Summary

Inch-scale thin laminations in unconventional shale formations play an important role in defining the in-situ rock mechanical properties and therefore the formation's behavior during hydraulic fracturing. Borehole sonic measurements, typically used to quantify rock in-situ mechanical properties, tend to average thin laminations to yield a description in terms of an anisotropic stiffness matrix (Cij). The Cij values are used in building mechanical earth models that are critical inputs to hydraulic fracture modeling. To better quantify the sonic averaging effects arising from lamination and gain insight into the scale at which the lamination is expressed, both traditional sonic measurements and an advanced ultrasonic measurement were performed in a vertical pilot hole across an interval of highly laminated, kerogen-rich, limestone source rock of Upper Jurassic age in the northern Gulf of Mexico basin. The ultrasonic device operates with frequencies about 10 to 20 times higher than the sonic band, which enables inch-scale axial and azimuthal resolution and yields both compressional (P) and shear (S) rock slowness images. Analysis of the ultrasonic data shows contrasts in slowness of the order of 40% for P and 20% for S between compliant and stiff elements of the siltstone/limestone sequences logged. Contrasting the sonic to the ultrasonic data reveals that due to its spatial averaging, the sonic misses the inch-scale slowness contrast by as high as 10% for P and 5% for S. One consequence of this is that in a crossplot of dynamic Young's modulus and Poisson's ratio over a depth interval of hundreds of feet, the ultrasonic data exhibit much wider excursions between brittle and ductile rock than can be gleaned from the sonic data. Further, several geomechanical manifestations in the near-wellbore area can be observed for the ultrasonic images. These inform on a first-order basis for the interplay between rock strength and local stress. Breakouts, occurring in the limestone layers and reflecting rock compressive failure under a large deviatoric horizontal stress, are typically arrested at the interfaces with adjacent siltstone layers. In the "shaly" limestones where no breakouts are visible, the ultrasonic data reveal markedly detectable slowness azimuthal variations reflecting nonuniform stress concentrations in the near-wellbore area. These latter are exploited in conjunction with the sonic data within an acousto-elastic model to quantify the maximum horizontal stress magnitude and rock third-order nonlinear constants. Another rock manifestation relates to the correlation between Young's modulus of the relatively compliant layers and the increase in their radius derived from the acoustic caliper indicating these layers retreat into the formation.

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