The field stress parameters, direction and magnitude of horizontal and vertical stresses, are important factors in the design of hydraulic fracturing treatments in unconventional reservoirs. It is well understood that an induced hydraulic fracture propagates in the direction of maximum horizontal stress, suggesting that horizontal wells must be perpendicular to this direction for efficient well-reservoir hydraulic connection during treatment and production. The horizontal stress anisotropy also affects the optimum well spacing and stage length.
Numerical studies show that in naturally fractured reservoirs, higher horizontal stress anisotropy promotes more planar stimulation patterns that extend farther away from the well. For the same treatment parameters, low horizontal stress anisotropy leads to near-wellbore complexities. The magnitude of stresses also controls the minimum horsepower requirement for treatment. The bottomhole injection pressure must exceed and be maintained above the minimum reservoir stress for a hydraulic fracture to initiate and propagate through the formation. A relatively accurate estimate of the vertical stress magnitude can always be obtained by integrating the density logs to the reservoir depth. The minimum horizontal stress magnitude can also be determined from well tests, such as a mini-frac test or diagnostic fracture injection test. There is, however, no direct and easy way to measure the magnitude of maximum horizontal stress (SHmax) at the reservoir depth. Wellbore breakout analysis is commonly performed to constrain the SHmax magnitude based on observed wellbore breakouts. The result of such analysis, however, is more representative of the well-scale stresses rather than the reservoir-scale stress state, unless several wells within the same region can be studied simultaneously.
In this paper, we present a new methodology to estimate the direction and magnitude of maximum horizontal stress using microseismic focal mechanisms. The moment tensor inversion technique is applied to establish a moment tensor for all acquired microseismic events. The event focal mechanism and fracture orientation is determined by the fault plane solution of the moment tensor. Each focal mechanism is treated as an independent field experiment by which we can back-calculate the magnitude of SHmax. The field SHmax magnitude is then determined by analyzing all calculated values for all qualified events. The described methodology is used to estimate field SHmax for five cases in three different formations; the Marcellus, Eagle Ford, and Wolfcamp. Using the known orientation of fractures and full-field stress tensor, the minimum failure pressure is calculated. An example of this analysis is provided for one of the studied Marcellus cases.
