Wellbore breakout prediction in transversely isotropic rocks using true-triaxial failure criteria

Abstract

This paper presents a unified approach through which the influence of the elastic and strength anisotropy on wellbore instability can be thoroughly examined. The stresses at the wellbore wall are first calculated using the Lekhnitskii-Amadei solution, which accounts for elastic anisotropy. Then, shear failure is treated by combining the Mogi-Coulomb criterion for intact rock, with the Jaeger plane of weakness concept. The developed model accounts for all three principal stresses in predicting the onset of shear failure. The results of the specific case investigated show that rock elastic anisotropy induce higher stress concentrations. The difference, compared with the stresses found using the isotropic elastic model, could reach as high as 25% for the highest degree of anisotropy that might be expected for rocks of practical interest. The strengthening effect of the intermediate stress, as reflected in the Mogi-Coulomb criterion, reduces the required mud weight density by approximately 1.0 poundsper-gallon (ppg). Furthermore, it is demonstrated that the risk posed by bedding slippage, for a wellbore with an inclination between 15°-50° from the vertical, is masked when an isotropic elastic stress model is used. In contrast, the fully anisotropic model shows that an extra mud weight of approximately 4.5 ppg would be required, in order to avoid bedding plane slippage for the case under investigation. Although these results apply for a particular choice of strength properties and elastic properties, they give an indication of the implications of fully accounting for anisotropy and the effect of the intermediate stress when doing borehole stability analysis.

Heterogeneity versus Anisotropy and the State of Stress in Stable Cratons: Observations from a Deep Borehole of Opportunity in Northeastern Alberta, Canada

“…Although not universal, one recurring theme that appears in many of the above studies is that BO azimuths often change along a given borehole. Even though the underlying assumptions employing the equations are widely used in the literature, it has long been known that the drilling induced failure pattern may not reflect true far-field stresses and is complicated by deviations of the borehole from a principal stress direction (Mastin, 1988), geological discontinuities (Lin et al, 2007;Sahara et al, 2014;Shamir & Zoback, 1992;Yale, 2003;Zakharova & Goldberg, 2014), contrasting mechanical properties in different lithology (Agheshlui & Matthai, 2017;Pham et al, 2020), and rock strength anisotropy in the layered sediment or foliated crystalline basement (Setiawan & Zimmerman, 2018;Vernik & Zoback, 1989;. Analyses based on the equations have led to interpretations of variable stress states between the sediment and basement, such as a ~30° change in the SH orientation in Basel and Rittershoffen , and a stress state change from normal faulting stress regime in the sediment to strike/thrust faulting stress regime in the basement in Songliao Basin, China (B.…”

Influence of the Weakening Effect of Drilling Fluid on Wellbore Stability in Anisotropic Shale Formation

“…Setiawan [23] introduced a new complex variable function on complex variables, which was expressed as Eq. 8:…”

“…Therefore, it is determined that the direction of the weak plane is closely related to wellbore failure [22]. As a result, the isotropic wellbore stress distribution model is established [23,24], and it is found that Poisson's ratio has little influence on borehole stability in shale formation [25,26]. However, these studies often adopt assumed values, or only give experimental results, for the anisotropy of shale strength.…”

Heterogeneity Versus Anisotropy and the State of Stress in Stable Cratons: Observations From a Deep Borehole in Northeastern Alberta, Canada

“…One recurring theme appearing in the above studies is that BO azimuths often change along a given borehole. Even though the 2D Kirsch (1898) equations are widely employed, it has long been known that BOs and DITFs are complicated by deviations of the borehole from a principal stress direction (Mastin, 1988), geological discontinuities (Day‐Lewis et al., 2010; Lin et al., 2007; Sahara et al., 2014; Shamir & Zoback, 1992; Yale, 2003; Zakharova & Goldberg, 2014), contrasting mechanical properties in different lithology (Agheshlui & Matthai, 2017; Pham et al., 2020), and rock strength anisotropy in the layered sediment or foliated crystalline basement (Setiawan & Zimmerman, 2018; Vernik & Zoback, 1989; W. Wang et al., 2022). Analyses based on Kirsch (1898) equations have led to interpretations of variable stress states between the sediment and basement, such as a ∼30° change in the S H orientation in Basel and Rittershoffen (Azzola et al., 2019) and a change in the stress state from a normal faulting stress regime in the sediment to a strike/thrust faulting stress regime in the basement in Songliao Basin, China (B. Wang et al., 2020).…”