REENTRY DRILLING PROGRAM

Reentry drilling in hole GB-ER started on December 13, 1967. As planned, the drilling program consisted of removal of the sand and cement from the center of the 7-in. casing used to emplace the explosive (gas was used to remove the drilling chips). No deformed casing was encountered above 3800 ft, but the reentry operation was hampered by extremely cold weather and stuck drill pipe. Below a depth of 3300 ft the circulating fluid had to be changed to a lightweight drilling mud, since the cement was sufficiently moist to preclude further gas drilling. A differential temperature log established a water entry at a depth of 3553 ft through a cementing slot, with the water evidently coming through the open cement stage tool at 3548 ft. The rate of influx was about 2 cubic feet per hour. Chemical analysis of samples taken established the water to originate in the Ojo Alamo formation. Traces of radioactive xenon-133 were found in the water, as were barely detectable amounts of tritium.

At a depth of 3858 ft, a 6-ft void was encountered, and circulation was lost. No radioactivity was encountered, and a pressure of 48 psi was measured at the surface. Both the 7- and the 20-in. casing appear to have parted at this point.

A water level probe run into the hole at that time showed water standing at a level of about 1748 ft in the hole. Thirty barrels of water were then put into the hole, which should have raised the water

level by some 800 feet. A re-run of the probe established the new water level at 1739 ft.

Drilling was then resumed (still through the 7-in. casing) with mud being injected at the rate of about one barrel per minute without circulation being established. On January 10, 1968, a 9-ft void was encountered at a depth of 3907 ft. At the bottom of this void the drill bit encountered steel, making further drilling impossible. About 20 min. later, radioactive xenon was detected at the surface. At that time it was concluded that connection with the chimney had been established. The distance of this point from the detonation center agrees well with the lower limit for a failure radius calculated preshot; the fracture data from GB-1 and from the geophysical logs in GB-ER indicate a fracture radius about 100 ft larger, or about 40 ft greater than the maximum fracture radius calculated preshot. These points should be cleared up by further postshot work being proposed.

GEOPHYSICAL LOGGING

A limited number of logs were run in the region below 3800 ft inside the 7-in. casing. These consisted of density, gamma-radiation, collar locator, and caliper. A borehole camera was also run in an attempt to examine the void between 3907 and 3916 ft. Only the gammaradiation tool was able to reach a level of 3912 ft; its detector, located 6 ft above

The static gas pressure was read at the surface with a Heise pressure gage calibrated with a dead-weight gage and capable of a reading accuracy of better than 1/2 psi. The extremely cold weather and the presence of moisture made repeated heating of pipes necessary in order to keep gage lines open. This difficulty is probably responsible for some of the scatter in the pressure readings.

After the void at the depth of 3907 ft was penetrated, the pressure at the surface was monitored frequently. The time history of the buildup of this pressure is shown in Fig. 7. A maximum of 833 psig was read at the surface after about 20 hr. On January 23, 1968, a surface pressure of 840 psig was recorded.

Fig. 7. Time history of surface pressure buildup, January 10, 1968.

A formation pressure of 1050 psi at the 4150-ft level was estimated preshot from the buildup curve of GB-1.

PRELIMINARY FLOW TESTS

About two days after penetration of the void at 3907 ft, a very limited number of low-volume flow tests were instituted. The purpose of these tests was to obtain a clearer idea of the concentration of

radionuclides in the gas, and, if possible, of the nature of the connection with the chimney. In Flow Tests I and II, conducted on January 12, 1968 and January 12-13, 1968, respectively, gas was withdrawn from the 7-in. casing directly. Flow Test III, on January 17, 1968, took place after a production packer and 2-7/8-in. tubing had been set at 3797 ft. In all cases the gas was flared at the top of a 50-ft stack after passing through a bank of charcoal filters and a number of

liquid traps. A series of regulators between the well head and the filters kept the flow limited to the capacity of the filters and held the flow approximately constant. An orifice meter measured flow rates. The gas stream was continuously monitored at a number of places between the well head and the stack for the presence of radioactivities.

The decrease of the surface pressure as a function of time during Tests I and II is shown in Fig. 8; the start of Test II followed the start of Test I by 10 hr, 45 min. After Flow Test II, the pressure buildup was monitored, but icing of the

gage resulted in erratic readings and the attempt was concluded after 1/2 hr.

After tubing was installed, Test III was carried out about 5-1/2 days after Test II. Flow rates during this test were

about 1.5 M2cfd, and a bottom-hole pres

sure gage and a thermometer were used. The pressure gage showed a decrease from 953 to 950 psig during the first 40 min of flow. A maximum temperature of 152°F was recorded during this run, which is about 25°F above the preshot ambient temperature. Figure 9 shows the pressure-time behavior of Test III.

Periodic gas samples were taken and analyzed at the site during and after the flow tests to establish concentrations and to document the total release. In addition, surface and bottom hole gas samples were collected in evacuated bottles for a more thorough analysis at LRL.

The only radioactive constituent detected in the gas at the site during the flow tests was xenon-133, which emits gamma rays of 80-kV energy with a half life of 5.3 days. Its concentration measured at the flaring stack during Flow Test I was 4.5 X 10-2 μCi/cm3, and μCi/cm3.

during Flow Test III, 1.1 X 10

-2

3

No iodine was detected in the samples analyzed at the site or on the charcoal filters in the gas stream, even though the detection efficiency for iodine is about greater than that for xenon.

105

As previously mentioned, some tritium was detected in water samples taken while investigating the water influx into the 7-in. casing at the 3550-ft depth. The levels were very low-approximately three times the natural background. No tritium was detected in any of the samples taken during and after the flow tests and analyzed at the site.

Laboratory analysis of the gas samples is currently in progress, but no results are available at this time.

Regulators begin to freeze shut Stop run-1551

DISCUSSION

Results available from the GB-ER reentry and exploration are not very extensive at this time; more will accumulate as data are analyzed and evaluated. The fact that connection with the chimney was established is clear from the pressure and radioactivity encountered. The dynamic fracture data and the results of the geophysical exploration indicate that the extent of rock failure was about as expected. The flow data obtained cannot be considered significant relative to either reservoir evaluation or chimney volume determination, nor were they meant to be so. Their main purpose was

to explore the general nature of the connection of GB-ER with the chimney region. It is this author's opinion that the results available at this point in time from the total postshot program are not consistent with a picture that depicts GB-ER communicating cleanly with a region of high permeability. This opinion is not shared by the EPNG participants in the technical program who contend that the data are too limited and sufficiently confusing to preclude any conclusions at this time. Whatever the resolution of this question may be, it is clear that more work is needed before a conclusion can be reached with respect to the gas stimulation objective.

This report could not have been written without the cooperation of many individuals and organizations in making early results available to the author. In particular I wish to thank W. Perret and B. Murphey of the Sandia Laboratories for much of the close-in surface and subsurface data, D. Edwards and R. Kinnaman of the Atomic Energy Commission and K. King of the U. S. Coast and Geodetic Survey for the more distant ground motion and structure response information, and H. Kendrick of El Paso Natural Gas

Company for information on bottom-hole pressures, temperatures, and data on the surrounding wells. Throughout the chimney investigation and particularly with respect to the flow tests, the help and advice of C. Atkinson of the U. S. Bureau of Mines was welcome and valuable, as were the many conversations with L. Truby, W. Cutler, and W. Martin of EPNG. Lastly, I wish to express my

appreciation to my colleagues at the Lawrence Radiation Laboratory for their unstinting cooperation.