Hydraulic Fracturing has had a global impact on the distribution of hydrocarbons. The United States has gone from being an importer of natural gas to potentially an exporter. Some believe that enough oil resources have been developed to make the United States the country with the largest quantity of reserves in the world. The combination of precision horizontal drilling and fracturing the reservoir rocks is responsible for this bounty.
The fracturing process results in micro-earthquakes which can be mapped to determine the extent and progress of the process. Most hydraulic fracturing operations are done without concurrent monitoring of seismic activity, but the use of microseismic monitoring for this purpose is growing. Today, approximately 4% of hydraulically fractured wells in the USA are monitored. Determining the magnitude and location of the fractures provides useful feedback to the asset team (consisting of a completion engineer, geologist, and geophysicist) to help evaluate the effectiveness of the frac operation.
There are three principal methods used to record the microseismic events:
with a string of sensors in an adjacent borehole,
with a surface array of sensors, or
with sensors permanently buried in an array of shallow boreholes.
A conceptual drawing showing all three methods is shown in Figure 1.