At the Tight Gas Workshop held June 9 at the European Association of Geoscientists and Engineers (EAGE) Convention in London, the theme of Marlan Downey’s keynote address set

Figure 1. The time-lapse compressional (P) wave seismic response of the Cameo Coal Interval in 2003 and 2006. The highest EUR wells are shown in hotter colors, from blues .5 Bcf up to 10 Bcf in red. The total area is 2.5 sq miles or 6.5 sq km. (Images courtesy of the Reservoir Characterization Project at the Colorado School of Mines)
the stage for a fresh new look at this resource. Downey defined tight gas as gas production from rock with permeability less than .1 milliDarcy. He went on to point out that the main feature is the pore throat size, which is in the micron range; i.e, pore throats aren’t visible to the naked eye. In the micron range only gas can squeeze through the pore throats and not water because of the smaller molecular size of gas. Thus, water in the system is truly bound water, and the system is “closed.”

He also pointed out that when looking for tight gas, one needs to look for no free water in the system. Otherwise, he indicated, “You have a conventional gas reservoir and are doomed to fight water problems.” There are plenty of those reservoirs around as well, but they should not be lumped into the category of “tight gas,” he added.

Tight gas reservoir engineering
Tony Settari of Taurus Reservoir Solutions and the University of Calgary pointed out that many engineers use decline curves to try to evaluate tight gas reservoirs, but this approach needs to be rethought. He pointed out that the system fuels itself and that non-Darcy flow comes into play. Pressure-testing in tight gas is more of an art than a science, and zonal pressure testing doesn’t give the full story. Apparently it takes a long time to clean up wells, and drilling and fracturing fluids are a source of potential formation damage. An integrated approach is necessary to optimize development of this resource.

Integrated technology
Throughout the workshop the theme concentrated on the main point that tight gas requires an integrated approach. Since the audience was largely a geophysical audience, the question arose as to what role, if any, geophysics can play in tight gas exploration and production. The answer at the workshop was portrayed through a wide range of applications. Downey said it best: “How else are we going to look in all the tight places?”

To do this, we also have to look in the right places. Rulison field in the Piceance Basin of Western Colorado is a good example. Expected ultimate recovery (EUR) of wells varies from .5 bcf to as much as 10 bcf.

By using time-lapse seismic data we can probe the rock mass more completely. We can bust
Figure 2. Seismic shear wave azimuthal anisotropy. High anisotropy relates to high fracture density.
the rock up hydraulically, but more importantly we need to use what Mother Nature gives us and to do our work more smartly and efficiently. Mother Nature gives us natural fractures, and they help us access this resource. But natural fractures are where you find them, and to locate them you need to “listen to your mother,” something we all have been told but seldom heed.

Mother Nature gives us overpressured cells, connectivity and heterogeneity. That’s a lot when we just want to know when and where to show up for dinner. It is the same for this resource.

Time-lapse seismology gives us a historical perspective on tight gas as wells change the pressure in the reservoir, and pressure is difficult if not impossible to measure and monitor from wells. By monitoring these pressure changes we can tell which parts of the reservoir are being drained and which aren’t. This helps us avoid costly well completions into zones that are already connected and helps us target new wells or re-completions into zones that aren’t. The reservoir changes with time, so we need to monitor it.

Through monitoring we can see where the changes occur, and that tells us what part of this complex reservoir we are accessing. In doing so we can work smarter and better locate and stimulate our wells with more complex fracturing, including shear fracturing. We can use the natural fracture systems more effectively to connect to the reservoir.

Our lives are about connections, and the same is true of wells in developing tight gas. At Rulison we use time-lapse seismic to see what’s changing and where. By probing the rock mass through monitoring pressure change we can optimize resource development.

Figure 1 shows the time-lapse compressional (P) wave seismic response of the Cameo coal
Figure 3. Sealing and non-sealing faults exist at Rulison as determined from time-lapse seismic monitoring.
interval in the Rulison field from dedicated seismic surveys shot for Colorado School of Mines’ Reservoir Characterization Project in 2003 and 2006. The highest EUR wells are shown in hotter colors, from blues .5 Bcf up to 10 Bcf in red. The total area is 2.5 sq miles (6.5 sq km).
In tight gas the weakest rock is that which is fractured. Areas of change represent drainage areas. The larger areas relate to the best wells. These wells accessed the natural fracture systems and are connecting them up over larger drainage areas.

To develop these resources, we need to find the fracture systems early and monitor the reservoir. To find the fracture systems we use shear wave azimuthal anisotropy (see Figure 2). To understand the cause of fracturing we use time-lapse seismic to find the faults and determine what faults are conduits for gas migration and what aren’t (see Figure 3).

What is the cost? At Rulison the cost to acquire and process these dedicated seismic surveys is less than half the expense of a well. That’s cost-effective, but will anyone use this technology? The answer is yes, if they listen to their mother and truly want to develop this resource economically.

In summary, we need to treat Mother Nature with respect through better reservoir characterization and look for gas in all the right places through integrated approaches.