Imaging Geometry And Growth Rate Of A Hydraulic Fracture Zone By Locating Induced Microearthquakes

Abstract

The hydraulic fracturing technique has become an important tool in the enhancement of hydrocarbons recovery, geothermal energy extraction, and solid waste disposal. The characterization of geometry parameters and growth rate of a hydraulic fracture zone is an important task for monitoring and assessing subsurface cracks. In this paper, we develop a location approach to determine the precise hypocenter locations for a cluster of seismic events induced by hydraulic fracturing. Two techniques were used in our location scheme: waveform correlation and grid search methods. The waveform correlation method allows us to obtain more accurate differential arrival times among events within in a cluster.. The grid search method is suitable when dealing with a nonlinear location problem. We applied our method to seismic waveform data from a hydraulic fracturing experiment at the Los Alamos Hot Dry Rock geothermal site and determined hypocenter locations for 157 induced microearthquakes. The maximum absolute and relative location errors were estimated to be 30-39 meters and 3-9 meters, respectively. Among the 157 events, 147 microearthquakes occurred in a tight cluster of a dimension of 40 meters, roughly defining a vertical hydraulic fracture zone. The length, height, and width of the hydraulic fracture zone were measured to be 40, 35 and 5 meters, respectively. The orientation of the fracture zone is estimated at about N400W. Analysis of the temporal-spatial pattern of the induced microseismicity revealed that the fracture zone grows significantly in a two-hour period during the hydraulic injection. Using seismicity distribution in time and space in this period, we determined that the fracture zone grows toward the northwest along the fracture zone strike with a growth rate of 0.1-0.2 meters per minute.