The creation of a fracture by injection of fluids is always accompanied by deformation of the earth’s surface and radiation of seismic energy from microseismic events. Both features are often exploited in the monitoring of hydraulic fracture operations by using arrays of tiltmeters or seismic receivers. Knowing the orientation, height, and length of hydraulic fractures is often important in the design of closely-spaced pairs of injectors and producers, in designing optimal fracture treatments for other wells and for optimizing reservoir management in fields with fracture-treated wells. In general, geophysical techniques are currently incapable of determining either the width (aperture) of a single fracture or the composite width of a multiple fractures. >> Oil & Gas Engineering Books <<
Monitoring during fracturing
Seismic receivers are used in a manner similar to that employed for passive seismic monitoring. Typically, they are deployed in one or more nearby wells, perhaps shallow wells drilled for this purpose, but they provide better observations the closer they are to the fracture depth. The receivers are usually multiple-component geophones that are clamped to the wellbore wall and deployed at multiple depths in the monitor well(s). The arrival times of the P-waves and S-waves are used to locate the events in space and time. Because the use of just one or two monitor wells does not permit traditional triangulation, it is usually necessary to supplement the arrival time information with the azimuth of the arriving P-waves, as determined from particle-motion analysis (or polarization), to help constrain the location of the events. In modern applications, the growth of the fracture can be monitored in real time, and information can be provided to the completions engineer on site. The events monitored consist primarily of shear events in the immediately surrounding rock after the fracture tip has passed; by accumulating the locations of these events, an image of the fracture as it grows can be obtained in three dimensions (see Fig. 1).
Fig. 1—Passive seismic monitoring results showing fracture locations in a cross-sectional view of seismicity and intervals of gas production from Clinton County, Kentucky (after Phillips et al.).
Use of tiltmeters
Tiltmeters can be deployed at the earth’s surface or in wellbores (see Fig. 2). Noise from wind and other ambient conditions can largely be eliminated by placing the “surface” tiltmeters in shallow holes (20 to 40 ft; 6 to 12 m), and most modern studies use these shallow wells rather than placing tiltmeters directly on the surface. Tiltmeters can also be deployed in a deeper monitor well to provide better estimates of the fracture parameters. Deformation is monitored with an array of several tiltmeters; predictable tilt features caused by the solid-earth tidal loading are removed, and the resultant signal is inverted in near-real time to provide an interpretation of the fracture as it grows. As a reservoir is produced from a hydraulically fractured well, the stresses may change over time, and a new refracture treatment may result in a new set of fractures or extensions of the original fracture at different azimuths. Tiltmeter studies have demonstrated that complicated refracture reorientations can sometimes be significant for reservoir management.
Fig. 2—Schematic view of a vertical hydraulic fracture and its associated deformation field. Example locations of tiltmeters are shown.).