A new methodology for estimating field horizontal stress from microseismic focal mechanisms

The full stress state in a formation is characterized by the directions and magnitudes of the three principal stresses. It is common practice to take the vertical stress, ϭv, and the minimum and maximum horizontal stresses, ϭhmin and ϭHmax, as the principal stresses in unconventional reservoirs. Considering the relatively high depth of most unconventional reservoirs, which results in a very high vertical stress magnitude, this assumption holds true in most cases unless a geological feature such as a fault or fold changes the stresses locally. The magnitude of vertical stress can easily be determined by integrating the density of overburden rocks from density logs. A good estimate of ϭhmin magnitude can be obtained from well test results such as a mini-frac test or a diagnostic fracture injection test. The magnitude of ϭHmax, however, remains the most challenging stress component to determine.