A mechanical earth model (MEM) is a repository of data—measurements and models—representing the mechanical properties of rocks and fractures as well as the stresses, pressures and temperatures acting on them at depth. Engineers and geoscientists use it to understand how rocks deform, and sometimes fail, in response to drilling, completion and production operations. Each data point in an MEM is referenced to its 3D spatial coordinates and time of sample collection.
Rocks deform in a variety of ways in response to stress:
- Some rocks, such as granites, are stiff and strong.
- Some, such as mudstones, are soft and weak.
- Some, such as salts, with sufficient time can flow.
An MEM provides information about mechanical behavior and strength by using relationships between rock properties, induced deformations and ambient conditions. Because of the layered fabric of rocks or the presence of fractures, rock properties are frequently anisotropic—their properties are not the same in all directions as they would be with isotropic media.
An MEM documents the ambient conditions, including the stress, fluid pressure, temperature and fluid content, relevant for a geomechanical analysis. An MEM may represent a snapshot at a time of interest—for instance, the virgin conditions of a reservoir—or it may track how conditions evolve as the reservoir is being produced.
Constructing an MEM
To build and calibrate an MEM, geoscientists characterize the reservoir and overburden and then use monitoring measurements and modeling techniques. Measurements come from a variety of sources such as seismic data, well logs, image data and in situ temperature, pressure and stress measurements. Additional sources are mud logs, cuttings data and laboratory mechanical test data on core samples (Figure 1). Knowledge from external databases, such as the World Stress Map—a compilation of present-day tectonic stresses—can also be included in MEMs.