Passive borehole microseismic activity emanates from a reservoir due to changes in stress and pressure and is monitored with a string of triaxial geophones in a monitoring well. Applications of passive microseismic monitoring include mapping the extent of fractures during hydraulic fracture treatments, fault mapping, and tracking a gas or water front during assisted recovery production. Microseismic seismic monitoring has been employed for some 40 years with resurgence during the last 10 years as evidenced by the number of service companies now providing this service. Published articles in the mid 1980’s discussed hydraulic fracture monitoring with a triaxial geophone located in the same borehole where the hydraulic fracture treatment was applied. Current technology generally uses two wells: a treatment well and a monitor well where the string of triaxial geophones is emplaced for monitoring the hydraulic fracture. To evaluate any hydraulic fracturing modeling and processes, the extension of the hydraulic fractures that have been generated must be known in terms of direction, length, height, and growth history. Such information can be provided by microseismic fracture monitoring. The moveout and the differences in time between P- and S- wave arrivals are used to calculate the distances from a monitoring well to the origin of a microseismic event. The event direction is calculated from a hodogram. Important elements of a monitoring system include the receivers, telemetry systems, and automatic processing of vast amounts of data. This paper summarizes the physics, mathematics, and uncertainties of the microseismic fracture monitoring process. Interpretation procedures and issues are discussed, and examples of the important results are illustrated. In particular, key elements of microseismic monitoring include development and confirmation of a velocity model and microseismic event detection from a continuous stream of data from 8 to 12 triaxial geophones. Each individual event is downloaded to an event file and processed automatically by an event locator to fix an event in time and space. In the process, the uncertainty in location of an event is also determined. Finally, by plotting all events temporally relative to the treatment and to the monitoring wells in 2D and 3D, the growth over time of a hydraulic fracture can be determined in terms of direction, length, height, and growth history. All of these key elements are illustrated in this paper.
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