The Stimulation of Hydrocarbon Reservoirs

With Subsurface Nuclear Explosions

John C. LorenzSandia National Laboratories

Albuquerque, NM 87185

Abstract

Between 1965 and 1979 there were five documented and one or more inferred

attempts to stimulate the production flom hydrocarbon reservoirs by detonating nuclear

devices in reservoir strata. Of the five documented tests, three were carried out by the US

in low-permeability, natural-gas bearing, sandstone-shale formations, and two were done

in the USSR within oil-bearing carbonates. The objectives of the US stimulation efforts

were to increase porosity and permeability in a reservoir around a specific well by

creating a chimney of rock rubble with fi-actures extending beyond it, and to connect

superimposed reservoir layers. In the USSR, the intent was to extensively fracture an

existing reservoir in the more general vicinity of producing wells, again increasing

overall permeability and porosity. In both countries, the ultimate goals were to increase

production rates and ultimate recovery horn the reservoirs. Subsurface explosive

devices ranging from 2.3 to about 100 kilotons were used at depths ranging from 1208 m

(3963 ft) to 2568 m (8427 ft). Post-shot problems were encountered, including smaller-

than-calculated fracture zones, formation darnage, radioactivity of the product, and

dilution of the BTU value of tie natural gas with inflammable gases created by the

explosion. Reports also suggest that production-enhancement factors from these tests fell

short of expectations. Ultimately, the enhanced-production benefits of the tests were

insufilcient to support continuation of the pro-grams within increasingly adversarial

political, economic, and social climates, and attempts to stimulate hydrocarbon reservoirs

with nuclear devices have been terminated in both countries.

DISCLAIMER

This report was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.

DISCLAIMER

Portions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.

IntroductionMC22 2WI

During the three decades following the end of the Second World War, bo&s*r. I

United States and the Union of Soviet Socialist Republics dld research and field testing to

develop nuclear explosive devices for non-military applications. In the US, this work

came under the Plowshare program (as in “beating swords into plowshares”), initiated by

President Eisenhower in 1957 and fimded through 1978. In the USSR the program was

called Nuclear Explosions for the National Economy.

The stated goals of these programs were to find peaceful uses for the huge

amounts of energy provided by nuclear explosions (Sylves, 1986). The devices used

were commonly referred to not as “bombs” but by the somewhat incongruousterm

Peacefid Nuclear Explosions, or more conveniently by the acronym “PNE”. The energy

released by such devices was considered to have great promise for use in projects other

than purely destructive ones, the main attraction being the potential economy of scale.

Ideas that were considered included the excavation of large volumes of earth and rock for

mining and underground storage purposes, and for the creation of harbors and canals

(including the possibility of blasting of a larger canal parallel to the Panama canal)

(Sylves, 1986).

The stimulation of hydrocarbon reservoirs was another application considered for

nuclear devices, and several pilot projects were carried out in this area in both the US and

the USSR. The goal was to increase both the rates of production and the ultimate

recovery volumes, by exploiting the zone of massively fractured rock documented to

surround other underground nuclear tests.

As shown by military underground te- at America’s Nevada Test Site, and by

the 3. l-kiloton Project Gnome experiment detonated in 1961 in bedded salt in

southeastern New Mexico, the high temperatures and pressures of a nuclear explosion

within rock strata first vaporize and melt large volumes of rock. This produces a

spherical void or cavity tens of meters in diameter, lined at the bottom with a glassy melt

of fused rock containing much of the radioactive fission products from the blast. The

shock of the blast also produces a zone of damaged rock surrounding the cavity. This

rock has been compressed and compacted on the underside of the cavity due to the.

. ..

unyielding nature of the underlying strata, but it has been heavil y fractured above the

cavity where the shock of the blast was directed toward the free land surface.

A “chimney”, a hundred meters or so tall and filled with rock rubble, forms within

minutes to hours of the explosion as the shattered rock overlying the point of detonation

collapses into the cavity. The surface area of a chimney is significantly greater than that

of a typical oil-field wellbore, and was considered to have potential for enhancing

diffusion of hydrocarbons fkom a surrounding reservoir into a collecting wellbore.

