Title: Green River Basin Symposium Stimulates Energetic Discussions
Authors: C. Elmo Brown and Michele Bishop
Publication: The Outcrop, November 2003, p. 1, 6-7
“These pipelines have wheels,” said John Harpole in the first of a daylong series of thought-provoking talks at the RMAG/PTTC symposium on Petroleum Systems and Reservoirs in Southwest Wyoming held on September 19th. By “wheels,” Harpole meant the proposed projects would happen—that pipeline construction is rapidly moving into the Rockies as area gas is now displacing diminishing Canadian supply, particularly to markets in California and Arizona. The next 10 years are expected be most interesting in Wyoming, with price differentials evaporating as demand increases. The Greater Green River Basin will be an important supplier of gas to fill the new and expanded pipelines.
The 384 participants registered at the symposium were expecting to hear both sides of the debate concerning whether or not the Greater Green River Basin (GGRB) is a basin-centered gas system. As shown by the first talk, the attendees received much more. The symposium chairmen, Steve Goolsby and Mike Wilson, designed an encompassing and balanced program with a luncheon keynote address and eleven talks that outlined the history, the present state of knowledge, and the future prospects of this resource-rich basin.
In his keynote address, Robert Weimer detailed the chronological exploration of the GGRB. Beginning with an oil seep on the Mormon Trail used in the 19th century by pioneers who would mix the surface oil with flour and grease wagon axles and continuing on into the 1980s with the recognition of the basin-centered cell of overpressure in the Cretaceous and Tertiary, Weimer wove a tale of how each new step in knowledge and technology resulted in significant discoveries in the basin. He concluded that history shows that the key to new discoveries is to imagine where accumulations might be present which technology cannot now identify.
Several speakers detailed the present state of knowledge beginning with a pair of presenters illustrating how seismic responds to the over-pressured, gas-saturated zone. Another talk focused on how using slickwater fracture techniques in tight gas sands has shown to be an effective and low cost method for maintaining residual fluid conductivity and for reducing damage. Speaking of permeability, a talk by Alan Byrnes showed that lab-generated matrix porosities and permeabilities may dramatically diminish when placed under formation pressure. In the GGRB, Mesaverde-Frontier core porosity reduces by about 0.8 percent porosity and pore diameters may shrink by 50-70% in tight gas sands thus reducing core permeability by a factor of 3 to 40 times. This decrease in permeability combined with an increase in irreducible water saturation creates a gas/water transition zone of several hundred feet. He also demonstrated that near-borehole water damage might cause several months delay in optimum gas production as the water is slowly expelled. On the plus side, thin beds of high permeability matrix greatly enhance production from a tight reservoir.
As for the future potential of the GGRB, several authors discussed estimates based on the basin-centered gas model and whether or not the model is viable. Ronald Johnson presented the methodology used by the USGS in its new assessment of the Mesaverde total petroleum system in the Greater Green River Basin. Of the estimated mean of 84.6 TCFG, 131 MMBO, and 2.6 MMBNGL left to be added to reserves in southwest Wyoming during the next 30 years (with no economic constraints imposed), it is believed that 25.78 TCFG will come from the continuous-type reservoirs of the Mesaverde. Much of this addition is believed to come from infill drilling within known shallow reservoirs and discovery of new reservoirs (“sweet spots”) in deeper parts of the basin. Wayne Camp and co-authors showed that based on present production it appears that “sweet spots” are better described as subtle stratigraphic and structural traps within a basin-center gas setting and that future success in the basin is dependent on recognizing these traps. This presentation was voted Best Paper by the symposium attendees.
Keith Shanley and co-authors, chief proponents for conventionally trapped reservoirs in the GGRB, noted that water production is significant and widespread from these petrophysically challenged reservoirs. The authors showed that in tight gas sands, a very low Sw allows for maximum gas production. Since fluid flow is reduced as Sw increases, at some point if Sw is high enough, no fluid will enter the well bore. This explains why a downdip water leg has not been established in existing fields. These authors also argued that the only “continuous accumulations” in the GGRB occur when natural gas is adsorbed onto matrix particles, such as in coalbed methane and shale gas plays.
Ben Law contended that the deep GGRB still fits the definition of a basin-centered gas accumulation by being a gas-saturated, low permeability, abnormally pressured system that contains no water. He also stated that even though the USGS estimates seem high, reserve predictions are based on a 1% recovery rate. Though not stressed previously, the USGS has always believed that the east side of the GGRB contained heterogeneous reservoirs with good enough rock qualities to promote conventional trapping and accumulation.
Regardless of the type of system present, history of the GGRB shows that maximum field size keeps on increasing with each new advancement in technology or knowledge. Randal Billingsley opined that a mixture of resource recognition and technological innovation by risk-tolerant operators is required to tap into future opportunities in the 19,700-square-mile GGRB. He also showed that more than twelve years usually transpires between discovery and effective development of GGRB fields. Does this mean that the newest and biggest discoveries have already been drilled and are just waiting on the next cheaper borehole design or advanced completion technique?
All in all, the symposium was a resounding success with debates and discussions taking place during the after-symposium reception. Though debate continues on whether the GGRB is a basin-centered gas accumulation, the debate, and the science, has moved forward with some questions being answered and new ones proposed. For more information on the specifics of the symposium, the book of abstracts and presentations is available from the RMAG office for $20.