Reservoir stimulation by fracturing, one of the first applications in the oil field to use geomechanics methods, is still a major development area. Exploitation of shale reservoirs has caused a surge of interest in the mechanical anisotropy of rock, which was not widely appreciated until about 2000. To make improved predictions of fracture geometry and growth, models for stress and strength and interpretations of sonic and resistivity measurements must be modified to account for anisotropy. Advances in sonic logging tools and interpretation have made this possible.
One feature common to all of these areas is the mechanical earth model (MEM), which is a collection of the data needed to make quantitative and qualitative predictions of the subsurface geomechanical environment. These data include the stresses in the Earth, pore pressure, rock elastic properties, strength and fabric and nonnumerical data such as the presence of intense natural fracturing. An MEM can be simple or complex, be large or small and be 1D, 2D, 3D or 4D—three spatial dimensions plus time—according to the complexity of the field and phenomena of interest. The most important defining feature of an MEM is that its data are related to the rocks that are being drilled, fractured or otherwise affected by field operations, rather than a particular well or set of wells. A second feature is that it is designed to be updated as new data become available from ongoing operations. Data sources for an MEM include any that give information on stress and mechanical behavior; such sources include wireline and LWD logs, cores, cavings and cuttings, regional geology and all types of seismicity.
Ongoing challenges for geomechanics include improvements to
- sources of data for predicting in situ stress and rock properties
- the use of anisotropic information for predicting deformation during exploitation of unconventional resources
- the treatment of fault and fracture displacements within numerical models.
In addition, to help improve the application of geomechanics to various sec-tors in the industry, engineers must have a better understanding of the relationships between rock failure and operational failure for wellbore instability and sand production.
Geomechanics in the oil field has come a long way from its early days as an adjunct to sonic logging. It is recognized as an important part of nearly all aspects of petroleum extraction and has been crucial in improving efficiency and driving down costs. The application of geomechanics in new reservoir types and mature ones and its integration into operators' workflows, along with the introduction of new measurements and techniques, will ensure its continuing role in the industry. From here, its operational impact will only grow. The application of geomechanics for revitalizing mature fields is imperative and will affect activities such as infill drilling, compaction mitigation and refracturing.