Fracturing and shattering of the rock also extends beyond the chimney into the formation

for some distance. This fracturing had additional potential of providing conduits for

hydrocarbons between the formation and a wellbore drilled into the chimney after the

blas~ or to break up the formation and increase reservoir permeability on a broad scale

(Howard and Fast, 1970, Kreith and Wrenn, 1976).

In the US, the chimney/fracture complexes created by high-energy nuclear

devices were expected to provide a method for unlocking the large natural gas resources

locked up in “tight” (less than 0.01 millidarcy permeability) sandstone reservoirs,

primarily in the Rocky Mountain region. The problem was (and still is) that although the

volume of natural gas in such reservoirs in the Rocky Mountain region is measured in

tens to hundreds of trillions of cubic feet, it is typically disseminated within low-porosity,

low-permeability, interbedded sandstone and shale intervals thousands of feet thick.

Conventional vertical wells drilled into such reservoirs commonly do not produce gas at

rates sufficient to pay the costs of drilling and completing wells within an economic time

frame.

There are various non-nuclear techniques for stimulating low-permeability

reservoirs, all aimed at enlarging the surface area available for percolation of gas through

the formation and into a wellbore. The need is to access more of the reservoir than that

penetrated by just the surface area of a 20-cm (8-in) diameter hole in the rock. However,

the volumes of rock involved in the Rocky Mountain reservoirs are typically so large that

it is difficult to get enough energy into the formations to successfidly stimulate more than

a small fraction of the vertically-stacked reservoirs penetrated by a well, or more than a

short distance out horn the wellbore.

,

This problem has since been partially solved by the use of horizontal wells,

advances in hydraulic fracturing, and not insignificantly, rises in the price of natural gas

which make it economically viable to drill more closely spaced conventional vertical

wells. However, natural gas in the mid-60’s was a vast resource without a universally

successfid, economically viable production strategy. The lure of nuclear technology as a

potential solution to the problem was high in an era of can-do optimism.

In addition to the numerous military and non-military nuclear test shots petiorrned

at the Nevada Test Site and Project Gnome in southeastern New Mexico, the US

performed three nuclear-stimulation tests of natural-gas reservoirs in 1967, 1969, and

1973, under the Plowshare program. There were reports of plans to develop the affected

gas fields with hundreds of nuclear stimulations pendhg successfi.d completion of these

tests. However, such plans were not developed beyond the three initial tests due to a

combination of factors, including ambiguous to disappointing test results and changing

economics and politics (LOOK magazine ran a story titled “The Nuclear Threat Inside

America” in December of 1970). The appeal of nuclear technology was tier

diminished by the fact that no-one was williig to underwrite the liability of such

programs. Finally, the growing environmental movement of the 60’s and 70’s in this

country led to significant negative public reactions to the tests.

In contrast to US practice, the Soviets targeted oil reservoirs as the most likely to

benefit flom their nuclear stimulation efforts. (They also experimented at least twice

with nuclear explosions for the purpose of controlling runaway or blow-out oil wells:

Nordyke, 1975). Only sketchy details of the results fiorn two of the USSR nuclear

stimulation tests have been published, although there have been allusions in the literature

to at least one other, acknowledged verbally by Soviet investigators (e.g., Nordyke,

1975). In addltiom information derived from seismic records and the known geology in

the area of one Soviet nucIear detonation has pointed to what was probably the third or

fourth and last Soviet reservoir-stimulation test in 1979 (Raichlin and Clarke, 1980). The

reasom”for termination of the Soviet program of nuclear stimulations have not been

published.

A listing and map of the 122 acknowledged non-militay Soviet nuclear

explosions, their locations scattered across the former USSR have been published by

.

Sultanov et al. (1999). Tentative correlations have been made here between that list and

some of the published details of the USSR stimulation tests (such as approximate nuclear

yields) to determine the most probable locations, names, and dates of the Soviet tests.

Descriptionsof the tests

PROJECT GASBUGGY: As reported by Howard and Fast (1970), Bradford

(1970), Holzer, (1970), and Kreith and Wrenn (1976), a fusion device designed for 26

kilotons but with a post-shot calculated yield of 29 kilotons was emplaced into a 17.5-

inch (44.5 cm) wellbore and detonated on December 10, 1967. This test, designated

Project Gasbuggy, was designed to both stimulate production and increase the reserves of

natural gas in the sandstone layer of the Cretaceus Pictured Cliffs Formation at a depth

of 1292 m/4240 ft in the San Juan basin, northwestern New Mexico.

