Understanding hydraulic fracture growth, effectiveness, and safety through microseismic monitoring

Abstract

Microseismic monitoring is a very useful tool for optimizing fracture treatments, evaluating completion schemes, and assessing well layouts and spacing in unconventional reservoirs. The microseismicity is induced by the reservoir changes resulting from the hydraulic fracturing process. The dimensions and orientation of the fracture can usually be deduced from the microseismic distribution, and it is often possible to determine other features of the fracturing process, such as complexity, asymmetry, and interaction with geohazards. It is important to understand the geomechanical process that occurs during fracturing to best interpret the microseismic distribution and to fully understand the value of any source analyses, such as moment tensor inversion. The perturbations imparted to the reservoir during fracturing are usually very large and can result in unexpected behaviour, if ignored. Microseismicity monitoring has provided a very large data base from which environmental impacts of fracturing can be assessed. With thousands of fractures monitored, there is clear evidence that fractures do not extend the thousands of feet vertically to the shallow depths of typical aquifers. Fractures are generally much longer than they are tall as a result of the rock mechanic barriers that result from sedimentary structures. Microseismicity monitoring has also provided evidence that hydraulic fractures are not likely to generate felt earthquakes in anything other than the rarest circumstances. Most of the seismic activity induced by a hydraulic fracture has energy levels that are 1,000 to 1,000,000 times smaller than events that would be felt at the surface, and even much farther below those that might cause damage.