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Borehole breakout initiation, progression, and stabilization are modeled using a semi-analytical method based on Melentiev’s graphical conformal mapping procedure, and Kolosov–Muskhelishvili complex stress potentials. The only input data required are the elastic moduli of the rock, the Coulomb strength parameters (cohesion and angle of internal friction), and the far-field stresses. The stresses around the borehole wall are computed, the region in which the rock has failed is then “removed”, creating a new borehole shape. This process is iterated until a shape is obtained for which the breakout will progress no further, and a stable state has been reached. This modeling shows that stresses around the flank of the breakout evolve so as to reduce the propensity for shear failure, which helps to explain why the breakout width remains relatively constant throughout the process, even as the breakout region deepens radially. The failed area around the borehole becomes smaller and more localized, as the breakout tip sharpens and deepens. Using the Mogi–Coulomb failure criterion, a good match is obtained between the modeled breakout geometry and the geometry observed by Herrick and Haimson in laboratory experiments on an Alabama limestone. The new method leads to a correlation between breakout geometry, rock strength properties, and in situ stress. The paper ends with a critical discussion of the possibility of inferring the in situ stress state from observed breakout geometries.

A unified semi-analytical solution based on graphical conformal mapping and complex variable methods is proposed to calculate the in-plane stress around an arbitrarily-shaped hole in isotropic or anisotropic materials. The method requires only the outline coordinates of the hole, the elastic moduli of the material, and the magnitude and direction of the far-field stresses. Comparison with many published results for a wide range of shapes, such as triangles, squares, ovaloids, and ellipses, has been carried out to validate the method. The method has also been applied to a highly irregular geometry that has been observed in a breakout of a subsurface borehole. The solution is essentially closed-form, in the sense that it can be explicitly expressed in terms of the mapping coefficients, and parameters that depend only on the elastic moduli of the materials. With such a degree of flexibility, the method will be useful to study the effect of hole geometry on the stress distribution around holes in isotropic or anisotropic materials.

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 pounds-per-gallon (ppg). Furthermore, it is demonstrated that the risk posed by bedding slippage, for a wellbore with an inclination between 15° and 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.

A method of constructing high productivity and high wellbore dynamic stability wells has been developed for EOR field development application. It integrates a heuristic reservoir engineering modelling method for determining optimum drainage points with high dynamic flow and a 3D coupled reservoir geomechanical modelling method for identifying low sanding propensity regions within the entire reservoir. The reservoir geomechanical method couples dynamic reservoir modelling with geomechanical modelling. It can account for in-situ stress changes associated with reservoir pressure change, and predict any potential geomechanical-related physical events for the remaining life of the field. Correspondingly, the generated drainage map and 3D sand production critical drawdown cubes can then be combined to identify global optimum well placement locations within the reservoir, layer by layer. Drainage points selected by this heuristic reservoir modelling method can be correlated with their respective EUR (Estimated Ultimate Recovery) values, while the 3D critical drawdown cubes can identify reservoir regions with low sanding propensity. This combined approach can therefore lead to the development of multi-layer commingle wells having various angles of reservoir penetration, for optimizing well productivity and EUR value without the requirement of sand control. Illustrated by a case study in brownfield reservoirs, a non-linear well trajectory which maximizes reservoir fluid contact in a prolific sand layer can be designed without any sand control completion for sanding mitigation through optimization and management of production plan.

Oil and Gas exploration in deep water poses significant challenge of drilling through narrow mud weight windows with little margin for error. A thorough pre-drill planning complemented with more informed real-time risk detection and intervention while drilling the well, ensures efficient and safe drilling. This paper is a summary of one such recent successful drilling campaign.In 2010, Eni Indonesia one of the big operators in Indonesian deep water, drilled 4 Deep water wells in varying water depths as deep as 2700ft in Kutei and Tarakan basin, offshore East Kalimantan. Real-time geomechanics services were employed to provide estimate of safe mud weight windows while drilling to the drillers. Under this service in the planning phase, pre-drill pore pressure models generated for each of the planned wells, revealed over pressured reservoirs with narrow drilling margin between pore pressure and fracture gradient. In the execution phase, a customized workflow was followed to ensure that correct pore pressure is estimated in real-time using all the available log and drilling data generated while drilling and that only properly interpreted and validated safe mud weight windows is communicated to the drilling team on board the drill-ship and town through a flag-based communication tree.Geomechanics engineers working onboard the drill-ship ran proprietary software which streamed in real-time logging-while-drilling data – Gamma Ray, Resistivity, Density and Neutron porosity logs, caliper etc. Using these logs multiple pore pressure and fracture gradient estimates were generated in real-time. These estimates were interpreted in conjunction with drilling mechanics, mud logs, cuttings analysis, Equivalent Circulating Density and Equivalent Static Density to generate safe mud weight windows. This window was further calibrated during drilling with wireline and logging-while-drilling recorded formation pressure measurements and leak-off tests. Real-time update of safe mud weight window enabled several critical decisions like altering casing depths and mud weight management.

The Serang Field, offshore East Kalimantan, Indonesia was discovered in 1973. Production started in 1993. Although it is a mature field with 120 wells, drilling still faces challenges in the form of severe lost circulation and stuck pipe events. Side tracks often need to be drilled and drilling budget overruns can be significant. The 2006 drilling campaign of 5 wells saw the loss of several BHAs and needed 4 sidetracks, due to stuck pipe events. In order to improve the drilling operation and to uncover root causes of the losses and pack-offs, a detailed geomechanical evaluation was conducted. The study began with a comprehensive data audit and drilling event review. All the captured information and log data was then used to create a Mechanical Earth Model (MEM) for wellbore stability planning of future wells. Key findings of the study were that lost circulation occurred in high permeability zones in shallow carbonates (8-1/2" intermediate hole section) and were not caused by drilling induced fractures. A contingency plan was developed to drill to section TD with sea water, should losses occur. Breakouts, aggravated by low mud weights, lost circulation events, high deviation and slim hole contributed to poor hole cleaning, which caused the packoff incidents in the 6 1/8" sections. Safe and stable mud weight windows were established to mitigate hole collapse and stuck pipe. Since losses can occur even at low mud weights but do not usually have severe consequences, the team decided it was more important to focus on avoiding breakouts and improve hole cleaning with higher mud weights, improved drilling fluids with stress cage system to drill through depleted reservoirs, controlled drilling, ECD management to prevent formation breakdown and the use of Rotary Steerables to improve hole cleaning. The recommendations of the study were incorporated during a 3 well drilling campaign in 2009. All 3 wells were drilled and completed within budget and without stuck pipe incidents.