Thirty kilotons was calculated to be the upper limit of desirable yield due to the

necessity of avoiding fi-acture extension into the overlying water-bearing formation. The

device was placed 12.2 rn/40 ft below the target sandstone reservoir in order to let the

hoped-for post-shot chimney develop up into it. The test produced a 5.2 magnitude

seismic event and cost about $4.7 million 1969 dollars. The dimensions of the chimney

created by the sho~ derived secondarily from subsequent production rates, were

calculated to be on the orderof101 xn/333 ft high and49mI/160 fi in diameter. The

fracture zone surrounding the chimney was variously calculated to have extended 91

m/300 ft to 152 m/500 fi into the formation, similar to the pre-khot design dimensions.

The post-shot cavity (not actually entered by investigators as was the Gnome cavity), was

estimated to be 51 m/166 ft in diameter. The GB-ER well was drilled into the chimney

after the shot, and gas was flared from this well during the course of several tests

performed during the following two years.

The test was evaluated in several ways. Initial production was reported to have

been close to a million cubic feet of gas per day, or between 200’XOand 740% of the rates

of production from nearby unstimulated conventional wells depending on which well was

used for comparison. It was estimated that the ultimate recovery from the GB-ER well

wouId have been about 0.9 to 1.0 billion cubic feet over a 20-ye& life span, or 800°Aof

that of the average local conventional well. Finally, the stimulated well was estimated to

,.

be capable of recovering nearly twice the vohu-ne of the original gas in place that was

being recovered by conventional wells (22?40vs. 13%).

However, the gas from the GB-ER well itself was both radioactive and diluted.

Virgin natural gas in the formation is 99’XOhydrocarbon, mostly methane. Gas produced

fi-omthe GB-ER well six months after the shot consisted of 36% C02, 17’%0H2, and 4%

CO, created in part from vaporization of carbonate components of the host rock during

the explosion and resulting in a significant dilution of the BTU value of the original

methane. Two years after the shot, during most of which time the well was shut in but

after the flaring of 284 million cubic feet of gas during tests, gas from this well still tested

at only 80°/0of the BTU value of the gas from surrounding conventional wells.

The radiation introduced into the formation gas by the explosion initially tested at

700 picocuries/cc of tritium and 110 picocuries/cc krypton 85. These levels had been

reduced to 35 picocuries and 7 picocuries respectively by the end of testing (Kreith and

Wrenn, 1976). Other wells in the field, the nearest being 792 m/2600 ft distan~ showed

no indications of either damaged or enhanced production, and did not produce radioactive

gasses.

Enhanced fracturing of the Pictured Cliffs Formation beyond the chimney, one of

the primary purposes of the test, was “not detected” by Project Gasbuggy. Engineering

studies of the production rates suggested that the entire measured enhancement in

production could be explained by an increase in the effective diameter of the collecting

surface, i.e., the increase from that of a wellbore to that of the chimney.

The Gasbuggy site is still accessible by dirt roads, and is located in a remote and

rather bucolic section of the Carson National Forest nearly 100 krn/60 rni east of

Farmington, NM. The area is unpopulated: deer, elk, and cows occasionally graze their

way across the area. The location is depicted on the US Forest Service map, and

although the site itself has been reclaimed and re-vegetated it is marked by a small

fenced-off square, with a wooden Forest Service sign as well as a brass plaque mounted

on a concrete slab. The brass plaque, all in capital letters, is a curious mixture of tourist

information and obscure legs.1/environrnental warnings. It reads:

PROJECTGASBUGGY

NUCLEAREXPLOSIVEEMPLACEMENT/REENTRYWELL(GB-ER).

.

SITE OF THE FIRST UNITEDSTATESUNDERGROUNDNUCLEAREXPERIMENTFOR THE

STIMULATIONOF LOW-PRODUCTIVITYGAS RESERVOIRS. A 29-KILOTONNUCLEAR

EXPLOSIVEWAS DETONATEDAT A DEPTH OF 4,227 FEET BELOW THIS SURFACE

LOCATIONONDECEMBER10,1967.

