precision tests with and without sample ultrasonication and different dispersant liquids. Results of our work indicate a high sensitivity of particle size data from carbonate cataclastic breccias to the adopted analytical procedure. The large variability of analytical results impacts sealing predictions in faulted carbonate reservoirs. Accordingly, systematic investigations on such variability should be routinely included in reservoir modelling workflows as an additional uncertainty source in transmissibility predictions across fault zones.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009