A key to the success of long-term storage of CO2 in depleted oil or gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. This paper provides a review of the geomechanical factors affecting the hydraulic integrity of the bounding seals for a depleted oil or gas reservoir slated for use as a CO2 injection zone. Potential leakage mechanisms reviewed include fault reactivation, induced shear failure of the caprock, out-of-zone hydraulic fracturing, and poorly sealed casing cements in enlarged, unstable boreholes. Parameters controlling these mechanisms include the upper and lower bounds of pressure and temperature experienced by the reservoir, the orientation and mechanical properties of existing faults, rock mechanical properties, in situ stresses, and reservoir depth and shape. Approaches to mitigate the likelihood of geomechanics-related leakage include the identification of safe upper limits on injection pressures, preferred injection well locations, review of historical records for reservoir pressures, temperatures and stimulation treatments, drilling program design to mitigate rock yielding in new wells, and assessment of wellbore integrity indicators in existing wells. Introduction In order to achieve significant reductions in the atmospheric release of anthropogenic greenhouse gases, the implementation of technologies to capture carbon dioxide (CO2) and store it in geological formations will be necessary. Deep saline aquifers have the largest potential for CO2 sequestration in geological media in terms of volume, duration, and minimum or null environmental impact(1). The first commercial scheme for CO2 sequestration in an aquifer is already in place in the Norwegian sector of the North Sea, where 106 tonnes of CO2 are extracted annually from the Sleipner Gas Field and injected into the 250 m thick Utsira aquifer at a depth of 1,000 m below the sea bed(2). In light of the economic benefits of enhanced oil recovery (EOR) derived from CO2 injection in oil reservoirs(3), these types of reservoirs will be attractive CO2 injection targets and, most likely, CO2 storage in depleted oil and gas reservoirs (or in conjunction with EOR) will be implemented before CO2 storage in aquifers. An advantage of CO2 storage in depleted oil or gas fields is the fact that much of the infrastructure for fluid injection (e.g., wellbores, compressors, pipelines) is already in place. The Weyburn CO2 Monitoring and Storage Project in Saskatchewan, Canada(4) is an example of a large-scale application of EOR operations using anthropogenic CO2, in which the oil reservoir is being evaluated for subsequent use as a long-term storage zone. A key to the success of long-term storage in depleted oil and gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. The initial integrity of this "bounding seal" system is governed by geological factors. A considerable amount of effort has been devoted to the development of procedures for assessing fault seal capacity in potential hydrocarbon reservoirs(5).

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Wellbore instability can lead to expensive operational problems during the drilling, completion and production of horizontal and inclined wells. This paper reviews the direct and indirect symptoms of wellbore instability, its root causes, and various empirical and deterministic modelling approaches to predicting the risk of hole collapse or convergence. In general, linear elastic models that are only concerned with stability at the wellbore wall often give overly pessimistic predictions. An alternative approach, using the extent of the "yielded" zone around an unstable wellbore and the kinematics of rock detachment, is proposed for practical risk assessments. A case history for an open hole completed horizontal well in a limestone reservoir under high drawdown is described. General guidelines for conducting field-oriented stability assessments conclude the paper. Introduction Wellbore instability during the drilling, evaluation, completion and production phases of a well has become an increasingly important concern for many operators applying horizontal well technology. Traditional conservative completion methods for vertical wells are being challenged as operators attempt to reduce well costs and still derive the improved productivity and access to hydrocarbon reserves offered by horizontal wells. More recent horizontal well innovations include the use of underbalanced drilling techniques(l), slimhole completions, side track or re-entry wells with open hole build sections(2,3), and multiple laterals from a single vertical or horizontal wellbore(4). In applying these new technologies, there are often issues posed during the well planning stage where the risk of hole collapse in the short or long term must be addressed. In many cases, the selection of an optimal strategy to prevent or mitigate the risk of wellbore collapse might compromise one or more of the following other elements of the overall well design: the rate of penetration; the risk of differential sticking; drilled cuttings and mud disposal options; hole cleaning abilities; hole size, and consequently the completion and stimulation options available; formation damage risk; stimulation requirements; the ability to log the hole; and the selection of surface sand handling facilities (where sand production is anticipated). In many cases there may be insufficient experience with a given reservoir and the desired completion, hence the prior performance of vertical wells cannot be used, by itself, to guide the well design. This paper reviews the symptoms of wellbore instability and its fundamental causes. Published approaches to wellbore stability prediction will be described, particularly those which address the most common problems faced in developing normally to slightly under pressured oil and gas fields. Emphasis is placed on techniques and conditions applicable to Western Canada where there has been a rapid pace of horizontal well development, particularly with re-entry wells. Predictive techniques applicable to build, inclined or horizontal sections of a well, during the drilling, completion and subsequent production phases, will then be described. Selected case histories from the literature are cited and an example of a wellbore stability prediction for Shell Canada's first horizontal open hole completion is described. This paper will not review all the wellbore stability models which have been developed for vertical wells.

