Microseismic monitoring has been largely accepted as an important adjunct to the hydraulic fracturing of unconventional gas reservoirs. Both surface and downhole methodologies are now common for the performance of such monitoring, each with its advantages and challenges. This paper discusses the state of the technology today and where current work is being directed at reducing cost and enhancing the value of microseismic monitoring.
While the fundamental concepts of microseismic monitoring of hydraulic fracture well stimulation were captured by J. R. Bailey (1973) in his patent, it has really been within the last 10 years that the technology has become commercially and technically important, especially in the unconventional gas and oil plays where hydraulic fracturing is an essential element of every completion. Today, perhaps as many as 10% of the unconventional completions are monitored and technical arguments can be made for driving that percentage higher, if it can be done at reasonable cost.
There are two alternative microseismic monitoring techniques commonly used today: surface and downhole monitoring. As well, there are three general classes of techniques for locating microseismic (MS) events: hodogram techniques based upon the particle motion of direct arrivals, triangulation schemes based upon arrival times of direct waves, semblance methods based upon stacking of waves without arrival time picking. All three classes of location techniques can be employed in conjunction with either surface or downhole sensors. Since the first two classes are based on discrete arrival time and signal polarization picks, downhole sensor deployment is often necessary in order to resolve the location. On the other hand, the aperture and fold requirements of the semblance class of location techniques tend to favor a large areal spread of sensors as can be most conveniently achieved with a surface or near surface array.
In this paper we will briefly report on the current state of the microseismic monitoring practice as applied to unconventional gas exploitation, as well as listing some of the areas of current research and development that are directed at making the technology more valuable.