NOEXCAVATION,DRILLING,AND/ORREMOVALOFSUBSURFACEMATERIALSTO A TRUE

VERTICALDEPTHOF 1,500FEETIS PERMITTEDWITHINA RADIUSOF 100FEETOF THIS

SURFACE LOCATION, NOR ANY SIMILAR EXCAVATION, DRILLING, AND/OR REMOVAL OF

SUBSURFACE MATERIALS BETWEEN THE TRUE VERTICAL DEPTHS OF 1,500 FEET AND

4,500 FEET IS PERMITTED WITHIN A 600 FOOT RADIUS OF THIS SURFACE LOCATION IN THE

SE QUARTER OF THE SW QUARTER OF SECTION 36, T29N. R. 4 W., NEW MEXICO PRINCIPAL

MERIDIAN, RIO ARRIBA COUNTY, NEW MEXICO, WITHOUT UNITED STATES GOVERNMENT

PERMISSION.

UNITED STATES DEPARTMENT OF ENERGY

NOVEMBER 1978

The nearby US Forest Service sign notes, in part, that “The Atomic Energy

Comission currently monitors water sources in the surrounding area for radioactive

traces.”

PROJECT RULISON: Project Ruliso~ located in the Piceance basin of

Northwestern Colorado, was the follow-on test to Project Gasbuggy. A nuclear device

weighing 682 kg/1 500 lb was lowered into the gas-bearing interbedded sandstones and

shales of the Mesaverde Formation at 2568 m/8427 R depth and detonated September

10*, 1969. The target was less focused, the aim being to let the predicted chimney

connect several superimposed gas-bearing sandstone reservoirs 6-18 In/20-60 ft thick and

separated by equally thick shale layers.

Kreith and Wrenn (1976) reported that the fission device used in this test was

specifically designed to reduce the volumes of associated tritium, in response to

unacceptably high levels of this radioactive element found in the post-shot gases

produced at Gasbuggy. The size of the explosion (designed for 40 kilotons, with a post-

shot calculated yield of 43 kilotons: Morse, 1977) was also significantly increased,

probably due to the more difise target and probably as an attempt to increase

production-enhancement results beyond those seen at Gasbuggy.

The Rulison shot produced a 4.9 seismic event and, according to the Rifle

Telegram of September 9, 1987 (near the 18* anniversary of the shot), three minor after

shocks. Kreith and Wrenn (1976) reported that the flow in local streams increased by a

factor of three and became turbid within several hours of the shot, but that they returned

to normal within three days. The post-shot chimney dimensions were calculated (again

from production characteristics) to be 23 m/76 ft in radius and 107 rn/350 ii in height, not

significantly less than the 27 rn/90 fl and 115 m/376 ft respectively specified by the

design. However, the post-shot calculated radius of fracturing into the formation (67-113

In/222-370 ft) was only 45-75% of the design radius (149 m/485 fi).

Initial post-shot production rates were reported to be 200-400% of those of nearby

conventional wells. The ultimate recovery of gas from the test site was initially estimated

at 5-7 billion cubic feet but was later revised to 1.8 billion cubic feet. Nevertheless, it

still compared favorably with the average of one-half to one billion cubic feet of

estimated ultimate recovery from those same conventional local wells.

However, of the total of 455 million cubic feet of natural gas produced in four

stages after the shot (totaling 108 days of flaring over the course of 202 days), 26°/0

consisted of C02 and 110/0consisted of water vapor. No further production rates or

compositions have been reported, and the well was plugged and abandoned within a year

of the test.

Although not reported in the same format as Gasbuggy, the volume of some of the

radioactive elements generated by Rulison was “considerably less” than that from the

Gasbuggy test. A total of 1064 curies of krypton 85 and 2824 curies of tritium were

released during the flaring of 455 million cubic feet of gas over the course of 108 days.

Kreith andWrenn(1976) report that the cesium 137 and strontium 90 levels were

elevated in comparison to the Gasbuggy products.

However, whereas Gasbuggy had been widely accepted by the public, Project

Rulison was met with a groundswell of negative public comment and press, and was the

subject of several lawsuits.

