A methodology for wellbore stability analysis of drilling into presalt formations: A case study from southern Iran

Drilling into presalt formations has been a long-standing issue due to the rapid changes in the diameter of the borehole during drilling operations either because of creep or wash-out dissolution. There have been many studies on characterization of salts, with many mathematical models being presented to estimate the pressure induced due to the squeezing salt sheets. However, the results of none of these models have been fully validated against real field data and some recommendations have been made based on numerical simulations. In this study, attempts were made to introduce a methodology based on damage mechanics for wellbore stability analysis of a wells drilled in the southern part of Iran. The results obtained indicated that the presence of a thick salt layer in the well has resulted in significant wellbore closure in the intervals above the reservoir section. It was also found that the salt exhibits viscoelastic behaviour during drilling due to the homogeneous temperature which has not reached the threshold limit of viscoplastic boundary. A complicated change in the stress regime was also observed which could be linked to the existence of the thick salt layer or presences of a fault crossing the well. Therefore, it is recommended to further validate this model in other wells using the methodology presented

Classification Of Drilling-induced Fractures And Their Relationship To In-situ Stress Directions

Natural and drilling-induced fractures that can be identified on borehole walls are classified in terms of the failure initiation mechanisms that generate them and the principal stress orientations with respect to well trajectory. Fracture initiation mechanisms include tensile failure in vertical holes, tensile failure in inclined holes, tensile failure in elliptical boreholes, tensile failure resulting from plastic deformation, shear failure in vertical boreholes subject to low fluid pressures, shear failure in vertical boreholes subject to high fluid pressures, shear failure in inclined boreholes subject to low fluid pressures, shear failure in inclined boreholes subject to high fluid pressures, and failure in boreholes subject to horizontal stresses of equal magnitudes. Analyzing these situations assuming linear elasticity shows that the geometry of fracture traces on borehole walls depends on the in-situ stress tensor, the relative orientations of the stress tensor and the borehole, the fluid pressures in the borehole and in the surrounding rock, the rock''s cohesive strength, and its friction angle for shear. Furthermore, hydraulically induced tensile fractures are governed by the least principal stress, but shear fractures are governed by the intermediate and the least principal stresses, contrary to previous conclusions. A classification based on the fracture initiation analysis is presented and is illustrated by examples of different types of borehole wall fractures recorded by image logging tools run in wells in western Canada

A thermo-poroelastic analytical approach to evaluate cement sheath integrity in deep vertical wells

© 2016 Elsevier B.V.Failure of cement sheath due to casing expansion or formations pressure during completion or production stages of HPHT or deep vertical wells is a very common phenomenon. There have been many studies providing approaches to predict cement sheath failure, where theory of elasticity or thermo-elasticity together with the plane strain concept were taken into consideration to obtain representative results. However, sedimentary formations in subsurface layers are exhibiting a poroelastic behavior and theory of elasticity may not be able to fully describe their behaviors when changes in pore pressure and in-situ stresses are taking place. In this paper, an analytical approach based on the theory of thermo-poroelasticity was presented to predict the possibility of cement sheath failure in deep structures. A separate numerical molding was also performed to evaluate the application of the approach developed. The results obtained indicated that a thicker cement can withstand a higher load applied by the formations and protect the casing against a significant collapse pressure. The temperature was also found as a significant contributor in increasing the pressure applied by the formation and casing on the cement due to pore fluid and steel expansions. Although some discrepancies observed between the results of the numerical simulation and the analytical model, it seems that the approach presented is able to provide reliable results considering the fact that interactions of material interfaces could not be included in the analytical modeling

A methodology for wellbore stability analysis in anisotropic formations: A case study from the Canning Basin, Western Australia

© 2016 Elsevier B.V.Stability analysis of directional wells has been a long standing issue due to complexity of geological settings and changes in the stress coordinate system when wells are being deviated. Although there have been many studies developing strategies to estimate the safe mud weight based on wellbore geometry and anisotropy of subsurface structures, the issue of instability in deviated wellbores has been widely experienced in many fields around the world, especially in Malaysia and Australia. The aim of this paper is to present a workflow for estimation of geomechanical parameters and stress states of boreholes drilled directionally into anisotropic formations. A case study from one of the wells drilled into tight shales was brought into attention to evaluate the application of the proposed methodology. The results obtained based on the interpretations of data and reports of similar incidents in the field indicated that the well was deviated due to drilling parallel to the direction of bedding planes, causing creation of blocky, and tabular cuttings with parallel surfaces at shakers. The Horizontal Transverse Isotropic (HTI) model was assumed and the ANNIE model, developed exclusively for shales, was considered for estimation of stiffness parameters required to fully characterize shale formations. Determination of the safe mud weight window using two well-known failure criteria with proven applications revealed that a three-dimensional failure criterion should have been applied to prevent the instability of the wellbore wall. In fact, following the underestimation of shear failure provided by the Mohr-Coulomb criterion, a 13 ppg mud would have been selected to save the well, which in turn could result in enlargements of the well and significant increase of completion costs. Although some practical conclusions are provided, more studies are required to evaluate the application of the proposed methodology in other wells worldwide

Geomechanical and Numerical Studies of Casing Damages in a Reservoir with Solid Production

© 2015 Springer-Verlag Wien Casings damage is a usually reported incident during production in many fields. This incident is conventionally induced by compressional, tensional, burst or collapse forces applied to the casing string. Excessive anisotropic and non-uniform stresses, causing shear failure in unconsolidated reservoirs, are one of the main reasons reported for the casing failure. In this paper, geomechanical and finite element numerical analysis was applied to model hydraulic and mechanical interactions between casing, cement sheath and formations in a carbonate reservoir located in Southern of Iran. The geomechanical analysis indicated that significant in situ stresses induced as a result of the fault reactivation and pore pressure reduction due to reservoir depletion could be the potential reasons for the casing damage experienced in this field. To assess this, numerical analysis was carried out to simulate the casing in the presence of existing forces during drilling, completion and production phases. It was found that excessive and non-uniform stresses surrounding the wellbore together with pore pressure reduction caused the formation to loss its strength and fail. This shear failure results in solid production, creation of the cavities and deformation of the casing because of the excessive buckling force. In addition, a new empirical equation for prediction of ultimate strength of the casing was developed according to the parameters introduced by sensitivity analysis

Application of quantitative risk assessment in wellbore stability analysis

© 2015 Elsevier B.V. Elastic and strength parameters, together with pore pressure and in-situ stresses are key parameters required to be known for determination of safe mud weight window (MWW) in vertical wellbores. Estimation of these parameters, however, is subjected to wide uncertainties mainly due to lack of adequate calibration information including lab and field test data. While there are literatures on the applications of probabilistic and risk analysis on wellbore stability evaluation, limited numbers of publications report on the impact of the chosen failure criteria in estimation of safe MWW under uncertain condition. In this study, data corresponding to a wellbore located in south part of Iran was analyzed using quantitative risk assessment to consider the effect of uncertainty on estimation of safe MWW using different failure criteria. The results indicated that Mogi-Coulomb and Hoek-Brown are more robust against the uncertainty of input parameters and mud weight used for this wellbore could have slightly been increased to reduce the shear failure of the borehole wall. The uncertainty in the input data might also be very critical for casing design when only a simple margin together with pore and fracture pressures are used to select the grade of the casing against burst or collapse loads. It was also noted based on sensitivity analysis that the maximum horizontal stress is the most effective parameter in estimation of MWW. This emphasizes the importance of a reliable estimation of in-situ stresses for safe drilling. The results presented here are based on a single case study, and further studies are still required to get any ultimate conclusion