Underbalanced drilling techniques are often considered to avoid or mitigate formation damage, reduce lost circulation risk, and increase drilling rate of penetration. However, drilling with a bottomhole pressure less than the formation pore pressure will usually increase the risk of borehole instability due to yielding or failure of the rock adjacent to the borehole. Numerous theoretical models for assessing borehole collapse and fracture breakdown risks exist. However, until recently, it has been difficult for non-specialists to use many of these models because they are not easily implemented, or because they required input parameters that are unfamiliar or difficult to obtain. A userfriendly PC Windows TM -based software package called STABView TM has been developed to help the well designer determine the optimal range of bottomhole pressure for underbalanced drilling, i.e., the bottomhole pressures that are high enough to avoid severe hole collapse, yet low enough to avoid initiating hydraulic fractures. The software has been designed to perform rapid parametric analyses for all types of wells in most geological settings. Guidance in the selection of rock properties and in situ stresses is provided to the user with an online database of typical values and a comprehensive help utility. Applications of the software to underbalanced drilling of horizontal wells in a number of sandstone reservoirs are demonstrated. FIGURE 1: Shear yielding occurs for underbalanced conditions due to the absence of a support pressure on the borehole wall. FIGURE 2: Radial tensile fracturing occurs due to steep inflow gradient. PEER REVIEWED PAPER ("REVIEW AND PUBLICATION PROCESS" CAN BE FOUND ON OUR WEB SITE) Journal of Canadian Petroleum TechnologyFIGURE 3: Output from a 3D linear elastic borehole stability analysis, showing the equivalent circulating density required to prevent hole collapse as a function of well trajectory. Areas with red shading indicate well trajectories for which overbalanced bottomhole pressures are required. 34Journal of Canadian Petroleum Technology FIGURE 5: Effect of filter-cake efficiency on Normalized Yielded Zone Area (NYZA) for a range of equivalent circulating densities, Cardium Formation sandstones, West-central Alberta. FIGURE 6: Pressure drop across a filter-cake or wall coating for overbalanced conditions, and the definition of efficiency (ε). FIGURE 7: Effect of filter-cake efficiency on Normalized Yielded Zone Area (NYZA) for a range of equivalent circulating densities, Cardium Formation sandstone-shale interbeds, West-central Alberta. 36 Journal of Canadian Petroleum TechnologyFIGURE 10: Effect of peak cohesion on Normalized Yielded Zone Area (NYZA) for a range of equivalent circulating densities, Eocene sandstone, Lake Maracaibo, Venezuela. FIGURE 11: FLAC output showing the extent of shear yielding and tensile failure predicted around a borehole during underbalanced drilling of an Eocene sandstone, Lake Maracaibo, Venezuela. FIGURE 12: Effect of well azimuth on the fracture breakdown gradien...

The effects of pore pressure penetration and time-dependent rock strength on the growth of the yielded zone around a borehole are analysed using an elastic-brittle-plastic model. Solutions for two possible cases are developed:no change in permeability, anda significant increase in. permeability upon yielding. The extent of the yielded zone is sensitive to a number of mechanical parameters, of which the residual strength of the rock is most critical. In bothcases, the rate of yielded zone growth depends strongly on formation permeability (both prior to and after yielding) and mud filtrate viscosity. In case (2), the initial extent of the yielded zone is also critical to the rate at which it will grow. Model predictions are compared to field data from a western Canadian setting where time-dependent borehole enlargement occurred in a shale interval. Introduction Borehole instability in shales is a frequent problem experienced during drilling. Although the potential destabilizing effects of clay hydration and swelling have long been known and accepted, it is now recognized that borehole instability in shale sections is also strongly dependent on mechanical parameters, i.e., the state of effective stress around the borehole and the strength of the shale. The following discussion describes the mechanical factors affecting borehole stability. Hydration effects are only considered indirectly, as they may affect the strength of the rock. Elastic-brittle-plastic Borehole Stability Model A number of non-linear borehole stability models have previously been developed. These include damage mechanics models(1, 2), bifurcation models(3), non-continuum models(4, 5), linear elastic borehole breakout models(6), strain-hardening elastoplastic models(7) and strain-weakening, elastic-brittle-plastic models(8–10). The paper presented here is a new development of the latter type of model. The defining feature of the elastic-brittle-plastic model is the assumption that, once a rock's peak compressive strength has been exceeded when stresses around the borehole increase during excavation, the rock yields but does not fall into the borehole. Instead, an annulus of weakened rock (referred to as the yielded or plastic zone) develops around the well, peak stress is redistributed away from the borehole surface, and eventually a stable configuration is achieved (Figure 1). Since the yielded zone will be susceptible to spalling due to pressure surges during trips and mechanical erosion by the drillstring, the larger this zone is the greater the likelihood that hole enlargement and instability-related problems will occur. In order to develop a closed-form solution to this problem, it is necessary to make a number of assumptions concerning the mechanical behaviour of the rock. Firstly, it is assumed that deformation is linear elastic until peak stress is reached, then strain weakening occurs instantaneously and stress is reduced to a residual level (Figure 2). It is also assumed that peak and residual strength of the rock can be represented by linear Mohr-Coulomb criteria, plane strain conditions prevail, the material is homogeneous and isotropic, boundary stresses are uniform (so that the problem is axisymmetric), and the borehole axis is parallel to one of the principal in situ stresses.