Borehole microseismic monitoring is a method of inserting recording sensors down a monitoring well that is adjacent to a hydraulic fracturing well to record fractures created during the process. The fractures, or microseisms, create small-scale disturbances that can only be captured by a microseismic method. The data collected from this method allows models to be created that aid in future development of a field.
During the oil crisis of the 1970s, this method was developed as a way of monitoring and controlling the effects of hydraulic fracturing. Hydraulic fracturing gained popularity during this time as an interest in understanding geothermics increased, which was studied by the development of fluid injection into the Earth as a method of researching the temperature of the rocks. Hydraulic fracturing also aided in fluid circulation through the rock by fracturing the rocks. In the 1990s, the method proved to be rewarding in the Norwegian sector in oil fields like Ekofisk and Valhall. The results acquired from the Ekofisk field demonstrated the importance of microseismic, as the microseisms detected were at a depth that would not have been detected by a surface tool. 
The development of borehole microseismic monitoring has become a crucial method in shale gas exploration in the petroleum industry.  During shale gas exploration, microseismic monitoring allows the reservoir volume of the reservoir to be estimated, which can be assessed through velocity models. Through this method, properties of the reservoir such as geometry, height, length, and complexity, which give insight into the effect of the stimulation of the well, allow various models to be created, such as the one seen in Figure 1. 
Vertical Well Monitoring
When using microseismic to monitor a hydraulic fracturing well, vertical wells (Figure 2) have been the tool of choice. When hydraulic fracture wells are developed, the well initiates as a vertical well that is turned horizontal when it reaches the desired reservoir. To monitor the fractures that occur during the fluid injection (Figure 4), vertical wells are drilled in close proximity to the treatment well, encased and a line of sensors are inserted down that well. To properly assess the thousands of fractures that may occur, various vertical wells must be utilized, which can be a pitfall of this method. 
Horizontal Well Monitoring
Recently, because of the issues found with vertical well monitoring, an interest has increased in developing horizontal wells that run a path along the treatment well (Figure 4). Sensors are forced across of the monitoring well through the use of a wireline tractor, which may cause a limited distance which the sensors can cover. The use of this tool decreases the amount of wells that are need to be monitored.
Microseismic monitoring has become the most important method in analyzing the extent of a reservoir, mainly in unconventional fields where hydraulic fracking is utilized as the extraction method. The most important benefit that has been proven from this method is the ability to create a model that determines the maximum extent of a reservoir's potential, which can be used for further exploration in the future. A technical benefit is that it is facilitates the interpretation process as it displays the data in a single, simple display. Furthermore, the method can also help in distinguish faults that have been reactivated, which have larger magnitudes than fractures created by the hydraulic fractured event, which are smaller. 
While borehole microseismic monitoring has gained great popularity in the petroleum industry, research on this method has demonstrated the importance of being accurate in the principles of the methods used in the acquisition stage. One of the biggest pitfalls is the large amount of vertical wells that are needed to monitor a single hydraulic fracturing job. For horizontal monitoring wells, the pitfalls are that they are more complicated to develop, which also means increased costs. The sensitivity of the sensors needed to accurately record the microseisms is also an obstacle often encountered, as they are of such small-scale and the amount created can reach the thousands. 
Even though borehole microseismic is primarily used as a method of energy components exploration (oil and natural gas), monitoring wells have also been used as way of exploring other type of geological deposits. An example of this is the use of a monitoring well to investigate zinc and lead deposits in Mark Twain National Forest, Missouri. The survey was conducted as a way of analyzing water quality differences, affected by the deposits, and other relations between two aquifers that are found in the area. 
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