PROJECT RIO BLANCO: Project Rio Blanco, the last of the US tests, was

conducted on May 17*, 1973. As with Project Ruliso~ this test also targeted interbedded

sandstone and shale strata in the Piceance basin of northwestern Colorado, but was

designed specifically to encompass a much larger vertical interval that included both

parts of the Mesaverde Formation and the basal strata of the overlying Wasatch

Formation. The slightly different mechanical layout of this test consisted of three

devices, nominally of 30 kilotons each, emplaced at three different levels and detonated

simultaneously. The emplacement depths of 1780, 1899, and 2039 m (840, 6230, and

6690 ft) below the surface, were designed to stimulate and interconnect a thick section of

these gas-bearing strata. The use of a larger energy source (total post-shot yield of

approximately 33 kilotons for each of the three devices) was directed at the same goal.

Fortunately, tracers unique to each of the three detonations were designed into the test so

that post-stimulation gas derived from each level could be identified.

Post-shot production horn a well drilled into the upper cavity flowed at a

grati&ing 5.48 million cubic feet of gas per day, and flared 35.5 million cubic feet of gas

over the course of seven days (Toman, 1975). However, the pressure dropped rapidly by

40V0,from 2050 to 1260 psi (13.9 to 8.6 MPa) during the course of this short test. A

subsequent two-week production test, conducted after “adelay of about two months to’

allow reservoir pressure to build back up, flared 62.2 million cubic ft of gas but

demonstrated a similar rapid pressure drop.

Post-shot analysis for tracers suggested tha4 disappointingly, the produced gas

was coming exclusively flom the upper detonation in the Wasatch Formation. No

communication between the three predicted chimneys could be documented, nor was

there any production horn the lower two zones, despite the fact that the three shots were

separated vertically by only 119 m/390 fl and 140 rn/460 ft.

The natural gas produced from this test was again diluted by inflammable gasses

created by the explosion, consisting of 50-60°/0COZand 15°/0Hz when tested. fiere

were hints that the percentage of COZwas actually starting to increase toward the end of

the production test, possibly due to the ex-solution of explosion-related gasses from

formation waters as resemoir pressures were drawn down (Toman, 1975).

Radioactivity levels of the gas were reported to have decreased by 5-1OVOover the

seven-day course of the first test, during which 242 curies of krypton 85 and 23 curies of

tritiurn were released.

Investigators, sponsors, and investors must have been iiustrated at this point,

wanting to know why the various nuclear stimulation tests were not producing the

anticipated sustained high production volumes. Money was allotted for more detailed

post-test arxdyses than for the previous two shots, including drilling into the lower zone,

and nearby coring (Ballou, 1978).

Two out of three core runs taken near the lower chimney were successful,

recovering 8.2 m/27 ft and 4.4 m/14.4 ft of core. Although one core was taken 74-78 m

(243-256 ft) from the point of detonation and the other significantly closer, at 55-57 m

(180-1 87 ft) from that point, no differences in the degree of shock-induced

microfiacturing were observed between the two cores. Microfiacturing, documented in

controlled test samples of various types of rock exposed to the nuclear blast in the earlier

Gnome project, was felt to be a key to enhanced formation permeability, and thus to

enhanced production.

In fact, no shock-induced fracturing was observed in either of the Rio Blanco

cores. This puzzling absence led to the important conclusion that enhancement of

fi-acturing in the formation had not in fact occurred as fa out from the explosions as had

been predicted or desired (76 m/250 fi). According to Ballou (1978, p. 80) “Significant

explosion-induced permeability enhancemen~ in this case, does not extend as far as 2.6

Rc, the range of the closest cores” [Rc is the radius of the chimney, calculated to be 23.5

m/77 ft]. The physically documented maximum radius of the effects of the explosion

within the formation was insufficient for a successful stimulation at this site, casting

some doubt onto the calculated radii of effects for the previous Gasbuggy and Rulison

tests.

In retrospect was deemed that the Mesaverde and Wasatch formations at the Rio

Blanco site were not in fact suitable for a nuclear stimulation due to insufficient reservoir

press~es and to the relatively small, discontinuous nature of the sandstone reservoirs. It

was opined, afier the fact and with some misgivings, that the reservoirs at this site had

been insufficiently characterized prior to testing (Ballou, 1978, p. 87-88). Interestingly, it

was the hoped-for ability of nuclear explosions to comect such disseminated reservoirs

that had been one of the proposed justifications for nuclear stimulations. This, the

smaller radius of fracturing, the failure of the three shots to intercomect, and the large

.

public outcry over this test, were among the factors that combined to eventually stop the

program.

