A methodology for wellbore stability analysis in anisotropic formations: A case study from the Canning Basin, Western Australia
Graphical abstract
Introduction
There have been many applications reported for directional wells in the oil and gas industry, ranging from effective productions of gas shales to active well control in deep water drilling conditions. However, directional wells suffer from few severe problems such as excessive drag and torque of drilling strings (Huang et al., 2015), cuttings transportations (Yang et al., 2015), casing wear (Zhang et al., 2014) and wellbore instability (Ma and Chen, 2014, Willson et al., 1999), among which wellbore instability is the most important problem, costing more than 100 million dollars per year worldwide (Aadnoy and Ong, 2003, Fjaer et al., 2008). This instability is not limited to the drilling stage but it may take place during the production stage when depletion begins or a secondary recovery is initiated (Khishavand et al., 2016).
Wellbore stability analysis applied to a directional drilling practice is the approach used to determine the safe mud weight window by taking the effect of wellbore geometry, changes in geomechanical parameters, and in-situ stress states into consideration (Aadnøy and Looyeh, 2011, Ijeoma and Adeleye, 2015, Gholami et al., 2015a). However, due to complexity of geological formations and lacks of good understandings of mechanical anisotropy, on many occasions, material isotropy is assumed to determine geomechanical parameters of rocks and estimations of safe mud weights of a drilling practice (Bradley, 1979). This traditional assumption can easily result in under- or overestimation of elastic stiffness, strength and in-situ stress parameters particularly for a well drilled into laminated formations such as shales (Zhang, 2013).
Studies discussing the effect of bedding planes on the stability of deviated wellbores was initiated by Aadnoy (1988), who concluded that ignoring the anisotropy of rocks may result in significant errors in estimation of failure induced on the wellbore wall. Ong and Roegiers (1993) revealed that a deviated borehole is influenced by rock anisotropy, differentials in-situ stress and thermal conditions. Aoki et al. (1993) studied borehole failures in bedded formations, and found that shear failures within the rock matrix and failures along the bedding planes are significant factors leading to excessive shear failure. McLellan and Cormier (1996) found that wellbore instability in shale formations induced due to a complex interaction among bedding planes, in-situ stresses, well trajectory and rock properties. Last and McLean (1996) indicated that the design of trajectory is vital when wells are drilled into a foliated formation. Okland and Cook (1998) discovered that instability might be very severe when a well is drilled parallel to bedding planes. Zhang et al. (2006) studied the laboratory test data of rock strengths in weak rocks, and improved geomechanical modelling of wellbores considering rock anisotropy. Al-Ajmi and Zimmerman (2006) analysed borehole wall failure using a weak-plane model and indicated that it is affected by steeply dipping bedding planes. Wu and Tan (2010) indicated that weak bedding planes can be very unfavorable during drilling through shale formations. Li et al. (2012) established a stability model for shale gas reservoirs and emphasized the effect of foliations. Okland and Cook (1998) introduced the concept of the attack angle (i.e., the angle between the direction of the wellbore and the weak bedding plane) which was further studied by Bassey et al. (2013) as a factor affecting the selection of the best trajectory for wells drilled into shale formations. Liu et al. (2014) emphasized the influence of weak planes, in-situ stress and well tracks on stability of boreholes. Chenevert and Sharma, 1993, Horsrud, 2001 and Ma and Chen (2015) indicated that water contents in shales change the strength of weak bedding planes, causing drilling difficulties and wellbore wall failure. Liu et al. (2016) did an analysis on horizontal wells drilled into shale gas formations and indicated the importance of bedding planes and strength anisotropy.
Although the effect of weak planes on wellbore stability has been widely recognized and studied, due to the complexity of problems in deviated wells and many factors, which may have been included, the failure mechanism of wellbore in weak planes formations has not been fully understood yet. The aim of this paper is to provide a workflow for stability analysis of wellbores drilled directionally into anisotropic formations. A real case study from one of the wells drilled into tight gas shale formations and deviated due to presence of bedding planes was presented to indicate how a mechanical earth model can be built in this kind of severe condition.
