Pacific Section AAPG Convention

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Variability Of Elastic Properties And Applicability Of Brittleness Indicators, Miocene Monterey Formation, San Joaquin Basin, California


The combination of horizontal drilling and multi-stage hydraulic fracturing has been paramount to unlocking vast hydrocarbon reserves in tight shale reservoirs not previously considered to be economically productive. As a result, quantification and improved understanding of formation brittleness is important for completion zone identification and design of stimulation treatments that create complex fracture networks. Brittleness is commonly quantified through empirical correlations that combine elastic properties of rocks, which relate applied stresses and resulting deformation. Elastic properties are easily computed from well logs, but results are frequency-dependent and must be corrected to static conditions. This study addresses how elastic properties of the heterogeneous and diagenetically complex Miocene Monterey formation vary across the San Joaquin basin, and how these properties compare to those of other well-documented shale formations. By investigating the effect of mineralogy and burial conditions on Young's modulus and Poisson's ratio calculated from sonic and density logs, the extent to which a brittleness index can be used to delineate areas better suited for fracture stimulation is explored. Results from this assessment suggest that mineralogy is a more suitable brittleness indicator for Monterey regional work in the San Joaquin Basin than a generic index based on elastic properties sensitive to the variability of burial and compaction seen at the basin scale.