The follow-on Massive Hydraulic Fracturing experiments conducted near this site

were equally frustrating. Hundreds of thousands of gallons of fluid and hundreds of

thousands of pounds of sand proppant were pumped at high pressures into the formation

but resulted in minimal and even negative production enhancement. These results were

eventually explained as formation damage and multiple planes of hydraulic fracturing.

SOVIET TEST #l: The USSR allowed their scientists to report the results of two

nuclear stimulation tests, including one that was conducted in a carbonate oil and gas

reservoir at a location specified initially only as “Field ‘A’” (Orudjev et al., 1971;

Nordylce, 1975; Werth, 1970a,b; Anonymous, 1975). A comparison between the

published data for this test and the list of non-military Soviet tests published recently by

Suhanov et al. (1999) suggests that this experiment comprised the Butane 1-1 and Butane

1-~ shots, detonated on March 30*, and J~e 10*, 1965. The lati~de and longi~de

presented by Sukanov et al. place this test about 300 km/200 rni east of Kuybyshev, in

the southwestern part of the former USSR

This field had been producing oil for seven years prior to the test, and thus had an

established production history and decline curve. Two devices of 2.3 kilotons each were

detonated simultaneously in two wells, separated by 200 m/656 ft laterally and at depths

of 1348 m/4422 fi and 1378 m/4521 ft within the reservoir. These shots were placed.

between 120 and 200 m (394-656 ft) from the nine nearest producing wells. A third

device of 7.6-8 kilotons was applied to the same reservoir at the later date, at a depth of

1350 m/4429 ft in a separate hole 350 m/1006 ft distant flom the first shots (Nordyke,

1975).

Daily production rates, variously reported as for the nine producing wells nearest

the shot holes or for the 20 wells within 470 rdl 542 ft of the stimulation weIls,

apparently improved by about 34°/0. The estimated ultimate recovery for the field was

reported to have improved by 25-35°/0due to flattening of the decline rates.

Interestingly, the gas-oil ratios for these wells were reported to have decreased

after the shots, a counter-intuitive result given the carbonate reservoirs and given the US

experience of vaporized rock and high C02 contents following a subsurface nuclear

detonation. The GOR’S in the Soviet field went from 450 cubic meters of gas per ton of

oil to 150 cubic meters per ton, and were apparently sustained at that level for several

years following the shot (Nordyke, 1975). The compositions of the post-shot gases have

not been published.

Release of radioactivity to the atm&phere was said to have been prevented, and

the radioactivity of the oil produced after the test was described as an “insignificant” 1 x

10-9millicuries of cesium 137 and 4.2x 10-8curies of tritium per liter of oil (Werth,

1970b). This is consistent with the results of the Gnome experiment, where only 5% of

the controlled samples of oil exposed to nuclear radiation from that shot became

radioactive (Howard and Fast, 1970).

No magnitudes have been published for the associated seismic events. Orudjev

(1971) reports that the fractured zone created by the explosions extended to a radius of

300 to 400 m (984-1 640 ft) from the detonations, with individual cracks found

(presumably in cores) at a distance of up to 800 m/2625 ft.

SOVIET TEST #2: A second Soviet nuclear stimulation, described only briefly,

was conducted in “Field B“, another limestone and dolomite oil reservoir at slightly

shallower depths (Orudjev et al., 1971; Nordyke, 1975; Anonymous, 1975). Correlation

to Suhanov et al.’s 1999 list suggests that this test consisted of the Griffon-1 and Griffon-

z shots, detonated September 2nd and 8ti, 1969, east of the Ural mountains and about 250

km/1 50 mi south-southwest of Perrn. Two 7.6-8-kiloton devices were detonated

sequentially in the middle of the field at depths of 1208 m and 1212 m (3963, 3976 ft), in

wells separated laterally by 1200 m/3937 ft and separated from the nearest producers by

150-300 m (492-984 h).

Orudjev (1971) reported a production enhancement by 30-60’Hoin the seven wells

within 150-800 m/492-2625 ft of the test wells, and that injection tracer tests indicated an

overall increase in formation permeability, of unspecified magnitude. No pressure or

temperature increases were observed in the reservoir during or after the tests.

Radioactivity of the produced oil was reported to be “within admissible concentration

limits”, and without release to the atmosphere.