Section snippets
Geological setting
The well of this study, Asgard 1, is a vertical exploration well located in the onshore Canning basin, Western Australia. The Early Ordovician to Early Cretaceous pericratonic Canning Basin encompasses 506,000 km2, of which 430,000 km2 are onshore. The late Cretaceous and Tertiary sediments of the basin, however, are mainly controlled by the offshore portion. The basin has a maximum length of 15 km, which is concentrated exclusively in two NW trending depocentres. Fig. 1 shows the regional
Theory of transverse isotropy (TI)
According to Hooke's law, a complete characterization of an anisotropic rock requires 21 independent stiffness constants. However, if a material exhibits any physical symmetry, the number of independent stiffness constants can be reduced to as many as nine (an orthotropic model) or even five (a transverse isotropic model). Transversely Isotropy (TI) is perhaps the most common anisotropic model, demanding five independent elastic parameters for a complete rocks characterization (Amadei, 1983).
Methodology
Mechanical Earth Modeling (MEM) is a strategy taken to determine the safe mud weight window of drilling through coupling geomechanical parameters with in-situ stress conditions. This modeling is often started by estimation of elastic and strength parameters together with pore pressure followed by determination of stress states in the field. Hoop, radial and axial stresses around the borehole are then calculated by considering a constitutive law such as Kirsch's equations and linked to shear and
Results and discussions
Having followed the methodology developed to determine the safe mud weight window of directional wells drilled into anisotropic formation, the stiffness and dynamic elastic parameters of shaly interval were determined in the first step. Fig. 9 shows the stiffness and elastic parameters obtained through the above analysis.
Looking at the variation of stiffness parameters brought in the third track of Fig. 9, it seems that , indicating the fact that HTI is a sophisticated model chosen
Conclusion
In this paper attempts were made to provide a guideline as to how stability analysis and safe mud weight window determination can be properly done in directional wells drilled into anisotropic formations. A case study from a wells drilled through abnormally pressured and anisotropic shale formations was presented to show the application of the proposed methodology. The results obtained indicated the ANNIE model can provide a good estimation of stiffness parameters required to estimate the
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2022, International Journal of Rock Mechanics and Mining SciencesCitation Excerpt :Previously Khan et al.80 reported that ignoring the effect of elastic anisotropy can result in underestimation of wellbore stresses and overestimation of breakdown pressures, thereby affecting the safe mud weight window and, as a result, wellbore instability. Several researchers have also reported that strength anisotropy affects the safe mud weight window and therefore will influence the stability of the wellbore.5,18,80–82,84 In the current study, the compressive strength is fairly anisotropic in all of the studied lithofacies, as the compressive strength is similar in both parallel and perpendicular directions.
Deformation Rate Analysis: How to determine in-situ stresses in unconventional gas reservoirs
2021, International Journal of Rock Mechanics and Mining SciencesCitation Excerpt :It should be noted that the horizontal stresses shown in the fourth track of Fig. 11 were obtained from the MEM of the Well A using the poro-elastic equations which were calibrated against the LOT of the well. Poro-elastic equations have been widely used to determine the horizontal stresses with the details given in the study of22,23 and.24 The horizontal stresses obtained were further calibrated against the values determined by the Australian Worldwide Exploration (AWE) to ensure the accuracy of the results.
Thermo-poro-elastic analysis: The effects of anisotropic thermal and hydraulic conductivity on borehole stability in bedding formations
2020, Journal of Petroleum Science and EngineeringCitation Excerpt :That is, the physical parameters within the beddings are not the same as that in the vertical direction of the beddings. This anisotropic elastic theory has already been applied to calculate both the time-independent stresses (Aadnoy, 1987, 1996; Ong and Roegiers, 1993; Okland and Cook, 1998; Gaede et al., 2012; Liu et al., 2016; Gholami et al., 2017; Setiawan and Zimmerman, 2018) and the time-dependent stresses (Abousleiman et al., 1995; Abousleiman and Cui, 1998; Ekbote; Abousleiman, 2006; Dokhani et al., 2016; Cao et al., 2017; Gao et al., 2017a, 2017b; Kanfar et al., 2017) around the borehole. However, rather big limitations exist because this anisotropic elastic theory can only be applied to a specific condition when the borehole axis is perpendicular to beddings.
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2019, International Journal of Rock Mechanics and Mining Sciences