Modeling of seismic response to stimulation and depletion. In the red box pressure is shown in MPa. Original reservoir pressure is 25 MPa. Velocity changes are represented in the color bar at right. These time changes ath the base and the velocity changes drive the time lapse seismic interpretation. (Images courtesy of Talisman Energy)
Operators in gas shale plays in the US do not need to be told that horizontal drilling and multistage fracture techniques are the dual gifts that have made these plays economic. In general, seismic technology has taken a back seat unless it’s microseismic monitoring of hydraulic fracturing. But one company in Alberta is combining the benefits of all of these techniques to open up new horizons.

Pouce Coupe

The Pouce Coupe field in northwestern Alberta has seen gas production for many years. In the late 1990s operators began developing the Montney reservoir as a conventional low-porosity gas field. The mid-Montney marker in this reservoir had the highest porosity of the formation, ranging between 3% and 15%.

With improvements in horizontal drilling and fracturing techniques, it began to be possible to develop the lower porosity zones outside of the mid-Montney marker.

The Montney formation in total is about 820 ft (250 m) thick, but zones outside the mid-Montney have much lower porosities, not much more than 5%.

The Pouce Coupe region began to be identified as an area where unconventional reservoir development techniques could make a serious contribution. Talisman Energy, a leading Montney property holder in the area, determined that using these techniques in a geologically known environment with ample well control could help refine its unconventional exploitation methods. The company identified several geophysical methods that could be applied and leveraged for a competitive advantage.

Chief among these was 4-D seismic monitoring. “This is one of the very first unconventional 4-D surveys to monitor stimulation,” said John Logel, senior geoscience advisor for Talisman. “It’s probably the first 4-D survey to monitor active completions, to assess the effects of fracture treatment on rock properties as they occur.”

He went on to say that rock property analysis tied to seismic modeling suggests that the Montney reservoir’s acoustic and elastic properties should respond to both pressurization and depletion, brought on by stimulation and then production. To test this theory, Talisman reshot the 4-D seismic surveys between fracture treatments. Modeling has indicated that an amplitude anomaly “halo” should define the stimulated area, which should reverse over time. The analysis will also highlight areas with bypassed pay to optimize recovery.

Fracture monitoring is another key component of the analysis. “We’re hoping to see the pressure front of where the fracture is occurring and to what extent,” Logel said. “This will help us determine where there is overlooked or bypassed pay.”

This technique is expected to aid in improving the efficiency of operations and understanding ultimate recovery, Logel said. It also helps clarify the local and regional stress regimes in the area. Several methods of fracture monitoring are available, and Talisman is testing most of them, ranking them, and determining the most cost-effective method or combination of methods to best monitor fracture dimensions and drainage area.

The company established an observation well from which to gather microseismic data during the fracture monitoring phase. It also drilled four shallow-water wells in which vertical seismic profiling (VSP) technology was installed. This provides four surrounding points to record microseismic data and triangulate events in 3-D.

Finally, a surface microseismic array was deployed to test the accuracy of differing microseismic methods.

Preliminary seismic difference showing time lapse change in Pouce Coupe data due to fracture stimulation of the reservoir.
Now what?

Not surprisingly, all of this monitoring has generated an enormous amount of information, and Talisman is currently processing and beginning to analyze the data. It is also sponsoring a student in the Reservoir Characterization Project (RCP) at the Colorado School of Mines to aid in the interpretation. The original design idea for the 4-D was conceived from work completed by the RCP.

“The idea was to use it as a field test lab for our unconventional world. So we collected a lot of this data, and we’ve extracted the engineering information out of it that we need. But there’s still a lot of geoscience data to be analyzed. So it only made sense to give this data back to RCP,” Logel said.

Kurt Wikel, who recently joined Talisman after completing his degree through RCP, said that the involvement of that organization will help his company push the boundaries of the research being undertaken. “It’s not that Talisman doesn’t have the internal expertise,” Wikel said. “We just have so much data, and we want to see how far it can be taken beyond conventional time-lapse analysis.

“Internally we’re getting what we want out of it. But there’s more that we can do, and a lot of it will be scientific in nature. There’s a great opportunity for geoscience to benefit from this dataset.”

The ultimate goal is to push the science of developing these unconventional reservoirs a little bit further. “Geophysically, we’re trying to fill an engineering need,” Wikel said. “But the other aspect is to see the difference between surface seismic and microseismic, as there is no industry consensus on how far pressure fronts actually travel.

“Microseismic events occur when rock breaks or slips. However, where events occur may not necessarily be where proppant and pressure go.”

At the end of the day, of course, it’s about tapping additional pay. “I haven’t heard of any other dataset that even comes close to answering this question,” Wikel said. “If there’s a big difference between microseismic and time-lapse seismic, if we’re missing a lot of reservoir, how does that affect our ultimate recovery?”

Already, added Logel, the company, which shot three seismic surveys in two weeks, is seeing measurable seismic differences. “As a company, we’re breaking new ground,” he said.