Microseismic signatures of hydraulic fracture growth in sediment formations: Observations and modeling

We analyzed a microseismic data set from hydraulic fracture stimulation of the gas field in west Texas. We used an automated wave-picking algorithm and obtained a high-density image of induced microseismic events accompanying the hydraulic fracture growth. The microseismic locations delineated a planar fracture growing predominantly in the horizontal direction; the vertical growth was limited by shale layers. A strongly asymmetric fracture with a twice longer eastern wing containing 80% of the located events was observed. Owing to the planarity of the microseismic cloud, it was possible to reduce the location problem to two dimensions and to use only S waves for event localization. Thus, because of the larger amplitudes of S waves, a fourfold increase in the number of located events was achieved. We find that the length of the hydraulic fracture increased, for different depth intervals, both linear and nonlinear in time. We use hydraulic fracture models to explain the spreading of the microseismic front, whose nonlinear time dependence could indicate either a diffusive fluid flow or a two-dimensional growth of the hydraulic fracture. By the maximum-likelihood fitting of the observed fracture growth and by inverting for its parameters, we find that the fracture was 7 - 10 mm wide and that nearly the whole injected volume was used for creating the new fracture, that is a negligible diffusive infiltration of the injected fluid into the reservoir rock occurred.