SOVIET TEST #3: The last USSR example with any documentation at all is only

postulated to have been an attempted nuclear stimulation, and was located in the Middle

Ob region of western Siberia. This shot produced a 5.4 seismic event on October 4ti,

1979. USGS estimates from seismic records (Raichlin and Clarke, 1980) reconstructed

the device to have been on the order of 100-kilotons, although Suhanov et al. (1999) list

the Kimberlite-1 explosion of this date as being of only 21-22 kilotons.

TM shot was never described by the Soviets as a stimulation test, results have not

been reported by Soviet authors, and the purpose of the testis not listed by Suhanov et al.

(1999). Rachlin and CIarke (1980), however, inferred that hydrocarbon-reservoir

stimulation was in fact the objective of this test from its location within the bituminous

shale region of the Salyrn oil field. They reconstructed the emplacement depth as

approximately 3048 m (10,000 ft), although the Sultanov et al. list places the Kimberlite-

1 test at an emplacement depth of only 837 m (2746 ft).

If this was in fact a stimulation test, the goals may have included the cracking and

release of the oil in the local bituminous shales in addition to fracturing of the formation.

It is tempting to infer that whether it was a 21- or 100-kiloton explosion*, frustration at

marginal results in earlier tests led to attempts to achieve the expected results by

increasing the size of the nuclear detonations, as in the US.

*(An expIosion of 100 kilotons should have produced more than the reported 5.4

maetitude seismic event: Walck, personal cornrnunication, 2000.)

Differences Between the US and Soviet Programs for Nuclear Stimulation of

Hydrocarbon Production

The US and Soviet technical approaches to the problem were significantly

different. American tests were all done in relatively deep, low-permeability, natural-gas

bearing formations composed of interbedded sandstone and shale, where significant

volumes of gas are known to be in place but where the extremely low permeability of the

formation typically restricts flow from unstimulated wells to uneconomic rates. US tests

were therefore designed to stimulate the production of gas in fields where the rates were

marginal to begin with. In contrast, the Soviets targeted existing oil reservoirs where

production had been declining, hoping to revitalize production and improve recovery.

In US tests, the nuclear package was placed directly in the location where gas was

to be produced in order to create a chimney of rock rubble and associated fractures that

effectively provided a large-diameter wellbore. The degree of success was then

measured by comparison of production from this shot hole to the production rates from

several nearby conventional wells completed in the same reservoiriformation. Since the

formations that were stimulated are both marginal and heterogeneous, production from

the conventional wells used for comparison is highly variable, making post-shot

production enhancement dif%cult to assess.

Soviet practice was to place nuclear devices at depth into dedicated holes in the

vicinity of producing wells, hoping to fracture a large zone encompassing the areas from

which the nearby wells were producing. They then compared pre- and post-shot

production rates and decline curves from these fields. These comparisons are somewhat

easier to make, although Soviet reticence and the paucity of published details obscures

assessment of the results.

The tests reported by the Soviets made use of smaller devices than those used for

the first two American stimulations (2 to 8 kilotons vs. 26 to 40 kilotons). One reference

suggests that the low-yield Soviet devices were used to minimize damage to nearby

settlements and installations. However, the trend in both countries was toward

explosions with higher yields.

Results and Discussion

The results of the US stimulation tests in low-permeability sandstone reservoirs

were technically, if not economically, successful, in that they did in fact increase the

volume of produced gas. However, three basic problems became apparent:

1. The BTU value of the gas produced from these wells immediately after the

shots was decreased significantly due to the creation of large volumes of

nonflammable (predominantly COZ)gases by volatilization of the rock and

formation fluids under the high temperatures and pressures of the explosion.

. 2.

3.

The methane produced from the wells stimulated with nuclear devices was

sufficiently radioactive, at least during the short-term, post-shot production

tests, that dilution with gas produced from conventional wells would have

been required prior to safe marketing, diminishing the economic advantage of

the nuclear stimulation.

The post-shot production enhancement was not as high as the ten-fold

increase desired and predicted, and by the time the first two problems were

factored in was apparently insufficient to justify the extra cost of a nuclear

stimulation.

