Seismic monitoring of hydrofracture operations can provide valuable information to guide production strategy. We present an algorithm for processing seismic fracture monitoring data. The method is more closely related to seismic reflection imaging methods such as the diffraction stack or Kirchhoff migration than the commonly used methods that use the delay time between P-wave and S-wave arrivals.
Tight oil and gas bearing formations can contain large amounts of valuable hydrocarbon resources however low formation permeability can preclude profitable operation of production wells. Artificial permeability can be created within a reservoir however by pumping high pressure fluids into the formation in order to break the rock and form fractures. Fractures can propagate thousands of feet into the rock formation when the fluid pressure exceeds the breaking strength of the rock, thus providing a pathway for gas and fluids to migrate into the borehole and drain the reservoir. The purpose of seismic fracture monitoring is to determine the direction and distance that fractures propagate from the treatment well. Subsequent production wells can then be drilled in locations where artificially created fractures are not already draining fluids. Optimally placed wells maximize production at minimal drilling cost.
A method by which fracture locations can be estimated has been reported in multiple publications and open meetings (House, et al, 2004). The method involves using measured time delays between P-wave and S-wave seismic events generated when the fracture opens and is similar to earthquake location methods (Jeffreys and Bullen, 1940 for example). We propose an alternative fracture location technique using methods akin to diffraction stacks and Kirchhoff depth migration.
Figure 1 sketches the components of a downhole seismic fracture monitoring survey. Data is typically continuously recorded over many hours with 3-component downhole geophones that should be placed to optimize the accuracy data processing results. The frequency content of fracture monitoring data varies between 200 Hz and into the kilohertz range thus the sample interval at which data is recorded should be set to accurately represent the maximum frequency present in the waveform. The number of downhole 3-component geophones used in fracture monitoring projects varies from a small number such as 4 in a single borehole with numbers greater than 40 in a single borehole becoming more common. The number of observation wells in which geophones are placed is typically only one due to practical constraints of available adjacent boreholes however some monitoring datasets have been simultaneously recorded in multiple wells.
Seismic wavefields recorded in fracture monitoring experiments differ from seismic waves in the commonlyused reflection seismic method in that the seismic source in a fracture monitoring experiment occurs at an unknown location and at an unknown time. The goal of processing downhole fracture monitoring data is to determine both the location and time of the event based on seismic events recorded in adjacent boreholes. A common method of approximating the location of fracture formation via downhole seismic data is based onearthquake location technology.
