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) |
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. |
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. |
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.
Recommended Reading
Triangle Energy, JV Set to Drill in North Perth Basin
2024-04-18 - The Booth-1 prospect is planned to be the first well in the joint venture’s —Triangle Energy, Strike Energy and New Zealand Oil and Gas — upcoming drilling campaign.
EIG’s MidOcean Closes Purchase of 20% Stake in Peru LNG
2024-04-23 - MidOcean Energy’s deal for SK Earthon’s Peru LNG follows a March deal to purchase Tokyo Gas’ LNG interests in Australia.
Crescent Point Divests Non-core Saskatchewan Assets to Saturn Oil & Gas
2024-05-07 - Crescent Point Energy is divesting non-core assets to boost its portfolio for long-term sustainability and repay debt.
Permian Resources Adds More Delaware Basin Acreage
2024-05-07 - Permian Resources also reported its integration of Earthstone Energy’s assets is ahead of schedule and raised expected annual synergies from the deal.
Evolution Petroleum Sees Production Uplift from SCOOP/STACK Deals
2024-05-07 - Evolution Petroleum said the company added 300 gross undeveloped locations and more than a dozen DUCs.