Part of the reason for the problem of less-than-desirable production increases was

that the chimney-fracture complex created by the explosive shock was apparently smaller

than predicted. This may have been due to an inhibition of upward-growing shock-

generated fractures by the high overburden/confhing stresses encountered at depths of

the actual tests. The chimneys and fracture zones produced by nuclear detonations at the

Nevada Test Site and at Gnome, which were the models for the stimulation tests, were

the product of relatively shallow detonations: Gnome was expolded at a depth of only

360 m/1 184 ft. The zone of crushed/compacted rock, created by unyielding strata and

documented only beneath the cavities of the shallow nuclear tests, may in fact have

created low-permeability damage zones surrounding the entire cavity in the deeper tests

where the weight of the overburden was significantly greater: American nuclear-

stimulation tests were conducted in reservoirs at depths of from 1292 rn/4240 ft to 2568

m/8427 ft.

Such a zone of damage, never included in descriptions of test design or accounted

for in post-shot analyses, would significantly inhibit flow rates from the formation into

the nuclear cavity. The nuclear enlargement of a wellbore into a chimney may not have

been enough to compensate for a damage zone. Finally, fracturing of the formation

beyond the chimney, both predicted and required for success, was not observed in cores

taken from the reservoir rock adjacent to the shot after the Rio Bkmco test, nor was it

required in order to account for the limited production enhancement resulting born these

shots.

.

BTU dilution and radioactivity would have diminished during production over the

course of months to years, as the local effects of the nuclear explosion faded with half-

life attenuation of the radioactive elements, and as volumes of gas beyond the area

influenced by the detonation were drawn to the chimney. However, it was estimated that

a third of the reservoir volume would have been produced by the time these were

negligible considerations. The volumetric advantage of a nuclear stimulation would be

most apparent early in the production life of a well when production rates are highest, but

this would also be the time when the produced gases are most radioactive and have the

least BTU value. As these two problems were ameliorated, the difference in production

rates between a nuclear stimulation and a conventional stimulation would also diminish,

along with the advantage of the nuclear stimulation.

The Soviets were also able to enhance both production rates and the estimated

total recovery of hydrocarbons from reservoirs through the use of nuclear stimulations.

The shallower reservoirs targeted by the Soviets may have allowed more of the desirable

upward fracturing and chimney formation than did the deeper US tests. Moreover,

carbonate strata stimulated by the Soviets maybe more susceptible to shock-induced

microfiacturing, as documented by experiments during the Gnome test (Howard and Fast,

1970). Nevertheless, there have been no fbrther reports of nuclear hydrocarbon-reservoir

stimulations in the USS% suggesting that the success rate was no more capable of

justifjhg continuation of the Soviet program than the US program. Of course, a

consideration in this equation is that successfid production enhancement need not have

been the most important consideration in a market driven by other than economic factors.

Diminished gas-oil ratios reported after Soviet tests are surprising given their

stimulation of oil-bearing, carbonate reservoirs, and in light of the American experience

of having gasified some of the calcite-cemented sandstone reservoir rock to C02. A

significant amount of the oil in place as well as the host carbonate rock might have been

expected to have been converted to gas during the Soviet tests, thereby increasing the

gas-oil ratio. Other factors such as the types, amounts, or “acceptable limits” of

radioactivity in the produced oil were either not measured or not reported.

Taken at face value, the Soviet results, like the American results, suggest a “

technical success, but discontinuation of both programs suggests that the successes

ultimately were insufficient to support development of the program beyond testing stages.

Summary

Digging data out of the literature on the nuclear stimulation tests of hydrocarbon

reservoirs presents interesting contrasts. Much was written about the design and promise

of these tests prior to actual testing in the US, yet relatively little has been written about

the final results, probably because the results were ambiguous and poorly understood.

When reported, results are given in different formats, different units, and with different

slants. The fact that the projected large follow-on programs were not carried out suggest

that the tests, although technically successful, were not in fact economically successful.

Published results of the USSR tests are particularly hard to come by, although perhaps for

other reasons. The Gasbuggy, Rulison, Rio Blanco, Butane, Griffon, and Kirnberlite test

results were apparently insufficient y positive to justifj continued testing or development

of the nuclear stimulation techniques in either the US or the USSR. Because of this, and

combined with other factors such as increasing environmental awareness and changing

social, economic, political, and military conditions, the programs of nuclear stimulations

of hydrocarbon reservoirs were terminated in both countries. There have been no efforts

to revive these techniques during recent times of high energy prices.

Acknowledgments

This manuscript has benefited significantly horn reviews by Marianne C. Walck,

and Scott P. Cooper.

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