Shear-type failure at a wellbore or perforation is typically characterized by damage to the formation rock. For example during drilling, wellbore failure may occur with rock fragments breaking off from the wellbore wall, and for producing wells, the applied drawdown pressure can result in sanding. Perforations are more susceptible to such failures because, in general, they become less stable as the reservoir depletes. In this paper, a damage model using the Mohr–Coulomb failure criterion is formulated to better describe the failure of porous granular material. The damage variable is assumed to be related to the increase in porosity resulting from the loss of mass from the formation rock matrix. The comparisons of numerical predictions and triaxial compression tests data show that the model can accurately reproduce the mechanical behavior of sand materials. Numerical simulation of a sand production test on a perforated cylindrical specimen is performed. The proposed model demonstrated its capability to predict the accumulated sand mass and rate produced throughout the entire experiment. A threshold of equivalent plastic shear strain has been quantified to determine the onset of sand production in perforated wells; this threshold can be used for field application of sand production analysis on similar sand material. An application on a vertical well with horizontal perforations has been simulated numerically. The key objectives are to examine the critical drawdown pressure for different reservoir depletions and produced sand volume for different drawdown pressures. The critical drawdown window plot was generated and can be used to estimate the allowable drawdown pressure for different reservoir depletions. In addition, it was shown that the produced sand volume and sand mass rate increase with increase of drawdown pressure.

Wellbore failure can occur at different stages of operations. For example, wellbore collapse might happen during drilling and/or during production. The drilling process results in the removal of an already stressed rock material. If the induced stresses near the wellbore exceed the strength of rock, wellbore failure occurs. The production process also changes the effective stresses around the wellbore. Such changes in stresses can be significant for high drawdown pressures and can trigger wellbore failure. In this paper, the Mohr–Coulomb failure criterion with a hyperbolic hardening is used. The model parameters are identified from triaxial compression tests. The numerical simulations of laboratory tests showed that the model can reproduce the mechanical behaviour of sandstone. In addition, the simulations of multilateral junction stability experiments showed that the model was able to reproduce yielding and failure at the multilateral junction for different levels of applied stresses. Finally, a numerical example examining multilateral junction stability in an open borehole during drilling and production is presented. The results illustrate the development of a localized failure zone proximate to the area where two wellbore tracks join, particularly on the side with a sharp approaching angle, which would significantly increase the risk of wellbore collapse at the junction.

A new elastoplastic damage model, based on experimental evidences and using a thermodynamic framework, is proposed for rock materials under a large range of stresses. The elastic and plastic properties are affected by induced damage, which leads to material softening behaviour. After formulation of the model and determination of its parameters, triaxial compression tests on sandstone are simulated to show the model's consistency. In order to describe progressive failure process due to onset of strain localization, the elastoplastic model is extended by including a non-local damage formulation. Three representative cases are studied. The numerical results verify the well-known fact that the elastoplastic non-local damage model avoids spurious mesh sensitivity and provides a reliable description of the growth of the damage zone and the failure process.

Reservoir depletion/injection results in changes in effective stress, which may cause formation failure, propagation of existing fractures/faults and initialisation of new fractures/faults. A flow-deformation coupled reservoir geomechanical modelling approach has been applied to compute the change in reservoir effective stress and associated propagation of fractures and reactivation of faults. The computed inelastic strain change was used to predict the magnitudes of microseismic emissions associated with fault reactivations. The moment magnitude of microseismic events, consistent with the Richter scale, can be computed from the scalar value of their moment tensor. In a discontinuous model, the moment tensor can be calculated directly; in finite element models, the moment tensor is calculated by integrating the change in inelastic shear strain over the volume of the elements containing the failed fault. Coupled 3D geomechanical (deformation and fluid flow) simulations for Valhall field were conducted. Well rate and reservoir pressure histories were used as inputs to simulate reservoir depletion/injection. The 99 faults visible on seismic interpretations and 22 subregional fracture sets were included in the model. An elasto-inelastic cap model including water-weakening mechanism was used to model the chalk reservoir; the failure of faults and fractures were simulated by Mohr-Coulomb criterion. Prior to performing production simulation, preproduction stress modelling was carried out to reach an equilibrium stress state that was consistent with the in-situ condition, in terms of magnitude and orientation. Then, oil production at 112 wells and water injection at 15 wells for the period from 1982 to 2006 were simulated, during which the simulated microseismic events were in a good agreement with observations. The computed formation deformation in the reservoir and overburden was related to the in situ stress and faulting structure and was correlated over long distances. These characteristics are indicative of a near-critical process (and are also observed in flow rate correlations in the field). The coupled 3D geomechanical simulation provides a tool for validating modelled reservoir geomechanical effects against specific field data, and so enhances confidence in the implications predicted for drilling operations and reservoir management.

In order to ensure a successful drilling campaign, it was required to obtain knowledge of the possible location of natural fractures generated by different tectonic events affecting the Malay Basin through geological time to a high level of certainty. Acquiring this information would assist in the optimization of well trajectories to avoid these naturally fractured regions which would likely result in severe wellbore instability problems. As described by Petronas (1999&2007) the Malay Basin underwent an extensional phase during the Oligocene period with deposition of lacustrine source rocks. The Miocene was a time of fluvio-deltaic source and reservoir deposition and a compressional phase starting Early Miocene resulted in structural inversion. A major unconformity underlined the peak of the tectonic inversion in relation with the closure of Proto-South China Sea and the opening of South China Sea further to the North. In the Malay Basin, the geological history led to a complex structure trapping which requires further works and then an innovative approach based on the geomechanics was proposed to predict the locations containing natural fractures. The objective of the study was to model the heterogeneous paleo stress and strain states by the construction of a geomechanical model in this field and to relate the predicted strain and stresses to fracture characteristics. The first part of the study was to quality control the interpretation of the 3D structure of the area, which had been constructed from 3D seismic interpreted surfaces and faults. 1D Mechanical Earth Models computed from well data provided the required elastic parameters for the 3D structural restoration. The technique used to perform the restoration was based on the Finite Element Method, particularly suited for modeling complex geological structures. The second part of the study consisted in the computation of the paleo stress state via geomechanical forward modeling and relating the resulting plastic strain attributes with fracture density. In this step, the strains calculated from the displacements derived from the structural restoration were applied at the boundaries at each geological time-step in order to calculate the internal stress condition and plastic strain distribution accumulated through geological time.