Using Empirically Developed Rock Tables to Predict and History Match Fracture Fracture Stimulations

Abstract Rock tables giving values for fracture closure gradient, Young's modulus, and Poisson's ratio have been developed by the authors using lithologies determined from well logs, cores and mud logs, and substantiated by numerous mini-fracs. Log curves are layered and analyzed for lithological intervals as thin as one foot. The various rock types are arranged in the table as a function of increasing closure gradient, giving a basic order of sandstone, siltstone, claystone, shale, and the group limestone-calcite-pyrite-anhydrite, dolomite and coal, all below the weight of overburden. Included in the tabulation are mixtures of the various lithologies derived in similar manner from formation logs. An example would be a carbonaceous-limy-sandy-shale having a closure gradient of 0.93 psi/ft (21 kPa/m), a Young's modulus of 2.18x106 psi (1.5x107 kPa), and a Poisson's ratio of 0.33. By comparison, clean sandstone has a closure gradient of 0.66 psi/ft (15 kPa/m), a Young's modulus of 3.33x106 psi (2.3x107 kPa), and a Poisson's ratio of 0.23. Over seventy identifiable lithologies or lithology-mixtures are listed in the table. All values were developed through a combination of rules-of-thumb and interpolation. This empirical data set is analogous to a seven-component, programmed log evaluation. A normal log analysis done manually will yield about fifty layers of rock with distinct compositions per fracture height equivalent to 100-165 ft (30-50 m) of depth. Layer-specific formation characteristics are then assigned to a fracture model for deriving initial shut-in pressure (ISIP) and closure. Next, the calculated results are compared to actual mini-frac data. If corresponding values show little similarity, then there exists some stress difference from normal behavior in the reservoir under study, the most common cause being pressure depletion. Variance is also attributed to such factors as dilation around the wellbore (cracks in the rock), tectonic strain, compressibility changes, density increase from silica diagenesis, and reservoir gas over-pressurization observed in mountainous regions of North America. In-situ stresses can be adjusted to properly model these particular situations. A separate set of rock tables was compiled for southern California siliceous formations where Young's modulus for diagenetically-altered diatomaceous rock ranges from 0.2 × 106 psi (0.14x107 kPa) to 1.0 × 106 psi (0.69 × 107 kPa). Unconverted diatomite, a regional lithology susceptible to depletion-induced compaction, exhibits a Young's modulus as low as 0.04 × 106 psi (0.028 × 107 kPa). Diagenetic sequences such as siliceous shale and porcelanite require special application of the California Rock Tables for depths to 8,000 feet (2,440 m), after which the standard Canadian Rock Tables for post-Precambrian formations can be employed.

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