Geomechanics for Unconventional Resources

In this paper, we discuss the effect of geology on the resulting heterogeneous distribution of rock mechanical properties, and use this information to define key drivers of geomechanical problems of well construction and completion design. The paper explores the relationship of rock fabric on anisotropic elastic properties and on anisotropic strength, and provides examples of reasonable values across well known facies of North American organic-rich mudstone plays. The paper proceeds by defining the effect of mechanical anisotropy on nearwellbore stress concentrations, fracture width, breakdown pressure, solid production potential, and minimum horizontal stress. By discussing requirements of near-wellbore completion quality and far-wellbore completion quality and by providing means of evaluating these important sets of properties, we provide an effective workflow for hydraulic fracture design. Finally, the paper discusses important implications of time dependent behavior to organic-rich mudstone plays, including Biot’s poroelastic coefficient and creep. Principal conclusions of this work are that organic-rich mudstone plays are geologic plays, and understanding the geologic drivers of their vertical and lateral heterogeneity in mechanical and reservoir properties is of highest importance for exploration and production. Their content of stiff mineral constituents, compliant organic constituents, and compliant pores has a strong effect on the mechanical behavior of organic-rich mudstones. Thus, differentiating between the stress supporting constituents of the rock, and the non-stress supporting detrital minerals, is of highest importance to characterize their behavior. Confusion of these two constituents usually leads to misrepresentation of the shale facies. Organic-rich mudstones are also strongly anisotropic and their behavior cannot be approximated using isotropic models. For example, the near-wellbore stress concentrations in anisotropic rocks depend on the rock elastic properties. Thus, they are different for different rock types, and result in significantly different stress concentrations, breakdown pressures and fracture widths. As an additional example, changes in breakdown pressures and fracture width are strongly dependent on the ratios of elastic modulii (Eh/Ev and Gv/Eh) in the directions parallel and perpendicular to bedding. In addition, the far-field horizontal stress also depends on the anisotropic elastic rock properties, and thus changes from rock type to rock type. Finally, we found that the Biot’s poro-elastic coefficient in organic-rich mudstones is in the range from 0.2 to 0.6. These are low values and are in line of those expected for stiff rocks.