Hydraulic fracture height in cased wells

A new method determines the top and bottom of the hydraulic fracturing in a cased treatment well from the microseismicity induced within the fracturing. The method uses a wall-locking sonde to passively record three-component data over a range of depths within and outside the fracturing immediately following either the fracture treatment or subsequent fluid injection into the fractured formation. The processed data (i.e., the background motion after removing the obvious events) show an anomalous inversion as a function of depth that delineates the fracture height. Specifically, the ratio of the horizontal motion component, H, to the vertical component, Z, inverts subdividing the recording traverse into three regions: those above and below the affected zone defined by $\text{H}\text{Z}\text{< 1}$ and the affected zone with $\text{H}\text{Z}\text{>1}$. The hydraulic fracture treatment creates the horizontally elongated, in situ stress-aligned affected zone that is comprised of a dilatant communicating network of new and preexisting fractures, joints, pores, and weaknesses, nucleates from the casing perforations. The zone has anomalously reduced elastic properties (i.e., seismic velocities) and acts as an embedded seismic and hydraulic waveguide with the borehole running through it. Pressure gradients, temperature gradients, and stress recovery within the low-velocity zone induce a pervasive microseismic cloud for several hours after the pressurization. The microsource cloud and the low-velocity zone create the data anomaly and its alignment with the extent of the affected zone. Computer simulations of the in situ setting and recording corroborate the $\text{H}\text{Z}$ inversion and its interpretation.