Abstract
Detecting and locating microseismic events that occur during a hydraulic treatment of a coalbed-methane (CBM) formation can be challenging because of the special properties of the formation itself and the specific treatment parameters. Based on experience gained working with dozens of monitored CBM treatments, several specific challenges and their solutions can be identified. Events created during such treatments are usually small in magnitude as a result of the often small injection rates and petrophysical properties of the target formation. To detect these small events over the ambient noise, the sensors have to be in relatively close proximity to the injection. In addition, it is important to reduce the ambient noise as much as possible. Because options to reduce ambient noise are limited, it is imperative to ensure that the sensors and casing are securely coupled to the surrounding rock. In some cases, it is better to place the sensors higher above the zone of interest if a better coupling can be achieved. Another common challenge in CBM formations is the high velocity contrast between the target formation and the surrounding rock, which creates some mathematical challenges in the accurate localization of the events. Extensive model calibration, accurate logs, and clear events in known locations (e.g., perforation shots) can improve the overall location accuracy significantly. Most of these issues can be addressed in prejob modeling studies that quantify viewing distances, location accuracy, and waveform appearance etc., before any microseismic data is acquired. These studies help anticipate potential challenges for particular projects as well as document realistic accuracy levels, which are essential for a comprehensive interpretation of the microseismic-event distribution.
