A Truly Massive Stimulation Technique

A man wearing 1961-era protective equipment (coveralls and a hard hat), standing on the rubble pile that fell from the roof, partially filling the project Gnome blast cavern. Photo courtesy of llnl photos via flickr
A man wearing 1961-era protective equipment (coveralls and a hard hat), standing on the rubble pile that fell from the roof, partially filling the project Gnome blast cavern. Photo courtesy of llnl photos via flickr

After World War II, the United States suddenly found itself with nuclear explosives, a tool for which there were few obvious uses other than spectacular and indiscriminate destruction. The government therefore initiated a program to investigate benign uses for the tool – almost like a boy seeing what could be blown up with a new box of firecrackers.

This program of turning weapons to peaceful uses was called Project Plowshare – after the biblical reference to “beating swords into plowshares” – and the actual explosions were referred to as “shots,” or “PNEs” (Peaceful Nuclear Explosions.)

Most of the proposals envisioned large-scale earth-moving projects, such as the excavation of new harbors and the building of canals – including a new one across the Isthmus of Panama – but one of the few facets of the Plowshares program to actually be tested outside of the Nevada Test Site addressed the problem of unlocking natural-gas resources in low-permeability sandstones of the U.S. Rocky Mountain basins.

Large volumes of gas were known to be present in these reservoirs, but the gas did not flow readily into wells.

The main problem ultimately turned out to be that drilling and completion techniques of the day were damaging the all-important natural-fracture production mechanism – but this was not yet understood, because the technology (seismic, image logs) and data base (core, outcrop studies, detailed well tests) did not yet exist to allow recognition of the pervasive natural fracture system or support the development of effective completions.

This data deficit eventually was addressed and successful techniques were developed through characterization projects such as the U.S. Department of Energy-sponsored MultiWell Experiment (“MWX”), but before such approaches were tried – and before it was realized that what we didn’t know was hurting us – the philosophy seems to have been one of forcing engineering solutions onto the problem.

They Had a Blast

Three actual nuclear stimulation attempts were carried out in the United States, in 1967, 1969 and 1972, in the San Juan and Piceance basins of New Mexico and Colorado. The Soviet Union conducted three similar experiments during the same time period.

The technique in the United States was to drill a well into tight, natural gas reservoirs and place a 1,500-pound “nuclear device,” or “physics package,” at the bottom of the well.

The expectation was that blast-related microfractures would propagate a few hundred feet out from a blast-and-collapse induced chimney of rubble, providing myriad pathways for gas to flow back to a re-entry well drilled into the chimney. Success was measured by comparing the post-shot gas flow rates and estimated recoveries to the rates and recoveries from nearby conventionally drilled wells, although this turned out to be a difficult comparison because of the inconsistent well-to-well production characteristics in such reservoirs.

Information is scarce, but the Soviets took a different approach, focusing on oil resources in limestone reservoirs. Their technique was to emplace the nuclear device in a dedicated well in the middle of a producing field, with the expectation that a blast would open fractures across the field. Pre-stimulation production rates for the surrounding producing wells were then compared to their post-stimulation rates.

Both countries claimed technical success, but several factors contributed to the termination of these projects, and the last such test was apparently carried out in 1979 near Ob in the USSR.

In the United States, the economics were marginal at best: A 10-fold production increase was the target, but the reported increase was a factor of between two and five, calling into question the claim of technical success.

The essential, but as-yet-unrecognized natural fractures in the reservoirs were undoubtedly damaged by crushed and melted rock. Moreover, the blast-related microfractures expected to form in the wall rock surrounding the chimney apparently did not form. These microfractures were reported from thin sections cut from the evaporitic wall rock of the blast cavern of the 1961, three-kiloton preliminary Gnome experiment conducted at 1,200-foot depth outside of Carlsbad, N.M.; microcracks apparently did not develop as extensively in sandstones at the 4,000-8,000 foot depths of the actual tests. However, investigators did not physically enter and investigate the blast caverns as they had at Gnome.

Even though natural gas production was enhanced during these tests, the limited degree of improvement would have been insufficient to support the high costs of the contemplated field development program of nuclear completions.

In addition, not surprisingly, the produced gas was somewhat radioactive, although the degree of radioactivity is rarely specified in reports. Radioactivity would have diminished over time, but at least initially the economic balance would have been eroded by the need to dilute the test well gas with gas from conventionally drilled wells in order to bring radioactivity down to acceptable levels for commercial use.

Another problem was that the BTU value of the gas from the shot wells was significantly diminished, since the blast converted some of the formation rock and natural gas into non-flammable CO2 and water vapor.

Ultimately, however, the public was not ready to burn this gas: A national environmental consciousness was beginning to develop at about this time, there were significant liability concerns, and the World War II “get it done at any price” attitude was beginning to fade.

In the end, operators did not clamor for the technology.

An Understanding

In a purely scientific sense, the nuclear experiments in low-permeability sandstones did not significantly enhance gas production because they put the cart before the horse, applying a new technology in the hubristic expectation that the inherent power of that technology would make the geologic complexity and the unknowns of the reservoirs irrelevant.

Geology, if considered at all, was an after-thought: The few geologic reports in the Plowshares literature are simplistic even for the 1960s – and the significance, even the presence, of natural fractures in the reservoirs, was not recognized let alone appreciated.

This was an era when the very existence of open natural fractures at depth could be and was still being debated.

Although the nuclear tests applied an innovative technology to a recognized, long-standing problem, they reinforced the lesson that one needs to understand a problem in order to have realistic expectations of solving it.

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Historical Highlights - John Lorenz

John Lorenz, a past president and elected editor of AAPG, has worked for the Peace Corps (Morocco), the U.S. Geological Survey, Sandia National Laboratories and, for the last five years, has been a consultant in naturally fractured reservoirs (FractureStudies LLC). He has won multiple AAPG awards, including the Jules Braunstein Award, the Distinguished Service Award and two A.I. Levorsen Awards. He holds a commercial pilot’s license and a Ph.D. from Princeton University.

Historical Highlights

Historical Highlights - Hans Krause

Hans Krause is an AAPG Honorary Member, Distinguished Service Award winner and former chair of the AAPG History of Petroleum Geology Committee.

A History-Based Series, Historical Highlights is an ongoing EXPLORER series that celebrates the "eureka" moments of petroleum geology, the rise of key concepts, the discoveries that made a difference, the perseverance and ingenuity of our colleagues – and/or their luck! – through stories that emphasize the anecdotes, the good yarns and the human interest side of our E&P profession. If you have such a story – and who doesn't? – and you'd like to share it with your fellow AAPG members, contact the editor.

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