A grain-based rock model was developed and applied to study wellbore stability. The rock is represented as an assembly of tetrahedral blocks with bonded contacts. Material heterogeneity is modeled by varying the tensile strength at the block contacts. This grain-based rock model differs from previous disk/sphere particle-based rock models (e.g., Potyondy and Cundall, [1]) in its ability to represent a zero (or very low) initial porosity condition, as well as highly interlocked irregular block shapes that provide resistance to movement even after contact breakage. As a result, this model can reach higher uniaxial compressive strength to tensile strength ratios and larger friction coefficients than the disk/sphere particle-based rock model.

The model was first used to match the properties of typical rocks (such as sandstone) by simulating the direct tension test and uniaxial compression test. The calibrated model was then applied to study wellbore breakout and the thick-walled cylinder (TWC) test. The model captured the fragmentation process near the wellbore due to buckling and spalling. Thin fragments of rock similar to onion skins were produced, as observed in laboratory breakout experiments (e.g., Haimson, [2]). The results suggest that this approach may be well-suited to study the rock disaggregation process and other geomechanical problems in the oil and gas industry.


Wellbore stability is required to recover oil and gas resources safely. Wellbores can fail via several mechanisms, including stress-related failures such as breakout and tensile failure, subsidence (depletion), fault activation due to production, and complex geologic conditions.

Image logs have been used extensively (Barton and Zoback, [3]) to observe and investigate wellbore failures, and numerical modeling has been used for several decades to predict and simulate wellbore failures. The history of numerical methods for borehole stability has been presented by Salehi et al. [4]. In the late 1980s and early 1990s, several linear elastic and poroelastic models for borehole stability were introduced by Bradley [5], Detournay and Cheng [6], Fuh et al. [7], and Yew and Lui [8]. In the 1990s, nonlinear elastic, plastic and elasto-plastic models were developed by McLean and Addis [9], McLellan and Wang [10], and McLellan [11]. Several thermo- and chemo-poroelastic models were also developed by Li et al. [12], Wang and Dusseault [13] and others. Although more complex material behavior was modeled in the later models, all of the aforementioned models were two-dimensional.

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