Microsiesmic Monitoring & Fracking: Downhole or Surface?

seismic spreads with a large aperture and plenty of receivers (up to 1,500). High multiplicity of accumulation and special modelling methods that involve algorithms for high-resolution seismic emission imaging ensure high degree of confidence in highlighting weak microseismic signals from the deep frac zones on the background of intensive surface noise. Such monitoring systems were used for monitoring the hydrofrac reservoirs of Upper Neocomian zones and Achimov reservoirs on Western Siberia oil and gas fields.

The problems include the following:

• weather and seasonal factors
• influence of natural landscape conditions on installation of the tools (swamps, lakes, etc.)
• placing the receiver points near the sources of powerful technological interference (engineering structures, power lines, roads, pipelines, etc.)
• presence of imaginary and false sources
• restrictions on receiving aperture

To improve setup conditions, operators must embed the equipment, including in small wells. Imaginary sources appear due to waves (from anthropogenic surface sources) that were reflected at the boundary located at a depth equal to half the depth of the hydrofrac target formation. During hydrofrac in the vertical wells, they appear in the center of the fracturing zone. False sources are the variety of imaginary sources; they are observed in frac operations in deviated wells and usually form an intense loop directed towards the fall of the drill string. Often, this loop masks useful target sources. For rejection of the imaginary and false sources, operators use special algorithms with simultaneous positioning of sources and calculation of corresponding optimal velocity model: the effective speed for imaginary sources would be lower than for the targeted emission sources (because the reflected waves spread through upper, low-speed part of the formation). This paves the way for automatic rejection of false solutions.

The last mentioned issue happens because the optimum aperture is usually chosen as about double of the target horizon depth – which means that for the deep horizons, for example, for Achimovsky collectors, the signals of calibration shots from frac perforation gun cannot be traced over the whole length of seismic points. This prohibits a full calibration, including correction of statics for all geophone points, forcing to reduce the aperture thereby reducing optical resolution of the method.

Notably, large amount of data (one to two orders of magnitude higher compared to the downhole monitoring) handicaps the real-time processing options.

Conclusions

Obviously, the choice of optimal microseismic frac monitoring technology requires consideration of all the mentioned factors and the price-quality ratio. For example, well-monitoring technology is feasible for the deep target formations where the priority task is to monitor the vertical development of the fissure zone for forecasting the fissure breakthrough to the neighboring water-saturated horizons – if there are suitable wells for monitoring. Notably, simultaneous monitoring via two or more wells, even if they are not the old fund, is the safest solution for pilot projects.

If the main task is to solve the standard problem of defining the trend, sizes, and assessing the filtration properties of the frac fissure zone – in many cases, the best choice