Monitoring of hydraulic fracturing operations has proved particularly valuable in shale gas development. Despite modern geomechanical modeling technology, there is typically a huge difference between the theoretical paths of formation fractures and their actual paths. Most operators use monitoring techniques to observe the actual propagation of hydraulically stimulated fractures so they can optimize successive stages in multistage operations. In some cases, real-time monitoring allows geohazards to be avoided by halting the treatment before the hazard is intersected or by pumping a diverter to “steer” the fracture into other reservoir volumes.

Typically, monitoring equipment is hung off in a nearby offset well, but this technique can be problematic if none are available. MicroSeismic Inc. has introduced a surface sensor array called FracStar that can acquire and pinpoint the locations of the almost inaudible microseisms that are generated as the formation fractures. Using a broad array of conventional surface geophones and a beam-steering technique, the entire frac pattern can be mapped without traditional error factors that evolve as distance increases from the monitor well. Events from each frac stage can be color-coded so operators can judge the effective coverage of each stage.

The first two stages of a multistage treatment are mapped. Stage 1 propagation is depicted by yellow dots and Stage 2 by green dots. (Image courtesy of MicroSeismic Inc.)

The PSET method uses a beam-steering approach that is analogous to a dish microphone or radio telescope. With this technique it is possible to detect and locate microseisms using only a surface array of conventional geophones. A surface array allows operators to map the entire frac pattern extent, not just those events close enough to the offset monitor well used in other microseismic monitoring techniques. Event patterns are undistorted by error factors associated with offset well monitoring, and because an offset well is not required, PSET operations are less intrusive.

Recently, a well in the Delaware Basin of West Texas was hydraulically fractured. The pay zone was about 12,000 ft (3,660 m) deep and was penetrated by a 1,500-ft (456-m) lateral in which the stimulation was to be pumped. A 951-element FracStar surface array was deployed to provide continuous recording of the stimulation treatment. The treatment was performed in several stages, with each stage packed off and isolated during treatment. Each stage lasted three to four hours. The figure represents event locations associated with the pumping of stages 1 (yellow) and 2 (green). The Stage 1 fracture propagated in the northerly direction only. The predominant direction was north-northwest, which was expected in view of the principle plane of horizontal stress. The cluster of events near the well bore resulted from late-stage increase of proppant density and likely indicates screenout. The Stage 2 fracture also grew mainly in the north-northwest direction but propagated more symmetrically on both sides of the well bore. A notable feature is the large amplitude cluster that propagates longitudinally near the well bore late in the second stage.

The combination of event mapping and time-based monitoring allowed the operator to gain significant knowledge of the way the rock was fracturing, providing valuable insight for future stimulations in the field. Since the job described above, six new wells and three re-frac wells have been treated and monitored, each adding to the extensive knowledge base that helps the operator decide how best to complete wells in this play.

Over the past six years, about 170 frac jobs have been monitored using the FracStar array. The company estimates that any time the proposed monitor well is more than 1,000 ft (305 m) away from the frac pattern, the use of the surface array becomes imperative.