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Stress Criticality of Fractures
In-situ Stress and Fracture Shear Fractured reservoirs are believed to be commonly in a meta-stable equilibrium, ie a state of incipient shear failure. The stress magnitudes and directions interact with both large and small-scale structures and with fluid pressures within the reservoir. Shear failure on a fracture occurs when the shear forces caused by anisotropic stresses are greater than the frictional strength of the fracture. The frictional strength results from the roughness of the fracture and the effective normal stress across the fracture. Increasing pore pressure reduces effective stress and, hence, promotes shear failure. Shearing and Fracture Permeability In a "critically stressed" rock mass, a subset of fractures are always close to failure. Repeated shearing on such fractures can keep flow paths open that might otherwise be closed by mineralisation. Our experience shows that the most important flow paths are the subset of "critically stressed" fractures in the rock mass. Shearing and Reservoir Pressure A consequence of the state of incipient shear is that reservoir pressure at discovery will be limited by shearing on preferentially oriented fractures. Higher reservoir pressures will always be dissipated by additional shearing. This limit on reservoir pressure can create inverse (or anomalous) pressure distributions related to the different stress gradients. This may be an important signpost to future reservoir behaviour. Benefits The orientation of the most productive features will be controlled by the interaction of the stress field and the fracture system. This means that by understanding the stress/fracture system, you will be able to design wells that preferentially intersect the most productive features. In significantly anisotropic stress fields, wellbore stability problems often occur and shear can even occur across the wellbore, resulting in the potential loss of the well. You can foresee these potential problems at the design stage and avoid them by drilling targeted wells. |
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This material is also covered in our Fractured Reservoir Training Courses. Examples from our work Shown below are contoured stereonets of m the ratio of shear stress to effective normal stress for every fracture orientation. Each stereonet corresponds to a different field and stress regime. Fracture shear failure has been shown to occur at m values from 0.6 to 1 (rough fractures in strong rocks). In the first three cases it can be seen that the maximum m value is between 0.6 and 0.7. In the much stronger granite the maximum is about 0.9. All the stereonets are oriented with the maximum horizontal stress across the page. |
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s hmax= s v > s hmin |
s v> s hmax > s hmin |
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s hmax> s v > s hmin |
s hmax>> s v > s hmin |
You can read about a case history involving critical shear in an Algerian field by visiting our Downloads page.